A dynamo and separate control box were used on Mk1 MGBs. The first MGB alternators (MkII models in 1967) were the 16AC (remote regulator) and 16ACR (integral regulator) which are rated at 34 amps. It was changed to the 17ACR in February 1973 and finally the 18ACR in about June 1976. There is conflicting information about the output rating of these latter two, some sources say the 17ACR is 36 amps and the 18ACR 45 amps, others say the 17ACR is 43 amps and the 18ACR 45 amps. 43 or 45 amps ought to be sufficient for factory loads, the V8 has a Delco 46 amp alternator and that is sufficient to keep the battery voltage above 12.5v even with headlights, twin cooling fans and the heated rear window running, and at idling speeds. The problem is that people fit voltmeters wired to the green circuit, which can be a couple of volts lower than the solenoid i.e. battery voltage, then get paranoid. It's battery voltage which is important, and any volt-drop between there and the green circuit is down to ageing connections, and the best alternator in the world isn't going to cure them, although it may cover them up.

The B+ (or BATT+) voltage sensing terminal is wired back to the solenoid with a standard gauge brown wire. This is used to sense the voltage at the solenoid rather than the alternator for voltage regulation purposes, and would ensure that under high current conditions any volt-drop occurring in the main output wires (thick brown and black) between alternator and solenoid/body is ignored and the voltage at the solenoid (and hence the battery) is maintained at the correct level, this system is called 'battery sensing'. This was the case in the 5-pin 2-plug 16ACR from 69 to 71.

Initially the 3-pin single-plug alternators used machine sensing (i.e. the correct voltage is maintained at the alternator terminals, but could be lower at the solenoid and hence battery under high current conditions) with just a single thick brown and a standard gauge brown/yellow in the alternator plug. This is a '2-wire' alternator. Clausager states that a new version of the 16ACR with modified regulator and surge protection was provided in March 72.

Possibly because of problems with low battery voltage, in 1973 the alternators seem to have reverted to battery sensing again (Clausager states the 17ACR was fitted from February 73) now with an additional thin brown in the alternator plug wired back to the solenoid as before, and this seems to have remained the case up to and including the 77 model year at least. This is a 3-wire alternator, but can be used with a 2-wire harness by connecting the third spade to the output spade in the alternator plug. A '2-wire' alternator i.e. one with two large output spades can be connected to a 3-wire harness as-is i.e. each brown wire to either of the large output spades, although the harness plug may need to be changed to fit the alternator.

The final variation was the 18ACR. Clausager says the exact change point is unavailable, but thinks it was from June 76, borne out by the Parts Catalogue which shows the 18ACR being used before September 76. The schematics get confusing here, with UK 1979 from the WSM and 'later' models for both UK and North America indicating it had reverted to machine-sensing with two thick brown wires from the two large output spades to the solenoid. This would give increased current carrying capacity and lower volt-drop now cars had electric cooling fans, offsetting the loss in voltage caused by the regulator sense terminal moving from the solenoid back to the alternator again, and despite the three wires is effectively a '2-wire' system. However JCR Supplies shows this diode pack which is the 3-wire battery-sensing type, and I have seen the harness plug on a 78 which does match that. However they say it is for an 18ACR, which other sources indicate should be machine sensing. They also show one with the two large spades i.e. the Euro plug, correctly described as machine sensing, but saying it is for 15/16/17ACR which is only partially correct.

  October 2014:

And to beat it to death, put a bullet in its brain, and hang, draw and quarter it, there are additional changes to the above in the Parts Catalogue:

• The original 16ACR as detailed above - part No. 37H 4194
• A different 16ACR in Jan 71 - 37H 6983
• The modified 16ACR as detailed above in March 72 - 37H 7503
• The 17ACR as detailed above in Feb 73 - 37H 7959
• A different 17ACR (no date) - 37H 8208
• An 18ACR (no date, but prior to September 76 and used to the end of production) - AAU1013

'Two browns' (what a terrible thought) wiring will cope equally well with both battery sensing and machine sensing alternators, but battery sensing alternators must have the 2nd brown wire, or at least a link in the harness plug between the + and B+, to operate correctly. In addition to the brown/yellow Indicator wire a friends 72 only has one brown (large), my 73 has one large and one smaller brown, and another friends 74 is the same, so those at least conform to the above. For completeness my 75 V8 (AC-Delco) uses the same plug, both large spades are output terminals, however only one is wired (as per the factory schematic) with a heavy gauge brown, the other is unused (and has allowed me to use it as a direct output to the cooling fan relay).

So some care needs to be taken to determine just which type of wiring, plug and alternator you have when making changes, even swapping alternators which take the same plug. If by looking at the two large spades on the alternator you can see they are clearly connected together, then you have a machine-sensing alternator and can use either or both large spades for the output. But if the two are clearly insulated from one another, then you have a battery sensing alternator. On these you must have a large gauge brown wire on the output spade at the very least, and a smaller gauge at least on the sense terminal. If in doubt as to which you have, it may be possible to determine by voltage measurement. Turn all the electrical loads on you possibly can, alternator plugged in, engine running at a fast idle, then connect a voltmeter between the two large spades. If you can measure any voltage between the two (may only be in the order of tenths of a volt) then you probably have a battery sensing alternator. If there is zero volts between the two large spades, then you probably have a machine sensing alternator. Or simply provide large gauge brown wires to both large spades to cover both eventualities, and get the benefit of a lower volt-drop under high-current conditions if you have a machine sensing alternator.

Tip: If you carry an alternator as a spare at any time, then it's a good idea to make sure it already has a pulley fitted. The large nut is very tight and makes it very difficult if not impossible to remove the pulley from a failed unit as there is no easy way of holding the rotor still (except perhaps by wrapping a fan-belt right round the pulley and gripping it firmly). If your spare alt has a pulley, then compare the size with what's on the car. If it's the same size then all well and good. If it's a different size then check now by trial-fitting that it is compatible with your fan-belt! And remember, if the pulley is smaller the alternator will rotate faster than normal, so you may want to limit engine revs a little to avoid over-revving the alternator. If it is larger then it will rotate slower, so you may find the engine needs to be revved a bit higher before it starts charging, will stop charging sooner as the revs fall, and it may not charge at idle. The charge voltage and current during normal driving will also be lower than usual, but if you keep the revs up and/or the electrical load down it should still charge well enough to get you where you are going.

Rear: Both dynamo and alternator use their own versions of an angle bracket to attach a single mounting point on the rear end-plate to two bosses on the block. Early blocks only had two for the dynamo, whereas later blocks have one pair for a dynamo and another pair further forwards for an alternator. After-market adapters are available to mount an alternator to an early block which only has the rear-most pair of bosses.

Why 'Idiot' light? I don't know, but it seems to be an Americanism (that is, it's Americans that seem to use the term, not that Americans are idiots as one seemed to think I meant ...). The only thing I can think of is a point of view that says "Only an idiot would need a warning light telling them the ignition was on." Which shows a complete misunderstanding of the purpose of the light, so who's the idiot now? However someone else said that he has heard the oil warning light (provided in lieu of an oil pressure gauge) referred to as the 'idiot' light, because only idiots ignore it when it comes on then seize their engine. But another view has it that even idiots should be able to understand when a warning light comes on, whereas you need intelligence to understand a gauge. So maybe, in terms of the ignition warning light, only an idiot ignores it until the battery goes flat, and as Jochen Beyer has pointed out the ignition warning light also lets you know your fan-belt has broken before you boil your coolant out.

The ignition warning light always had two wires connected to it which must be insulated from earth as at various times one side receives 12v from the ignition switch and the other side 12v from the alternator or control box. Most of the other bulb holders on CB 4-cylinder cars only need one wire as they can pick up an earth from being plugged into to metal panel or instrument case. V8s and RB 4-cylinder cars have the other warning lights with two wires as well as they are fitted in plastic panels so need a wired earth.

The warning light is like a pair of balance scales between the ignition circuit and the charging circuit, and that is how it is connected - from the white of the ignition circuit, through the bulb, and to the dynamo/alternator via the brown/yellow. (Note that the lamp-holder is unique in that it has two wires - one to each side of the bulb - and the body of the holder should not be connected to earth like the panel and main-beam lamps are.) If both circuits have the same voltage then there is no potential difference across the bulb and it will not light. This is irrespective of whether there is 0v on both circuits (ignition off, engine stationary) or 12v (actually around 14v when charging) on both circuits (ignition switched on and engine running and charging). If the two circuits show a potential difference i.e. ignition switched on but engine stationary, or ignition switched on and engine running but not charging, then the lamp will light. This latter condition is a fault (and incidentally the main purpose of the light) which should be investigated before you get stranded. You may also note that when you switch off the ignition but while the engine is still spinning the ignition warning light glows again until the engine stops. This is because the charging system is still outputting while the engine is spinning down, so outputting 14v to the brown/yellow, but with the ignition off there is no longer any voltage on the white. By itself that wouldn't cause the warning light to glow, but as there are several other circuits also powered from the white, each with their own path to earth, current from the brown/yellow passes through the warning light bulb and these other circuits to their earths, and that is enough to make the light glow, until the charging system stops outputting. It is this feature that gives rise to the "won't switch off!" problem that can affect 1977 and later North American spec.

At the simplest level, a glowing warning light tells you that the ignition is switched on but the dynamo/alternator is not charging. It may be obvious that the dynamo/alternator isn't charging if you haven't even started the engine yet, but the beauty is that you can see the warning light itself is working. So if the engine is running and the charge does fail at some point, then you have a very good chance that the warning light will come on and tell you about it. On alternator-equipped cars the warning light acts as a priming system to start it charging from about 1000rpm. Without the warning it may need to be revved to several thousand rpm to start, and new alts out of the box may not do that. Dynamos are self-priming as below.

Change to LED? The short answer is no. As stated above the warning light on alternator-equipped cars is used as a primer to start it charging, using the current through the incandescent bulb. If changed to an LED the current is reduced to a fraction of its previous value and won't allow it to start charging at the normal point. The warning is also part of the on-board diagnostics, and current can flow in either direction under various conditions. An LED only glows with current flowing in one direction - unless you go to the bother of installing it with a full-wave bridge rectifier. Although this effect is not an issue with dynamos there is another aspect relating to the warning light acting like a pair of balance scales. It is inevitable that there will be some differences in voltage between the ignition supply and the dynamo/alternator output and this difference in voltage increases as the demand for current goes up i.e. lights, wipers, heater fan and so on are turned on. Because an incandescent bulb is relatively insensitive to small voltages it will not glow, unless there is a fault resulting in a bigger difference in voltage then there should be. But LEDs are much more sensitive to small voltages and will glow when incandescents wouldn't, including when there is no actual fault as such. Even some suppliers will tell you not to fit one in this position.

With the exception of RHD cars from 1977 on with the ignition relay, the white for the warning light always comes off the ignition switch. On RHD 1977 and later cars it comes off the output of the ignition relay which starts off white/brown, then changes to white at the multi-way plug behind the dash. See ignition schematics for more info.

On a dynamo system the warning light brown/yellow is connected to the dynamo output at the control box and hence has a low-resistance path to earth to light it when the ignition is turned on. The initial excitation for the dynamo field always comes from its own residual magnetism, which is why you have to 'flash' the field terminal to battery when you install a new dynamo or when you are converting from one polarity to another. NEVER, I repeat, NEVER flash an alternator's terminals to battery. This residual magnetism results in a dynamo output of a couple of volts, which is passed through low-resistance windings on the cut-out and current regulator relays in the control box to the field winding. This voltage now causes the dynamo to output its full voltage, which operates the cut-out relay to connect the dynamo output to the battery so charging it. The cut-out relay has a normally open contact which disconnects the dynamo when the engine is stopped, or the output voltage drops below a certain level. In fact it usually releases at idle, lighting or flickering the warning lamp. If this did not happen the battery would rapidly discharge through the dynamo, which would be acting like a motor trying to turn the engine. The cut-out relay has two windings, one of which ensures the relay releases as the voltage falls. IMPORTANT NOTE: If you manually operate the cut-out relay with the engine stopped it will latch in, connecting battery voltage to the dynamo, which will try to turn the engine. This passes a high current through the control box and dynamo which will burn them out in quite a short time.

Because it has connections to both the ignition supply and the alternator or control box the bulb holder is a special in that it always has two terminals which are insulated from the body of the bulb holder so cannot short to an earthed metal panel. Various other bulb holders also had two terminals at various times - Mk1 indicator tell-tales for the same reason as the ignition warning light, V8 and RB indicator tell-tales and main-beam warning lights as they are fitted to a plastic panel and so need a wired earth.

The current regulator relay operates on a similar principle, but it only comes into play when the maximum design current of the dynamo is reached. The relay operates, introducing the same resistance into the field circuit, which again reduces the field voltage and hence the output current, to protect the dynamo against overheating and damage. This reduction in current causes the relay to release again, so giving full current, which causes the relay to operate again and so on, giving an average current over time as before. The system is designed such that this average current (19 to 22 amps) is the safe current for the dynamo. With large non-original electrical loads connected to the system it will be the current drawn by these that will cause the current regulator relay to operate to protect the dynamo. The loads are still connected of course, and so is the battery, and it will be the battery that will be supplying them then, at least partially, so gradually discharging it, even though the engine is running and the dynamo is operating correctly.

I've heard a claim from someone who studied the workings of the control box at college (many years ago!) that there is a weakness in the system in that if the battery is less than half charged the characteristics of the control box are such that the battery will never recharge, and you would have to recharge it using a charger before you could use it normally again. Personally I can't see it, the cut-out operates independently of the battery voltage, and even if that then tries to take so much current that the current regulator relay operates, the dynamo is still going to be delivering some voltage. As long as that is more than battery voltage then the battery will charge. Subsequently perusing the Lucas Fault Diagnosis Manual I found the statement that if a battery has been fully discharged, the on-board charging system will never put back more than half the original capacity, so I think that is where the 'half charged' thing comes from. Boost charging will be required to put back the full charge, and this applies to cars with alternators more so than dynamos as the regulated alternator voltage is less than the dynamo regulated voltage under most operating conditions. It's particularly relevant to cars equipped with electronic alarm systems, including modern cars, where these are used infrequently i.e. there is a constant trickle discharge from the battery.

From this it can be seen that the ignition warning light is necessary to give the alternator its initial excitation, and some schematics do show a resistor wired across the warning light to ensure that this initial excitation current is available even if the bulb has blown or is removed. However, I have never known of this resistor being provided in practice, and also in practice a used alternator has a little residual magnetism that is usually enough to 'kick-start' it into charging, although the engine may have to be revved to 2000 or 3000 rpm before this starts happening. Once it has started charging, it will charge normally i.e. down to about 600rpm as before, but then need to be revved to 2k or 3k again to start charging again. A new alternator just out of the box may not have this residual magnetism and so may not be able to kick-start itself, in which case the ignition warning light circuit is essential. ON NO ACCOUNT should you try to generate this magnetism by 'flashing' the alternator connections across the battery like you would polarise a dynamo, you may well blow the diodes or other electronics.

The voltage regulator is a sealed electronic module which constantly varies the voltage fed back to the field windings from the output, according to the voltage of the output - i.e. a closed-circuit feedback system. There is no current regulator circuit as such, the books say that the inherent design of the alternator is such that current is automatically self-regulating. This is possibly from the thickness of the output windings and hence their resistance (higher output units having thicker wires), that being all that is required as unlike a dynamo an alternator has its output windings attached to the case, hence no brushes or commutator to limit current. It does mean that an alternator naturally generates an alternating current in its output windings (3-phase in fact), hence the requirement for a network of diodes to convert this to pulsed direct current at the output terminals and field windings.

For dynamo-equipped cars i.e. with a 22 amp output capacity parking lights, dipped beams, brake lights, reversing lights and indicators take about 22 amps. For alternator-equipped cars these lighting circuits plus headlamp flasher and heater fan give 35 amps. On GTs the HRW adds about another 7 amps, although mine takes 9 amps as it is on a relay, and my V8 twin fans add another 10 amps. With all these there is more than enough load (54 amps) to overwhelm the 46 amp alternator. But don't expect to see the voltage drop below 12.5 volts at exactly the calculated load on your dynamo or alternator. Even if your circuits seem to be working well there are bound to be some unwanted resistances in switches and connections which will reduce the current taken by the circuits, which means you may be able to take more than the theoretical output of your dynamo or alternator and still be at more than 12.5 volts. However if the voltage drops below 12.5v at less than the theoretical figure, then either your dynamo/alternator is giving less output than it should or there are bad connections between it and the solenoid. So take another voltage measurement right at the dynamo control box 'B' terminal or alternator output terminal, and if that is more than about 0.5v higher then you have some resistance between the two measurement points which is limiting the maximum effective output.

  The Workshop Manual quotes charge voltage for the dynamo at 3000rpm as follows:

For the alternator it quotes 14.3 to 14.7v, the closer tolerance due to temperature-compensated electronics. However some suppliers of replacement voltage regulators quote 14 to 14.5v for their products, and another quotes 13.6 to 14.4v. I remember reading some time ago that Mercedes had started using higher voltages to protect against premature battery failure, as the lower the voltage the less capacity the charging system will be able to restore. Lucas states that a battery having lost just 25% of its charge will never be fully recharged by the vehicles charging system - even at the higher of the above voltages, and a battery that has gone completely flat will only regain 50% of its capacity from the vehicles charging system, both situations requiring an external charger at a higher voltage and current for maybe several hours. All this came about from a pal finding that his car took several seconds of cranking to start from cold normally, and he never got more than about 14.1v at the alternator terminals, even after a 30 mile run, even revving the engine, but after being on a conventional trickle charger it fires up straight away. He wondered whether his voltage regulator was faulty, which led to finding the above figures for replacements. However Bosch regulators are available in 14.2, 14.6 and 14.7v flavours, and maybe as high as 15v, and it would be interesting to see if there is a higher voltage Bosch unit that would fit a Lucas alt. There are about 30 Lucas voltage regulator part numbers just for the 16/17/18ACR, and almost as many Lucas equivalent numbers for each Bosch regulator. One would have to try and match each Lucas number with each Bosch Lucas equivalent - patience required! It had occurred to me that perhaps one could modify how the existing voltage regulator was connected and so 'encourage' it to output a higher voltage. A diode would be the obvious way, these have a forward volt-drop of about 0.5v regardless of current, and if connected correctly should result in an increase of 0.5v in the output. Whilst looking for higher voltage Bosch units I came across someone with a similar problem as my pal, and had done just that.

Ignition warning light doesn't glow, and I can't turn the engine off! November 2013: Ordinarily at this point I'd advise removing the wiring plug from the alternator, connect an earth to the brown/yellow (NOT a brown wire!!), turn the ignition on, and the warning lamp should light. However another scenario has recently presented itself which would make earthing the brown/yellow dangerous. Peter Burgess reported that they had a car in the workshop with a non-working ignition warning light, but also that they couldn't switch off the engine with the ignition key. My first thought for not being able to switch off was a sticking ignition relay, but they were only provided from 1977 and this was a 76. Second thought was that the ignition switch wasn't disconnecting power from the white, and a third possibility was a fault on the ignition ballast bypass circuit, having 12v on it from somewhere when the starter wasn't operating. None of those would have caused the non-functioning ignition warning light, but that could have been from a completely separate fault altogether. Subsequently Peter mentioned the warning light socket showed some heat damage, and when they wiggled the bulb it came on, and after that they could switch off the engine. The light now working could have been due to a loose bulb, but that wouldn't have prevented the engine from being switched off. And it would have to be a significantly higher powered bulb and working some of the time at least to cause heat damage. So I think the likely cause is the wires in the warning light bulb holder shorted together. That would prevent the light from working, and the 12v from the alternator on a running engine would be fed back onto the ignition white in full i.e. without the resistance of the bulb in circuit to limit the current, as if the ignition switch hadn't been turned off. Note this is a different scenario to North American spec cars with an ignition relay i.e. 1977 and later not switching off with the key, where the warning light works normally, which is covered here. Normally the resistance of the bulb is more than enough to reduce the current and voltage on the ignition system to well below what is required to power the ignition. This fault also puts an unrestricted 12v into the alternator via the brown/yellow, so could have damaged that as well. The higher current from the bulb being short-circuited, plus the short probably having some resistance, could well have developed enough heat to damage the holder.

So only earth the brown/yellow if the car switches off normally. If it doesn't, use another test 2.2w bulb to connect an earth to the brown/yellow. If the warning light is operating correctly both bulbs will glow at half brightness. If neither glow there is an open-circuit (which you should have found in the previous test). If the test bulb glows at full brightness, and the warning light not at all or only very dimly, the warning light wires are shorted together.

If you have got this far you should have found any faults in the warning light circuit itself. If the warning lamp glows when you earth the brown/yellow but doesn't glow when connected back up to the control box/alternator:

It glows when it shouldn't:
Typically this is "It glows all the time" or "It glows dimly at night".

It glows all the time:
This usually means the dynamo/alternator is not charging, although it could be a fault in the warning light circuit. Check the system voltage with the engine running at a fast idle.

It glows dimly at night:
Usually only relevant to alternators. If the warning light glows dimly at night, and increasingly brightly as the load is increased, then faulty alternator diodes are indicated. Open circuit diodes will cause a reduction in output, either voltage or maximum current, so the battery charging may not be immediately affected. Short-circuit diodes are more serious, usually resulting in a reduced charging voltage, and can cause noticeably increased levels of heat and/or noise in the alternator. It may be possible to replace the diode pack inside the alternator, alternatively replace the alternator.

For heavens sake don't do what someone said and fit a diode to 'correct' i.e. hide this problem. If you do you may well have stopped the warning light from glowing dimly at night, but you have also stopped it telling you of complete charge failure. If you want to do that you might just as well unscrew and throw away the warning light bulb and save the hassle of fitting the diode!

However another cause can be bad connections in the white - ignition switch - brown circuit chain which causes a low voltage on the white side of the lamp.

  Low voltage: Update March 2010:
Mike Polan has reported how low voltage from his alternator was caused by corrosion in the assembly and mounting bolts of the alternator. When charging he discovered that whilst the front of the alternator showed zero volts relative to the engine and body, the rear showed -2v! Cleaning up the assembly and mounting bolts, and the spacer and mounting ears, solved the problem. Incidentally using an ohmmeter with the engine stopped showed no resistances, a reminder that you should only ever use volt-drops in a circuit carrying its design current when looking for bad connections.

In the event the old brushes were hardly worn, but the slip-rings were a bit manky, so I cleaned them and the brush faces and refitted the old ones. Short-sighted having new ones to hand? Well, if the flickering continued it would indicate some other alternator problem, which might need replacement of the whole thing. In the event cleaning seems to have solved the problem, but it has made me wonder about fitting a voltmeter ...

For the alternator the Parts Catalogue (in the electrical section) lists 12H2516 prior to the 77 model year, and 13H9514 for 77 and later. The first does not appear on a Google search, and for the second there is one hit from Amazon and one from eBay i.e. none from the usual suppliers. They list 12G1054 and BAU1461 respectively, with some saying the former is 2.75" and the latter 2.5", and others listing an alternative of AEU1238 at 2.5".

For the V8 the Parts Catalogue lists 37H8023, no Google hits for that, Brown & Gammons lists BHM7044 but NLA, no size found.

I didn't know how the above sources measure the size i.e. whether it is the bottom of the groove, the OD, or something in between, but subsequently to originally writing this section spotted in one of my Lucas catalogues a drawing showing it was simply the OD. The roadster has an A127 from an unknown donor so the pulley is an unknown quantity, and that measures 2.44" OD. Someone else has said theirs (original as far as they know) measures 2.875" OD. The V8 (original as far as I know) measures 2.96" overall diameter. I was hoping to use that to determine how they were measured, but it isn't given anywhere I have found. As far as the 2.75" given and 2.875" measured above for ostensibly the same pulley goes, I'm not prepared to remove the fan belts to measure the base of the groove, but I have an old alt from a Metro. That has a pulley of 2.7" OD and 1.76" ID, i.e. a groove depth a bit less than 1/2". Applying that to the measured 2.875" OD above gives an ID of 1.935", so the given measurements are definitely not the ID. And applying that groove depth to the given 2.5" as if it were an ID gives an OD of 3.44" so it definitely isn't that either. OD is not necessarily a very good measure as the flanges can extend anywhere from flush with the belt to several mm above it (very much so on the V8) and not have any effect on rotational speed. Likewise the ID isn't very good either as the belt doesn't sit in that but in the Vee of the sides, with a gap below. Maybe it's measured with a belt wrapped round the pulley, and a new spare belt wrapped round the spare Metro alt sits slightly above the flange, changing the OD from 2.7" to 2.725". Belt tension and stretch in an old belt may well have an effect on measured diameter.

The pulley size determines how fast the rotor spins for a given engine speed, and as there is a maximum rotational speed for both dynamos and alternators the pulley size is chosen to suit the maximum engine rpm. I can't se a speed given for the dynamo but the WSM specifies 12500rpm for both the 16AC and the 16ACR and 15,000rpm for the 18ACR. The crank pulley size also has a direct effect on dynamo/alternator rpm at a given engine rpm. An alternator is said to spin safely up to 18,000 rpm and a 2:1 ratio of alternator speed to engine speed is often quoted, which would be safe up to 9000 engine rpm (6250 or 7500 in the case of MGB alts). The maximum for a dynamo is typically 1/2 to 1/3rd of the alternator due to its construction. A dynamo rotor has heavy output windings as well as the commutator carrying output current, with the segments constantly connecting and disconnecting that current. The alternator has a much stronger rotor carrying lighter, lower current field windings, and slip-rings carrying that current as a constant flow. For that reason the pulleys for a given engine will be larger for a dynamo (to give a lower dynamo rpm) than the alternator, i.e. 3" as opposed to 2.75" or 2.5". Dynamo and early alternator engines used the same pulley. The oddity is that the V8, with a slightly lower revving engine, has a larger (as measured) pulley, despite having a significantly greater electrical load from its twin cooling fans and HRW, when 1977 and later 4-cylinder cars that had a single cooling fan got the smaller (higher output) 2.5" pulley in place of the 2.75". But then the V8 has a 6" pulley compared to the 5" of the roadster. I.e. 20% bigger, which would imply a 20% bigger alt pulley for the same maximum alt rpm, which would be 3". 2.96" as measured would be logical for the slightly lower maximum rpm of the engine, but the fly in that ointment is that the V8 belt sits well down in the Vee of the pulley and gives an effective diameter of 2.56", so the V8 alt at maximum revs is spinning some 20% faster than even the smaller 2.5" roadster pulley. The old Metro alternator with its 2.7" OD pulley gives noticeably less output than the 2.44" (measured, 2.2" effective) A127 on the roadster.

Schematic

Probably the main reason for converting is to get a higher charging current as the dynamo is limited to 22 amps. Although this is usually adequate for most normal, and particularly 'classic' use, when stuck in traffic with headlights, heater fan etc. on the charge will almost certainly not be adequate which means you will be discharging the battery and on a daily driver this can rapidly reduce the battery to a point where it will no longer start the car. Bad connections will limit current flow, and will contribute to a drop in system and charging voltage, and I think it is this which people are seeing as much as insufficient output from the alternator. Even 'normal' volt-drops up from the solenoid and particularly with a voltmeter on the green circuit can be enough to reduce the indicated voltage below 12.5v, even though the voltage at the solenoid and hence the battery is above this critical point. A good example of a little knowledge being dangerous. Of course if you are going to significantly increase the electrical load then you may well need to consider an even higher rated alternator from another source.

One common misconception seems to be that fitting a higher rated alternator is automatically going to push more current through the wiring, and people get paranoid about uprating it. The maximum current that will flow depends (to the largest extent) on the electrical load, not the maximum capacity of the alternator fitted. If you only have 40 amps of load then only 40 amps of current will flow, even with a 60 amp, 80 amp or 100 amp alternator fitted. Having said that high-rated alternators like the 80 and 100 amp will be better at maintaining sufficient charge at idle if you have added electrical loads, as well as when running.

About the first thing to say about the process of converting from dynamo to alternator is that unless it has already been done you almost certainly will have to convert from positive earth to negative earth. Positive earth alternators will probably be very difficult if not impossible to find, a negative earth will be tricky to convert to positive, and the availability of used, rebuilt and new negative earth alternators of various types is almost infinite. Also it might be safer to take things one step at a time and do the polarity conversion first, check everything works OK, and only then do the alternator conversion. Getting the polarity wrong with an alternator connected will probably destroy it, and there is only one very simple step which will be 'wasted'. These notes only cover use of a Lucas alternator, there are too many variations in Bosch and GM Delco alternator connections, although the alternators themselves are quite suitable for use.

The usual advice is before starting any of the following work disconnect the battery earth strap first, and only replace it as the final step. However on cars with the remote solenoid it is very easy to remove the brown wire from the spade on the battery cable terminal on the solenoid, and as long as this can't drift back towards its spade while you are doing the work then this is fine.

Rewritten November 2012 (following the opportunity to work on Chris Mottram's car):

There is an alternative method if you are adding a lot of significant loads to the cars electrics, and consequently fitting a high-power alternator, and that is method 2.

Method 1:

Remove all the wires from the control box, and the control box from the panel on the inner wing (easier said than done with those fiddly nuts behind the panel!). The standard gauge brown/yellow from the WL terminal, and the standard gauge brown/green from the F terminal have their spades cut off, a portion of insulation carefully stripped so no strands are cut, twisted together, and soldered. Put a couple of lengths of heat-shrink tubing just over the soldered section, each about 1/4" longer than the soldered joint, heat-shrinking them one at a time. Then fold the 'spare' bit of tubing over, and fit a couple of lengths of larger diameter tubing over that and an inch or so of the insulation on the wires. This circuit supplies the 'priming' voltage to the alternator to start it charging, and indicate charge failure.

Method 2 is very similar but to carry higher currents, if you are adding significantly to the cars electrical load and hence using a high-output alternator, it would be best to use a new heavy gauge (of the appropriate size for your loads) brown wire from the output of the alternator to the battery cable stud on the solenoid at least. You may also need to use similar gauge wire from there to your non-standard, high power loads, the brown from the spade on the solenoid going to the control-box should be enough for all the factory loads. Again, you MUST remove the battery earth connection before starting work on the battery cable stud on the solenoid. After that you can leave the browns on the control-box B terminals, and the brown/yellows on the WL and D terminals, using the heavy gauge brown/yellow at the alternator for its INDicator terminal. The standard gauge brown/green should be removed from the control-box F terminal and its spades taped back and insulated at both ends to prevent them coming into contact with anything else. Alternatively you can discard the control box and join the three browns from the B terminals, and join the two brown/yellows, and insulate the earth wire, using the techniques described above.

If converting the polarity at the same time leave the alternator unplugged when connecting the batteries the new way round for the first time, if you get it wrong and the alternator is connected you will blow its diodes and burn wiring. Confirm the polarity is correct before continuing by connecting a voltmeter between a brown in the alternator plug (meter +ve) and an engine earth (meter -ve).

After confirming that the polarity is correct connect an analogue voltmeter (digital meters may give unpredictable results) on its 12v scale in place of the battery earth strap. There should no voltage registered. If there is, it will probably be a full 12v, and means some circuit on the car is switched on (courtesy lights? Boot light?) which should be found and switched off before proceeding. When no voltage is shown plug in the alternator. You may now see a few volts registered, which will be the normal microscopic leakage current of the diodes and can be ignored. If a full 12v is shown the alternator diodes are faulty. If the reading is correct, replace the battery earth strap. If you don't have an analogue meter use a low-wattage 12v bulb instead, such as one from one of the gauges. If there is any glow at all from the bulb, current is flowing, proceed as above. Only a significant current flow will cause the bulb to glow, connecting the alternator should not be enough.

You now have the alternative of going for broke and reconnecting the battery earth strap, or taking a smaller step. For this connect a high wattage bulb e.g. a headlamp bulb (e.g. an old one with one filament gone - "If you haven't found a use for something yet ...") in place of the earth strap. This will allow a safe amount of current to flow while you turn each thing on in turn. If any circuit is faulty and is full short the bulb will limit the current and prevent damage to wiring and components. Some things (low current items) will work almost normally, higher current items probably not. Low current items might cause a dim glow from the bulb, higher current items a brighter glow. Only turning the key to crank (nothing will happen) should cause the bulb to glow at full brightness, nothing else. When you are happy, reconnect the battery earth strap.

With the ignition off there should be no glow from the ignition warning light. A glow now indicates faulty alternator diodes or voltage regulator, or incorrect connections to it.

With the ignition on the warning light should glow. If no glow remove the plug from the alternator and connect an earth to the brown/yellow terminal (NOT the brown!). If the warning light glows now the alternator is faulty if not then the circuit is broken back towards the warning light, possibly where the brown/yellows are joined where the control box was, or a blown bulb. There should be 12v on the white at the bulb holder and an earth on the brown/yellow to light the bulb.

With the warning light glowing start the car, and with the engine revved above 1000 rpm the light should go out. If the light remains on the alternator is faulty. Only if the revs drop below about 600rpm should the light come back on, stay off till about 1000 rpm, then go out again as before. Early cars had an idle speed of 500 rpm and if the light comes on at idle, particularly with lots of load switched on, then you would be advised to increase the idle speed to, say, 700 rpm to keep the light out at all times. While the light is on the alternator isn't charging and the battery is discharging, which largely negates the effort of converting!

With the engine at about 1000 rpm, and all loads switched off (and the warning light off), measure the voltage between the brown at the fusebox and earth. You should see about 14.5v, much less or more than this indicates a faulty alternator. Now turn on headlights, brake lights, heater fan etc. The voltage will probably drop, possibly to less than 13v with one of the smaller Lucas alternators. Increase the revs to about 3500 and the voltage should rise above 13v again, indicating the battery is still being charged even with everything switched on. If the voltage doesn't rise above 12.5v check the voltage at the alternator output terminal(s), and if similarly low here it indicates the alternator has a low output current fault, however note that the smaller Lucas alternators will probably not be able to supply anything above the standard factory loads at best. If the voltage is closer to 14v at the alternator then there is a bad connection somewhere between the alternator and the brown at the fusebox, check the voltage on each brown wire and the battery cable at the solenoid.

Schematic

First remove the battery earth strap, and don't replace it until you have made all the wiring changes.

The alternator should have the following wires:

The voltage regulator should have the following wires:

Of the other wires the heavy gauge brown goes to the output terminal of the new alternator (See here for Lucas 16/17/18ACR terminals) and remains on the solenoid.

The two black wires and two brown/green wires at the alternator and old voltage regulator are no longer required and should be taped back out of the way of anything else.

Note that the earthing point to the body for the black wires from the regulator and alternator is also the earthing point for four other wires (heater fan, instruments, wipers and headlights), even if the regulator and alternator earth wires are removed the other four wires must still be earthed, you will get some very strange results without it.

That should leave the brown going from the voltage regulator to the solenoid, and this is also no longer required. If the two standard gauge wires at the solenoid (there should only be two apart from the large battery cable and the heavy gauge brown from the alternator) have separate spade connectors it should be easy to determine with an ohmmeter which goes to the voltage regulator and which to the fusebox. The one going to the fusebox must remain connected, but the one going to the old voltage regulator can be taped back both ends. But if these two brown wires terminate in the same spade connector then you have a 50/50 chance of cutting the right wire off. You could cut one wire off and then test, and if you have cut the wrong wire reterminate it on a new spade connector and tape the other one back, or cut both off and test and then reterminate the wire going to the fusebox, taping the other one back. But the best thing to do would be to cut both wires from the existing spade connector, identify the one going to the old voltage regulator and tape that back both ends, then for the other wire that goes to the fusebox reterminate that on a ring connector that will go on the stud with the battery cable. This makes a more secure connection than the spades, as all the current for the cars electrics goes through it. While you are doing that you can do the same with the output cable from the alternator, for the same reason. Later starters did have all the browns terminated with ring connectors on the battery cable stud as standard.

With all the wiring changes done, the new alternator mounted but the brown and brown/yellow not yet connected (make sure they can't short out on anything), and everything in the car switched off including doors etc. closed so the courtesy lights aren't on, check to make sure you haven't shorted any of the brown wires to earth/ground. The safest way to do this is to connect a voltmeter on its 12v scale in place of the battery earth strap. If the voltmeter shows any reading at all, there is something drawing current. Anything less that 12v shown is a tiny current, could be a clock or the 'keep alive' circuit of a radio. If it shows a full 12v it could be a small current e.g. something simple like a courtesy light left on, or it could be a full short. Connect a test-lamp or other 12v bulb (an old headlamp bulb is best) in place of the earth strap, and if it glows brightly it is a full short which must be investigated and fixed before you proceed.

A cruder check is to tap the battery earth strap very briefly on the -ve post of the battery. You should not get any kind of a spark. If you get a small spark maybe one of the courtesy lights or similar is still on. If you get a big flash then it looks like one of the browns is shorting to earth somewhere, which again must be investigated and fixed before you proceed.

With the brown at the alternator still not connected and protected from shorting to anything, and with the battery earth strap reconnected, connect the brown/yellow to one of the standard sized spades and turn on the ignition. If the warning light glows you can proceed. If it doesn't then turn off the ignition, move the brown/yellow to the other standard sized spade and try again. If the warning light glows now again you can proceed. If not, disconnect the brown/yellow from the alternator and connect an earth to instead end and try again. If the light glows now then possibly the alternator is faulty, or possibly the wire should go to yet another spade if you have non-Lucas alternator. If the light doesn't glow with the earth connected however, then either there is a problem where the brown/yellow joins the brown/black, or the bulb has failed, or there is some other open-circuit between the end of the wire at the alternator and where the white from the bulb joins the others at the ignition switch. This must be found and fixed before you proceed, or the new alternator probably won't charge.

With the bulb glowing with the ignition on, carefully connect the heavy gauge brown to the output spade, remembering it is live and unfused. You may prefer to disconnect the battery earth strap again while you attach the brown output wire, then go through the same tests for a short as before. With other types of alternator there can be different connection arrangements, some have an output stud as well as an output spade, use the stud as it will have a better current carrying capacity.

With both output and indicator wires connected to the alternator, start the engine revving it as little as possible, and watch the warning light. The warning light may still be glowing, so slowly raise the revs, and at about 900 rpm the light should go out. Now use a volt-meter on the brown at the fusebox and you should see around 14v. If so the new alternator is charging. With the engine idling turn on the lights, press the brake pedal, switch on any other electrical loads you can, and the voltage will drop to some extent, possibly towards 12v. Rev the engine to about 3k and the voltage should rise again above 12.5v. With everything turned off, and a fully charged battery, and the engine revved to about 3k, you should see a maximum of about 14.5v. With the engine idling again select 4th gear, handbrake and footbrake on, and slowly lift the clutch pedal up so the revs start to drop. The warning light should come on again at about 600 rpm. Dip the clutch again and take it out of gear, slowly raise the revs again and the light should go out again at about 900 rpm. If your normal idle causes the warning light to come on again anyway, it might be an idea to raise the revs a bit so it stays out, that way the alternator will still be charging at idle, rather than the electrical loads of the car draining the battery.

If the light doesn't go out when revved, and you have two standard sized spades on the alternator, switch off, and move the brown/yellow to the other standard spade. Turn on the ignition and if the light glows start up and try the tests above again. If the warning light doesn't go out when the engine is revved, with the brown/yellow on any of the standard sized spades that it glows on with the ignition on, or the voltages don't show as above, then possibly the alternator is faulty.

My roadster came with a single 12v which needed replacing in 1994, but it was too big to lift out through the hole in the shelf. Looking underneath I noticed that the carrier had been modified to take the bigger battery, and I wondered if they had welded it up after getting it in from underneath! But then I had a brain-wave and found that if I turned it over to lie on its end (it was sealed) I could just get it out from the top. It had just been loose in the cradle, so along with the new 6v batteries and interconnecting cable I got two clamp kits. It was apparent that originally the interconnecting cable went through some flexible armoured tubing, but with the clamps already crimped onto the end of my new interconnecting cable there was no way I was going to reuse that. It was rotten anyway so I pulled it out and just put the cable through on its own, installed the earth clamp in the other box, installed the batteries and clamps, checked the volt-drops, and away we went. Not too long afterwards I had occasion to remove the interconnecting cable, can't remember why, and saw with horror that it had been hanging down and rubbing on the propshaft! It had marked the insulation but fortunately not rubbed through. So I installed my own tube to support it up out of the way.

Added April 2009

I've seen a couple of comments from people who have flattened the battery, then charged it up in reverse, which seems a really iffy process to me, if not downright dangerous if someone else should go by any + and - markings for reconnection, boosting or even charging. Also some sources stating that +ve and -ve plates are made of slightly different materials which aid battery performance, which would work against you if the polarity is reversed.

From 1st July 2018 it has become illegal to supply sulphuric acid to members of the public without the appropriate EPP licence. So whereas mail-order supplied a dry-charged battery and a set of acid packs, in future and depending on the retailer, either only the dry-charged battery will be supplied and you will have to find a supplier of acid locally and get them filled there, or collect wet-filled batteries over the counter. July 2019: At least, that's what advice at the time seemed to say, but wet-filled batteries seem to be available for delivery by courier from some sources now (see below) including the MGOC.

June 2019: Another sudden failure, and it's a case of "déjà-vu all over again"!

October 2014: Sudden failure of one of Bee's batteries and subsequent replacement of both.

Twin 6 volts in chrome bumper cars, single 12v in rubber bumper. Some years ago two 6v were only slightly more than a 12v for a rubber bumper, but now a single 6v is nearly as expensive as a 12v. Nevertheless I intend to stay with 6v. It is frequently said that modern batteries benefit from more recent technical advances but why shouldn't that also apply to 6v as well as 12v? And at least one company offers 59Ah, 73Ah and 88Ah versions in the same package size. Certainly mine last well enough averaging 10 years for each set, and that with very little use for several months over winter with no recharging. The single 12v in the V8 doesn't last any longer much less in fact since I stopped the daily drive to work (after the 2nd battery failed in as little as 18 months I fitted a battery cut-off switch and it has been fine ever since). If you do opt to replace the twin 6v with a single 12v then you have the potential to move the fuel pump into a far more accessible position as Peter Mayo did.

The Workshop Manual quotes the original battery capacity as 51 Ampere-hours at the 10-hour rate or 58 Ampere-hours at the 20-hour rate. These days automotive batteries are also often quoted in 'Cold Cranking Amps' (CCA) or 'Cranking Amps' (CA) as starting an alternator equipped car is its main use and not a continuous discharge over a period (unless you regularly park with lights on). However for a dynamo equipped car Ampere-hours is more of an issue as at idle or low revs, especially with lights, wipers etc. on, the dynamo won't be charging and you will be discharging the battery, theoretically an issue if you drive in heavy stop-start traffic. CCA represents cranking at 0 degrees F/-18C, for warmer climates CA is more applicable as it represents cranking at 32F/0C. Divide CCA by 0.8 to get CA. One source (unverified) quotes that 6v batteries for the MGB should be 66Ah and 360 CCA. If you regularly start the engine in temps below freezing you may need 360CCA. If normally started above freezing you only need 360CA i.e. a 288CCA (or the next one up) battery. There is little benefit going for a battery with a higher CCA or CA than this unless you have a high-compression (i.e. higher than even the factory high-compression) engine. Incidentally this also indicates that modern 6v batteries have benefited from modern technology increasing performance in the same package size, and not as some aver. For complete originality I think the original 'tar top' batteries with exposed links can still be obtained, but they are also available in a more modern construction with internal links and a single fill cap.

  July 2019:
6v batteries for chrome bumper:
Several places are doing 77 and 80Ah 600CCA i.e. significantly more powerful than the originals at about £65, some including delivery, delivered filled, with variously 1, 2 and 3 year guarantees.

MGOC have D004W 80Ah 600CCA at £65, concealed links, single fill channel, wet, 2yr guarantee, delivery £15 for a pair; and D003 65Ah at £90 (!) plus P&P, supplied dry so you have to source your own acid at additional cost.

Brown & Gammons have GBY3031DZ at £99, concealed links, dry, acid GBY3031ACID listed at £32 but shown as 'Request stock alert' on one page and 'collection only' on another. GBY3031WZ at £90, wet, exposed links, collection only and GBY3031HD 'heavy duty' at £90, concealed links, apparently wet as collection only.

Leacy show two types: GBY3031D dry, no acid at £92 and GBY421D dry with acid pack also £92 but showing out of stock, and the acid packs are also showing as out of stock, so perhaps no longer available anyway. They are showing GBY3031W at £83 wet collection only but out of stock, and GBY421W at £86 wet, 'original style' (exposed links?) and apparently available for delivery. No capacities listed for any.

Moss Europe list a 57Ah at £92 and a 63Ah 'heavy duty' at £102, both dry but no acid listed. Wet versions of both (although the cheaper one is shown as 56Ah) at the same prices for collection only.

Alpha Batteries are showing a pair of 421 77Ah at £135 including delivery, wet, concealed links and single-fill, 2 year warranty.

This eBay seller has Exide 421 80Ah 600CCA at £68 including delivery, concealed links and single-fill, but no indication of wet or dry.

Halfords don't list a battery for a chrome bumper MGB.

421 seems to be the designation used by a lot of suppliers, quite a few possible suppliers in this Google search.

12v batteries for rubber bumper:
Note that the original batteries have the terminals on one of the long sides, not diagonally as on the 6v, so terminal layout is important. As originally installed the terminals were closest to the tunnel, with the positive post closer to the front of the car. The generic UK (at least) designation for this battery is 072. 075 would appear to fit but have the terminals the other way round i.e. positive towards the rear of the car when they are closest to the tunnel. If the wrong type of battery is used, or the right type but the wrong way round, you will destroy the alternator and damage wiring.

Leacy are showing GBY072W as wet for collection only, £78 inc VAT, no spec, 'out of stock'.

MGOC are currently showing no prices for 12v batteries.

Moss Europe show a dry at £128, nothing about acid, on back-order, or wet £128 collection only. No capacity indicated. They also have a 'heavy duty' at £112 (?) which is also wet for collection only.

Halfords list HB072 with a 3 year guarantee at £99, 68Ah, 550 amps 'starting power' (looking at Yuasa specs for one of Halfords Yuasa batteries 'starting power' seems to equate to CCA), sized at 261mm long by 175mm wide by 220mm high, and HCB072 with a 4 year guarantee at £109, 70Ah, 570 amps 'starting power', sized at 269mm long by 174mm wide by 225mm high.

12v for a chrome bumper?
October 2020:
Bottom Gear Bob on the MGOC forum mentioned HB/HCB202 which is a direct replacement (including the clamp) for a 6v battery at 175mm x 175mm and 190mm high and around £80. Check the cranking capacity though as some are lower than the original 6v at 300 amps as opposed to 360 amps, and note the 550 and 570 amps quoted for an RB 12v and 600 amps for 6v. Ampere-hours tends to be about 20% lower with the HB202 but that's only an issue if you need to lave it parked with the lights on.

Americans often quote 'group 26' as a suitable 12v battery for a chrome bumper MGB and this site quotes the dimensions of that as 208mm long by 175mm wide by 197mm high. The Performance Batteries site lists no less than 14 batteries that might squeeze in, but the most powerful is only 52Ah which is quite a bit less than the original 6v, albeit it at up to 520CCA (624CA). The dearest of these is £62 inc VAT. Some batteries come with one or two lips on the bottom edge for clamp brackets on modern cars, which can make the difference between fitting and not fitting. I've seen it suggested that these can be cut off, which may well work, but remember you have probably nullified the guarantee by doing so. MGOC are showing a conversion kit including battery and clamps but WITHOUT acid, £120. The battery is also only 50Ah, so even less capacity, and no cranking amps given.

Note that 12v batteries have the posts down one long side and can have the polarity either way round. Rubber bumper cars require the posts to be adjacent to the tunnel and the +ve towards the front of the car when installed. Type 069 and 072 batteries seem to have the correct orientation and size. You need to check carefully, REVERSE CONNECTION WILL DESTROY THE ALTERNATOR AND BURN WIRING. I've seen type 063 recommended but these seem to have +ve and -ve reversed.

About the only good technical reason for converting from twin to single concerns access to the fuel pump by fitting it in the near-side which would mean extending the 12v cable. Peter Mayo said that when fitting a geared starter it needed a longer 12v cable from the battery, so at the same time he moved his 12v battery to the near-side and made the cable long enough to reach that. He went even further by moving the pump up into the off-side box making access to it even easier. Make sure you use the link cable bracket at the top of the prop-shaft tunnel to hold the cable up out of the way, and always fit a clamp to hold it down.

I've seen one person doing this had cut the lid in half, which on the face of it might seem to make sense making it like the RB cover, and only having to remove half to either inspect the battery or access the storage bin, but it left one half of the cover with only two fasteners on one end.

And finally... The batteries, particularly the single 12v in a rubber bumper, can be a tight squeeze through the hole. If your new battery doesn't come with a handle it is a good idea to put strong cord or webbing around the battery before you fit it, and leave it in-situ. The next time you come to remove the battery you will congratulate yourself on your foresight.

But back to drains when you don't have an alarm, which could be from many causes, and may even be from the battery itself. However that can be determined by disconnecting the battery earth strap after a decent run, leaving it until you would normally expect problems, then reconnecting the earth strap and trying to start the engine. If it still won't start, then it's the batteries (or maybe the connections ...) but if it starts just fine you have a drain.

To diagnose this it's best to use a voltmeter, but connect it like an ammeter - very confusing to the uninitiated! Disconnect the battery earth cable, and connect the voltmeter in its place. Why not use an ammeter? An ammeter gives a very low resistance path to current, so if you have a high drain a high current will flow. If more than the typical 10A max of a hobbyists meter it could damage the meter, but more importantly create a spark when it is connected and disconnected, which could ignite battery gases. Yes, I know that disconnecting the earth strap might also create a spark, but there is little you can do about that unless you have a cut-off switch, and just one spark is better than several. By contrast a voltmeter offers a very high resistance path, so negligible current will flow even if there is a very large drain, and no spark. For an early positive earth MGB the positive of the meter goes to the earth terminal of the battery, and the negative terminal goes to the car body. For later negative earth MGBs, and those early cars converted to negative earth, the meter is connected the other way round. Digital meters don't usually mind being connected round the wrong way, and will just show a minus sign in front of the reading. If there is any current drain at all, you will see a reading on the meter. There should not be any drain at all on a dynamo-equipped car, unless you have added a modern radio with stations stored in memory (and not all of those), a car-powered clock, or an alarm (in most cases but not all). With an alternator equipped car there will always be a very slight drain, which is from the reverse leakage current of the alternator diodes. This is in the order of micro-amps, and a battery should be able to support this for several months. If you use an analogue meter, the alternator drain may register anything up to 12v on the meter, and you can eliminate this as a cause by unplugging the wiring from the alternator, and an analogue meter should drop to zero. But a typical digital meter is very much more sensitive than a typical analogue meter, and will almost certainly register some voltage even with the alternator unplugged. As long as this is less than 12v then it is an insignificant insulation leakage that can be ignored, a little dampness or dirt across the solenoid or other 'always live' component can be enough to cause that.

But if registers 12v then it could be significant, and if you don't have an analogue meter to compare it with (which should show zero volts) you will need to switch your meter to current. If that shows something in the tenths of a milli-amp or higher then it really ought to be investigated, not so much that it will flatten the battery (although it will over several weeks) but in case it is from damaged insulation on something that might suddenly get worse and cause a catastrophic short-circuit.

You can use a test-lamp in place of the volt-meter, but as well as even a low-wattage bulb causing a spark on a high drain, a small but still significant drain may not be enough to make even a low-wattage test-lamp bulb glow. A high-wattage bulb should not be used while looking for a drain as it will generate a significant spark on connecting and reconnecting. However a high-wattage 12v bulb in place of the earth strap is very useful when testing a replacement wiring harness as it protects the harness from damage if there should happen to be a short-circuit on any of the unfused circuits, but will allow most circuits to function to some extent.

  If you have a battery cut-off switch then you can do these tests without disconnecting the earth strap, which makes things easier as well as safer. Turn the switch off and remove any bypass fuse.

• If your cut-off switch is in the 12v cable going to the starter, then connect the meter between the 12v post of the battery and the brown at the fusebox, and continue as above.
• If your cut-off switch is in the earth lead from the battery going to the body, then connect the meter between the earth post of the battery and a good body earth elsewhere.

Originally there were just lead-acid or 'flooded' types with screw caps on each cell or lids covering all cells. These need periodic checking and topping-up to replace the distilled water lost through gassing and evaporation, which occurs during cranking and charging i.e. normal use. These must be operated in a well-ventilated space and you should not make the last connection of first disconnection or a charger or jump-leads directly on a battery terminal as it can ignite the gasses. There are 'sealed' version of these which are nominally maintenance free i.e. you can't top them up, but they can still gas. You definitely should not operate these in the plastic so-called 'battery boxes'. Some modern cars have the batteries within the passenger compartment or boot and these should have a vent tube leading to the outside, which must always be connected to the battery, so it follows that batteries used in this situation must have the facility to connect the vent. These are often sold with a little red angled tube taped to the top of the battery.

There are so-called 'sealed' versions of flooded which no longer have a removable lid to check and top-up the electrolyte, but they still gas and again must be used with the same precautions in ventilation, charging or jump-starting, and if inside the car must be used with the vent connected.

'Calcium' variants of the above have a higher capacity for the same physical size, at about 12% more expensive than lead-acid. They are probably all 'sealed' but can still gas so the warnings above still apply.

More recently 'gel' batteries became available, in which the electrolyte is a jelly rather than a liquid. These cannot leak, even when the case is damaged. Under 'normal' use they do not gas, so are safe for use in enclosed spaces, which is why they are used as backup batteries in the event of mains failure for burglar alarm systems and the like. However they must be charged at a lower voltage/current than flooded types or voids can develop in the gel which reduces capacity, conventional automotive chargers will damage them unless they have a special 'low-rate' switch position. They also have higher internal resistance so are not as effective as conventional flooded cells when used as starter batteries, so are rarely used in automotive applications. In hot conditions they can still lose water (somehow), which can limit battery life to as little as 2 years. They can stand heavy discharge better than flooded, which is why they are often used in charge/discharge applications like golf carts.

Following gel batteries Advanced Glass Mat batteries were developed. These have liquid acid, which is kept in place by a fibreglass mat which acts like a sponge. They have a higher cranking capacity for a given physical size compared to flooded. However because they need a higher acid concentration than lead-acid they need to be charged at a higher voltage, which may have implications on MGBs with standard alternators and high electrical loads, and especially dynamos. The big draw-back is that they are 4 to 5 times more expensive than flooded types so are hardly a practical proposition in conventional automotive use.

Some gel and AGM batteries are described as 'Valve Regulated Lead Acid' (VRLA) types which means they have a valve to the outside that maintains a positive pressure inside the battery, which normally prevents any gas escape. However under very high charge rates gas pressure will build up to open the valve, so again these should really be used in well ventilated spaces. They are also more sensitive to high 'ripple' charging currents, which is another reason why conventional automotive chargers cannot be used. And if you take into account the very high and peaky voltage and current output that can be seen from an alternator when the battery is disconnected (never run an engine with the battery disconnected) it seems to me that these shouldn't be used in automotive applications either!

References:

  Update February 2014:
Another important thing to remember, from the Lucas Fault Diagnosis Service manual:

I'd take this further and say that if ever the battery becomes discharged enough to need a jump or a bump start, i.e. not fully discharged, an external charger capable of outputting more than the 14.3v to 14.7v of an alternator should be used to fully recharge the battery. It should be quite safe to recharge up to 15.5v with the battery still in the car as this voltage can be reached by dynamo systems under normal use. But a higher voltage boost charger should be used with the battery out of the car and in a well-ventilated space (see above). Again from the Lucas manual, external charger current should be limited to one tenth of the ampere hour capacity of the battery at either the 10 or 20 AH rate. For the original batteries this represents about 6 amps, but replacements from some sources can be 75AH and even 88AH (see above), giving charge current of 7.5 amps and 8.8 amps respectively.

I have battery cut-off switches on all three of my cars, but normally the ZS is only turned off in the summer as in the winter it gets used more. However for various reasons this winter it's only been doing a few miles a week, and I have noticed the cranking speed gradually getting lower. So I took the battery off the car and charged it on the bench using my high-output charger. Initially taking just under 6 amps and showing just over 13v, after several hours the current had dropped to about 4 amps and the voltage risen to 15.5v. Put it back on the car (and checked it would start) and left it over night with the cut-off switch not turned off. Next morning, i.e. with the overnight load of the alarm and ECUs, it was cranking much faster than it had previously. I'll have to start turning the switch off in winter as well now.

One of the perennial questions is "How do I charge two 6v batteries". The answer is you treat them as if they were a single 12v. Forget any 'buts' about different current or voltage in each, it is exactly the same as having six 2v cells in a 12v battery - they are all charged in series so get the same current. Yes, they may exhibit different voltages as they age, but that is down to individual cells and applies equally to 12v batteries as to 6v.

As to how to connect the charger it would be easy to get confused by the interconnecting cable and end up connecting a 12v charger across just one of the batteries, which wouldn't do it a lot of good, and it is a right faff getting the battery cover off anyway. If your car has a cigar lighter you can forget the batteries, just buy a cigar lighter plug and connect the wires from the charger to it - observing the correct polarity! The +ve wire from the charger must be connected to the +ve circuit in the car, and the -ve to the -ve. This applies to both +ve earth and -ve earth cars. The cigar lighter was an option from the beginning and standard from the 1973 model year, but was wired differently over the years, each needing a different approach as follows:

• Up to and including 1968 it was connected to the brown circuit at the ignition switch hence not fused inside the car. In these cases it would be advisable to add an in-line fuse at the lighter (or at the ignition switch if you can identify the correct wire). A standard 17amp continuous, 35amp blow will suffice.
• In UK 1969 and 1970 cars it was connected to the 'accessories' contact of the ignition switch. Not only was this unfused as on earlier cars, but also needs the ignition key to be in and turned before one can use it for charging. This is likely to be a problem so rewiring the cigar lighter to the purple circuit (fused, always on) as on later models might be the best option.
• In UK 1971 cars it is wired to the green/black circuit, and although this does have an in-line fuse it is still on the accessories circuit so needs the ignition key to be in and turned as above. Again rewiring to the purple circuit may be the best option.
• From 1972 (UK) and 1969 (North America) all cigar lighters were wired to the purple circuit which is fused in the main fusebox and doesn't need the ignition key.

There are two types of cigar lighter plug - fused and unfused. The fused type might seem the safest option but the current travels through a very fine spring to get to the fuse which makes the plug get quite warm during charging. A better option is the unfused type which has a higher current carrying capacity. With a fuse in the car and another in the charger you are quite safe using an unfused plug.

Another option is to use a different plug and socket with the socket in, say, the engine compartment connected to the purple fuse and earth and the plug on the charger wires, but still needs the bonnet to be opened and closed to connect and reconnect. One thing to be aware of is that cranking the engine whilst the charger is connected may blow the charger and/or cigar lighter fuse.

Trickle chargers supply current to the battery continuously (not the same as 'constant current'). As long as they don't raise the battery voltage over 15v you should be OK to leave them on overnight as an exception, but not for long periods or regularly every night. This level of charging, even though it is the same as when the car is being driven, will cause the battery to 'gas' (incorrectly called 'boiling') to some degree which will cause the distilled water in the electrolyte to evaporate lowering the electrolyte level. Also while in a garage, or even out in the open unless there is free air circulation around the battery i.e. lid off and windows open, you will get a build-up of gas in the battery compartment which could cause an explosion and corrosion of metal parts. For chargers that raise the battery above 15v this evaporation will occur to a much higher degree and could even damage the battery i.e. warp the plates. For long-term battery maintenance e.g. when the car is not being used for some months get a 'conditioning' charger. These sense the battery condition and vary the rate of charge accordingly over time.

  Added July 2009: My son is in the market for a charger to keep the battery in his 'occasional use' classic BMW topped-up and looked at the Halfords Fully Automatic Charger. On the face of it this charger is intended for extended connection and has a 'maintenance' mode with reduced current, but if you read the Customer Q&A two posts state that the battery must be disconnected from the car for charging, also its maintenance charge level is 1.5A which is too much in my opinion. In fact if you look at the Q&A for all the Halfords chargers they say not to use them with the battery in-situ or connected to the car. Utterly pointless, for an intermittent-use classic car if you are going to disconnect them to charge them you might as well just disconnect them anyway, they will hold their charge for more than two months like that, remember when you buy them they have been sitting on a shelf at least that long. If you want a charger that you can leave connected all the time, while the battery is still in the car and connected, then one of the few suitable ones seems to be the Accumate battery optimiser (cheaper from the MGOC though) which charges down to less than 200mA and is specifically designed to carry the load of alarms, radios/CDs etc. Note that the Accumate can be switched between 6v and 12v, but the twin 6v batteries of the chrome bumper MGB should always be changed in series on the 12v setting. This Optimate 5 charger (also from Accumate) additionally has a recovery mode for deeply discharged batteries. Two others are recommended by AutoExpress here. However for an MGB at least I think a cut-off switch (without bypass fuse!) is much cheaper, more convenient, has an immobilising function and more importantly an emergency disconnect function in the event of an alternator fault or short in the many unfused wires. More recently I have come across this Battery Brain which can be used to disconnect the battery more conveniently than undoing clamps, and will disconnect itself if the battery drops below 12.1v. Still less preferable to a cut-off switch in an MGB, but a definite possibility for my ZS which gets little use in summer and where the fitting of a cut-off switch is more involved and the use less convenient, although I did fit one to that as well).

A warning on optimisers though. These are designed to be left permanently connected, to keep the battery topped up. I think that is fine for long lay-ups without using the car, but if you continue to use it when the car is being used on a regular basis you are never going to know when your batteries are on their last legs, or if the charging system is down on output. Go for an overnighter somewhere, without the optimiser, and it may not start next morning. So put it away whenever the car is being used more than, say once a month.

Another question is 'Can I charge two batteries at the same time?'. Firstly if we are talking about twin-6v batteries in an MGB then the answer is that is how they should always be charged, and in series as described above. As far as charging two 12v batteries at the same time then it all depends on the charger: