What none of this takes into account though is the effect that letting our batteries drop to such a low voltage, sulphating of the plates start to take place somewhere around 12.4 volts and if left unchecked some say this will permanently damage the battery and reduce its life and it is also suggested in some circles that letting the voltage drop below 12v on a regular basis will damage the battery quite quickly. (Mine when little used has shown 11.7v or so for extended periods giving warnings when entering the car and sometimes after a journey when switching off, but connecting a CTEK zipped through stage 1 'Desulphation' in seconds implying that at least has not been sulphated)

12 days on from the last time I drove the car and the voltage has dropped to 12.4v, as this is the point at which sulphation of the plates starts to damage the battery I have given in and charged it up again. (As above)

What my testing has shown me is a car that is being driven daily will never have a fully charged battery, from what I have found and read the only time a battery is fully charged is when it is charged using a charger when the float voltage will be about 12.8v. Quite what the SOC is when relying on the cars systems I do not know and cannot speculate on because I do not have the equipment needed to measure this but the voltage seems to be 12.5v and this corresponds with what I have read about Stop/Start systems, for some reason they do not try to fully charge the battery. (The concept that cars with Stop/Start systems don't fully charge the battery - the implication being that non-Stop/Start systems do - when that system puts more load on the battery is laughable)

The voltage seems to be 12.5v and this corresponds with what I have read about Stop/Start systems, for some reason they do not try to fully charge the battery. (As above)

My testing showed that charging with the CTEK the car could be left for 7 days longer than if it had been driven. I believe that this is because cars with stop/start do not try to fully charge their batteries for some reason. (As above)

You are wasting your time, the "Smart charging" system in your car is designed to only charge the car to that level. The more you charge it the less it will bother. (As above)

Something I have noticed but not looked into (I'm fed up with batteries now 😠) is that I mainly do stop start driving & before lockdown in anything other than coasting the onboard voltmeter showed about 12.5 to 12.7 volts, coasting 14.5+ volts. Now that I have fully charged the battery with the CTEK it normally shows 14+ volts except when stopped at lights with the engine off. It almost seems that having fully charged the battery the electronics are happy to keep it charged but when it is part charged that's where it keeps it. ('Coasting' is slowing down without using engine braking and is the same as idling as far as the engine is concerned. If these cars DO have a regenerative deceleration function (consumption meter in the 'Charge' section) then it won't be activated when 'coasting' or at idle (only when using engine braking) ... but it should be charging at the full rate anyway! Mine and a pal's of a similar spec and age have never shown anything other than the full 14.8-14.9v all the time the engine is running.)

In post #30 I said that the on board volt meter normally showed 12v+ during normal driving with this rising to 14v+ when coasting. (As above.)

Further, the battery drain since using the charger has reduced the overall battery drain by around 2/3rds... what tha.. what has the charger done to improve the battery performance? Remember I have already had the battery replaced under warranty. But still had excessive battery drain. So why has a “new“ battery been given such an improvement under a charger, maybe my new battery was an old battery, that worked, but needed desulphation or something. Or maybe I have something else wrong in the electrical system, that the charger compensates for? (How can a one-off use of an external charger then cause the on-board charging to 'improve'?)

However, as @veeeight has said and as I have experienced the car starts very easily at 11.8v so I'm sure it can stand for many more days without failing to start. (Down as far as 11.6v in my case, with no indication at all that cranking ability has reduced.)

Even if you are driving often, if the driving has a lot of stop-start and the engine cutoff function is activated, it is surprising how low the Voltage is. Mine is around 12.0 to 12.1 if I do not charge it with the CTEK. (As above, 12-12.1v is not 'low'.)

Initially the on board voltmeter showed 14.4 - 14.9v almost all of the time irrespective of whether I was accelerating, cruising or on overrun. But as soon as the Stop/Start came into play the voltage would quickly drop to 12.3v but then only regain 14+v on overrun. In the end I switched the Stop/Start off as it was clearly killing the battery, having done this I managed to obtain a steady 14.4v at all times. (Of course it will drop to a nominal 12v with the engine stopped because it's not being charged. Mine and a pal's always shows 14v+ when the engine is running, including afer an Auto Start/Stop. The concept that following an Auto Start/Stop when the starter motor has just been used it only charges at 12.5v is nonsense. Does it do that when it's been manually switched off and restarted?)

What it says is that although a fully charged AGM battery should have 12.8v cars smart charging systems aim to maintain 12.5v which is exactly what I have found, this as you will see below brings into question the longevity of batteries. (12.6v with the engine stopped is fully charged regardless of whether it is AGM or not. AGM offers a greater capacity and more cranking power, not a higher standing voltage. However AGM batteries are typically charged at 14.8-14.9v as opposed to 14.5v for non-AGM)

When I switched off though and left the car for a couple of hours I then got a volt meter reading of 12.55 which was exactly the same as the voltage before I started driving. What this seems to show is that the so called Smart Charging electronics is not effectively keeping the battery fully charged which is disappointing. (If that's the voltage with the engine switched off for a few minutes then it's fully charged)

I do not accept that we need to connect the car to a charger, for the car to be drivable/livable, its as if we are driving an EV but with non of the sophistication, no neat plug and cable, but rather bonnet up, need to attach positive and earth cables cables running down the side of the car, charger on the floor beside the car, and a cable along garage floor to PowerPoint. And you need to keep attaching and reattaching, simply to charge the battery. (Exactly!)

EV owners suffer from “range anxiety”, I.e. how far can I travel on the current battery charge. We suffer from “parking anxiety” . I.e how long can I park the car on the current battery charge. (As above)

I don't think I have that feature but I checked the voltage on the dash today & it was 11.3v. I expect there was a small load on the battery but that is quite low considering it has only been on the drive for 8 days, before that it had had a long m-way drive. (That IS low, especially one day after a long motorway drive and implies a fault, but the question is did it start? After a charge or a long drive, and then intermittent use or short journeys, mine only gradually drops from 12.something to 11.something over a period of several days and the lowest I've seen it is 11.6v with no indication that starting might be affected.)

I left mine at Heathrow recently for 17 days and it started perfectly. Its now been on the drive for 8 days or so, the voltage is 11.8 and it started perfectly as well. (Ditto.)

Most modern cars do not have the electric motors but do use the wasted braking to charge the Battery. (How? That's rubbish.)

By fitting an original battery that is bigger than the car needs, and telling the "Smart" alternator to charge it to "say" 80% it leaves 20 space for regenerative braking. It works like this, if the alternator sees less than whatever battery voltage is equal to 80% (or whatever has been designed in say 12 volts) then it produces 14.7 volts to charge the battery ASAP but once the battery reaches set figure the alternator backs off and just produces 12.5 volts to run all the ancillaries and trickle charge the battery. During engine braking the "Smart" alternator pushes the voltage back up to 14.7 to grab as much waste energy as possible. If you want to see this in action set the Media Display to "Engine Data" and watch the voltage reading, 14.7 for a while depending on the state of charge, then it drops to 12. something except when you come off the throttle when its back to 14.7 ish. Downhill engine braking gives you the most time to look for it. I have not looked during "Wheel braking" to see if our cars do that as well. (Nothing to do with the alternator 'seeing' the battery voltage or being'smart', it's the electronics that are 'smart' (or maybe not going by the number of complaints) and tell the alternator what voltage to charge at. I've seen no indication that the charging voltage 'backs off' to 12.5v when the battery is fully charged, mine and a pal's are always at 14.8-14.9v when the engine is running. 'Wheel braking' slows the car quicker than decelerating so it will spend less time on the overrun and hence less time with the boosted charge - IF overrun DOES boost the charge which mine and a pal's don't.)

When I took my car to the dealers in June they replaced the battery under warrenty, presumably because they found that the SOC was not right, i now find that unless the lights are on the voltage during normal driving is 14v+ and during coasting 14.9v, the only time it drops below this is when the engine is stopped when it drops to 12.7v. (As mine and a pal's)

About a week ago I started to get a message on the dash saying 12 volt battery see user manual, spoken to Mercedes me who advised me to leave the car running for about half an hour, next day happened again and then also had a message saying to stop the vehicle and leave running, did that also, the stop start function was also not working at this point, went to the car on Monday and it would not start. (If the systems are clever enough to raise warning when the battery is low (as mine does) why aren't they clever enough to boost the charge at that time? Mine always shows the full charging voltage, warnings or not, but if you start getting warnings then yes running the engine (normal driving in my case) or putting a charger on will stop them appearing. The key point is that you should not need to drive around aimlessly or leave the engine idling when not needed to charge the battery. The only time an external charger should be needed is for cars that are very little used and stand fo two or three weeks at a time.)

Yes, the 80% point is enough to operate all the cars function, including S/S ….. as long as the battery SOH is at or near a figure determined by the energy management system, which we don’t know. Thereafter things stop working and warning messages appear. :) Hence why all the numerous problems with batteries. Everything works fine with new batteries, older batteries, even when fully “charged” (but at a lower capacity) don’t work successfully with the smart charging system. (As above.)

Well that explains why our batteries are failing so quickly then because batteries degrade rapidly if they are not kept fully charged. Yet another home goal for the environmentalist, we pollute less by employing smart charging but pollute more by manufacturing twice as many batteries. (Yup, or driving round more, or leaving it idling ...)

I took V8's advice and MB replaced the battery but it's back to square one. If I don't drive it for 1 day I get the 'critical' text and if I'm not intending to use it for a few days I put it on charge. The messages often tell you to go for a drive, in this 'Green' day and age that seems like a total waste of expensive petrol. (Yup again, but with mine given even a half-hour run at a steady speed it can then be left for a several days or a couple of weeks before warnings reappear)

Many new cars, particularly with stop/start use a combined starter motor/alternator (I assume the A-Class is one) so there is not even a freewheeling alternator to spin up. (No it doesn't, it uses a conventional alternator.)

Reading the Mercedes site, it says I need to run the engine every 2 weeks for at least 15 minutes consistently. ('Drive it' you mean? If it's driven for less than 15 minutes over many consecutive two week periods then yes I would expect to start getting warnings, and may have to use a charger.)

It’s not so much that the batteries are rubbish, they aren’t, but this is a problem that affects all EU6 cars. (Correct in respect of the batteries, but affects all EU5/6 cars? Seemingly not)

Sadly for many this means the battery is rarely fully charged which although this doesn't effect day to day use will ultimately reduces the life of the battery. (As above)

The alternators do not charge the batteries to 100% any longer like in the good old days, instead relying on your driving pattern to charge the remaining 20%. Or an external charger if you don’t use the car much. (As above)

You should drive at least half an hour straight, preferably in CONFORT and make the car as long as possible with the green bar - CHARGE. Pick some downhill road ... speed up and take your foot off! Must solve! Silly it’s a software problem at Mercedes-Benz admit to. (So it's our fault for using the car on the wrong type of journeys? Not mine or a pal's)

If your driving doesn’t fit the prescribed pattern, it won’t charge the remaining 20%. (As above)

As I understand it the so called smart charging is designed to put as low a load in the engine as possible to help with fuel consumption, for this reason when you are using any amount of throttle the alternator gives out around 12.5v but when you are decelerating, ie. using no fuel, it pumps out 15v+ as this is what is needed to fully charge the battery. In truth it varies the current but you will see this as voltage on the dash. (14.8-14.9v not '15v+'. An alternator charges by applying a voltage to a battery and that causes a current to flow depending on the state of charge of the battery. As the battery charges up the current will reduce even though the charging voltage remains the same. It's a constant voltage system, not a constant current)

My guess is that as my car had only done 800 miles in 6 months it had been laid up for a long time which lead to the battery going flat and damaging it. Any how for now at least I seem to have a well functioning battery :giggle: and what I take from this is if the voltage drops to 12v+ during normal driving I will probably give it a quick charge with my CTEK just to keep it in prime condition. (You simply should not have to do that.)

So in summary to get over my anxiety, I have had a new battery installed under warranty. Bought a charger that seemed to condition the new battery and improved battery performance. Allowing it to hold a charge better. Had the car tweeked at the dealer and now have a smarter alternator, possibly due to new software. (So a software change has solved it? Maybe to charge at the full voltage all the time, like mine and a pal's!)

Short trips can drain the battery, but regular long trips to charge the battery can be inconvenient. (It shouldn't be needed, older cars never needed that unless without extended periods of non-use.)

My “smart” alternator is now way way more smarter. (Explain!)

How long a battery lasts is often a factor of how it is used. Car batteries last longest if they are kept fully charged so modern charging systems are no friend of batteries as they are designed to improve mpg by reducing the charging current from the alternator (ie load on the engine) in preference to the longevity of batteries. (Reduce emissions actually, but if that involves batteries needing more frequent replacement with 'normal' use then the concept is obviously flawed.)

The best thing to do with such a tech heavy car is to connect it to a smart charger, such as a CTEK 5.0. or equivalent once a month if the car is not being used. (Long periods of non-use maybe, not 'daily drivers' which most complainants seem to have)

"Also - does your facelift model have the on board voltage display? The pre-FL’s didn’t as they didn’t want to “worry” the owners with the negative energy balance numbers!"