With Euro 5/6 spec engines the alternator is said to charge at a reduced rate when driving at a steady speed which puts less load on the engine eking out the last bit of economy/emissions reduction. Whilst that doesn't seem very likely given the fraction of a horsepower we are talking about this article confirms that is the reason, and even for better performance when accelerating! It only goes up to the full rate on the overrun (and at idle when normally Start/Stop would activate) which means that if you do a lot of constant speed miles the state of charge of the battery can gradually reduce, and owners have complained that theirs need regular external charging or even annual replacement, with Start/Stop being disabled by the car's electronics when it detects the charge has fallen below a certain point. It's raised many complaints from people with leisure batteries in motorhomes etc. finding the auxiliary battery flat and the fridge defrosted after a long motorway journey, because the split-charge relay normally used to charge that battery when the engine is running drops out with the lowered voltage. These vehicles need a DC to DC charging system instead of a split-charge relay. Often referred to as 'smart alternators' the alternator is much the same as it always was it's the ECU that has the 'smarts' and tells the alternator what voltage to charge at (similar to Lucas 'battery sensing' systems in the 1970s ...).
But back to the A-Class. Some owners have said that this reduction in charging voltage is displayed on the MBUX (Vehicle, Engine) showing less than 13v in normal driving, only going up to 14.5-14.8v on the overrun. So far I've only seen 14.8-14.9v under all driving conditions including after spending two days on a CTEK charger and a 200-mile mainly-motorway round trip. After both those Auto Start/Stop did work for a couple of days (it doesn't normally) indicating the battery was sufficiently charged, failing to work again after a few days of non use. You can see the state of charge of the battery i.e. its voltage by pressing Start without depressing the clutch - or brake on autos - you may have to do this twice:
If you do this remember to push the Start button to switch off the ignition if you haven't started the engine, or you will not be able to lock the car!
But even when starting normally the MBUX when it comes on does show cranking voltage then the charging voltage. It's quite possible that once the ECU starts seeing low battery voltage it maintains the higher charge voltage in the hope it will restore full capacity. Or maybe not as that would mean it would be pushing out more emissions! When buying the car I noted that when the salesman lifted the bonnet the cover over the jump-start terminal on top of the battery had been pushed back, so maybe the battery had gone flat whilst with the dealer and they had to boost it. Information from Lucas in my MGBs era is that if the battery has become completely or almost completely discharged the on-board charging system will only get it back to 50% capacity, it needs boost charging at a higher voltage to get closer to full capacity. Modern 'reconditioning' chargers should be able to do that as they can charge in pulses of up to 20v, but the more basic 'maintenance' chargers won't as they only use a minimal charging voltage and current to keep the battery topped up. However the above article and others indicate that 'smart' charging systems may use pulses of 18v and hence a battery with the correct technology must be used i.e. Absorbent Glass Mat, and not a generic lead-acid battery, and those charge pulses may only be seen on an oscilloscope. I've had an oscilloscope on the battery posts and there was no sign of pulses, not even a ripple. Mains powered so only on the drive of course. But even pulses to do any good would have to raise the average voltage which would be seen as an increased voltage on a meter, just like lowering the voltage is said to damage the batteries. A reconditioning charger should fully 'recover' a battery, but even after using a CTEK for two days when the battery warnings stopped and Auto Start/Stop worked again briefly, also on another occasion following a 200-mile motorway journey, the MBUX voltage never dropped below 14.8-14.9v.
There are hundreds of posts on the A-Class forum (a selection can be found here) and others containing issues and problems with warnings being displayed and batteries needing frequent replacement, which include confusing and conflicting statements. Typical is that the battery isn't charged at the full rate until you are on the over-run, and when driving it only charges to 80% of capacity. Another is that 80% should be enough to stop warnings and allow Start/Stop to work. But that's 80% of some notional charge state, as the State Of Charge of the battery gets less and less that 80% isn't enough, hence the need to spend more time on the over-run i.e. go on the 'right type of journey'. Without that the battery gets more and more degraded needing replacement - some say annually, or put it on charge every month - and that's for a daily-driver. We have never seen that prior to Euro 5, and it's only occasional-use classic owners that talk about using a charger, and even that isn't an automatic requirement. What I don't understand is if the car is clever enough to do all that, and realise the battery is becoming discharged below a certain point and first disable Start/Stop then start giving warnings, why doesn't it use that knowledge to charge at a higher rate until the warnings stop? Well, maybe I do know why they don't do that, and it is because they would be putting more emissions out and not meeting Euro5/6 requirements, regardless of what it means for owners. After reading a few of these I commented that on that basis they weren't fit for (our) purpose, and the administrator got very snotty, earning a rebuke from the Moderator at one point.
Specific comments have been that the reduced charge rate is shown as a reduced voltage of 12-13v when running instead of a more normal 14.8v, I did ask if that was on the MBUX and for pictures but got no reply. Spending more time browsing old posts one person has said his MBUX voltage reading can be about 12.5v driving and 14.5v 'coasting'. Coasting to me is rolling along out of gear (or clutch depressed) which means the engine idling. Maybe he means 'on the over-run' which is when the momentum of the car is spinning the engine, and that IS when the consumption meter goes into the 'Charge' section. However idling also puts the charge voltage up to the maximum 14.8-14.9v even though the consumption meter is showing 'gallons per hour' i.e. effectively zero mpg instead of being in the Charge section.
Another comment has been that this 'charge' occurs under braking as well, with no indication of how that happens. Yes I know it would be possible with the right equipment on the car, and that when you are braking the consumption meter probably IS in the Charge section, but that is because you are decelerating as well. Braking slows you down quicker, so you are decelerating for less time, which reduces how long the consumption meter will be in the Charge section, so reducing how much extra charge can go into the battery ... if there even IS extra charge going into the battery on the overrun.
February 2024: More questions about battery voltage and Auto Start/Stop working and I mentioned that I had seen comments about the battery discharging when on a long steady-state journey, to which 'veeeight' responded that 'he couldn't recall having read that' - as if he is claiming that he could recall every comment ever made. Then a few days later to another question he responded:
Another (in a discussion on starter efficiency) has been that the A-Class has a combined starter motor/alternator, and again yes I know they do (or did) exist but mine at least has a conventional alternator. It may well have a modern geared starter and I know they take much less out of the battery when cranking than an old-style inertia or direct-drive pre-engaged, and that means the battery charge can fall to a lower level but still crank and start the engine. Additionally the Absorbent Glass Mat battery these cars should have offers more energy per unit volume, and they are pretty big batteries for the size of the engine, both aspects of which should allow the engine to start at a lower charge as indicated by the table below. Despite battery warnings, both when first getting in the car and even when I started getting them at the end of a journey at switch-off as well as at the beginning, with a battery 'resting' voltage of 11.6v, mine has shown no signs of failing to start, appearing to crank and fire up exactly the same as with no warnings.
Maybe the warnings just come on way too early. There is a further warning "Stop Vehicle Leave Engine Running" as in this video where the mechanic has a sophisticated battery/charging/cranking system tester that shows the battery at 60% capacity and everything else OK, i.e. double the capacity implied for mine, but the mechanics diagnosis is to change the battery. To be expected on a customer car, but is it really necessary? I doubt it, until it gets to the point where cranking is audibly slower, and if you carry a lithium jump pack even that isn't going to strand you.
Another comment has been that with Start/Stop switched off the battery doesn't 'deteriorate' as fast, but whether that is in all cases or only for those people whose journey-style involves lots of Start/Stop use wasn't explained.
It's impossible to get clear answers to questions on the forum as most people are only repeating hearsay often incorrect, the administrator 'veeeight' probably knows more than most but has been positively offensive in some of his responses, enough to get a rebuke from the Moderator in one instance.
When driving mine has never been other than 14.8 or 14.9v, whether that be short journeys with warnings or after a 2-day Ctek charge or 200-mile motorway run, both of which have stopped the warnings AND brought Start/Stop back - for a while so on the face of it mine doesn't seem to be charging at a reduced rate anyway.
One Sunday I was out for about an hour, the warnings had built up to getting them at switch off as well as switch on, and it stopped both of those. More surprising is that with virtually no use for a week they had not come back. Start/Stop still not working, but I wouldn't expect it to.
I had been waiting for the weather to warm up a bit and feel more like messing in the garage to make up an adapter so I can monitor the voltage on a separate meter to compare with the MBUX, and that was mid February. Plugged into the accessories socket it showed the same as the MBUX display whether it was ignition on at 11.8v which on the face of it shows it is significantly discharged, but not enough to get warnings. Idling and running 14.8v on both (as the MBUX has always been).
Next was to connect the meter direct to the battery which involved a wire coming from out of the bonnet and through the closed driver's door. Before starting the engine same voltage as before i.e. 11.8v. Idling was 14.8v again as before. But when driving it went up to 14.93v, varying by a couple of tenths either way, and that was the same in steady driving as on the over-run i.e. going into the Charge section on the consumption meter.
Connecting an oscilloscope showed no variation in voltage either, not even a ripple, although being mains operated that was only running the engine on the drive! That all bears out what I have found i.e. my warnings and loss of Start/Stop are solely down to lack of use and possibly a battery with reduced capacity from having done so few miles in three years, but steady driving is enough to boost the charge to stop the warnings, and even enable Start/Stop after a long enough journey.
It was only after two weeks from the 'one hour Sunday run' with just a couple of short journeys and several starts but no driving that the pre-journey warning came back, when the MBUX showed (ignition on not started) 11.6v. Does that mean 11.7v is the threshold for warnings? Maybe, but the table above indicates even 11.8v is only 30% charged.
I've positioned a small digital voltmeter by the dash, connected to the +ve battery connector via a fuse, with the -ve going to a relay which is powered from an accessories plug in the accessories socket in the centre console, the relay contact extending it's earth to the meter. This is only powered when the ignition is on, so the meter is only powered then and not all the time. That always shows exactly the same voltage as the MBUX i.e. ignition on currently 11 point something, then 14.8-14.9v under all engine running conditions. Idling:
Next step is to put a CTEK on again to get Auto Start/Stop working and monitor the battery voltage as before and see if it's still 14.8v in steady driving or has reduced, and if both displays still show the same.
In the meantime a pal has filmed his A-Class of a similar age to mine showing that on the MBUX at least the voltage is always about 14.8v apart from during Auto Stop/Start and the restart from that:
CTEK connected took about 12 hours over two days to complete recond. Next day a few miles into a short journey Auto S/S cut in, and apart from that over the whole journey both voltmeters showed 14.8-14.9v. But longer journeys needed to see if that situation changes, two coming up over the next couple of weeks.
I've changed how the secondary meter is connected though. I couldn't see an easy way of getting though to the cabin from the battery in the engine compartment, but there is a passage behind the top of the front wing, which means although the wire is not visible with the door shut it is lying across the inner door seal. A very spongy seal so shouldn't hurt it for the short period I intend to use the secondary meter, but I did find some water had come through after heavy rain. So I've cut the wire and fitted male (cabin wire) and female (battery wire) connectors so I can unplug them when parking up the car, reconnecting them when I get in. That means I don't need the relay, so have connected the -ve of the meter direct to the outer connection of the accessories plug. That has the additional benefit that I can see the battery voltage immediately after unlocking but before the ignition is on and putting more load on the battery.
Not used for five full days, on the sixth the aux meter battery showed 11.9v after unlocking but before ignition on. A 30 mile round trip, no Auto S/S on the way there but it activated soon after starting the return journey. Next morning aux meter showed 12.2v, Auto S/S activated after driving a few minutes, voltage 14.8-14.9v all the time the engine was running on both days. The following week we had a 200-mile round trip. 14.8-14.9v all the way there ... except for the last few miles where occasionally it went stepped down bit by bit to 14.5v, or went up to 15.0v, regardless of whether were cruising, accelerating, on the over-run or idling, MBUX and aux meter on the battery terminals reading the same to within a tenth of a volt at all times. Return journey just the same except the small variations started sooner, but the majority of the journey was at the usual 14.8-14.9v. Next morning 12.2v before firing up, 14.8-14.9v at the time the engine was running, with Auto Start/Stop cutting in within a couple of miles as before. After that I removed the aux meter, I'll probably put it in the roadster just because I have it, and because of that will put a more conventional 2" volt meter in the V8. A PO had cut an additional hole to the right of the fuel gauge containing a non-functioning temp gauge, I removed that and patched it as best I could although I've never been happy with the result.
The upshot? Despite all the talk about lowered voltage and batteries not being fully charged when cruising, as both mine MBUX and battery) and a pals (MBUX) show the same full (bar minor variations on my 200-miler) voltage at all times either our cars have had the same mod done by someone to prevent the problem, or MB themselves disabled it because of all the claims and complaints. It just remains to be seen now what happens over the summer when this car is (hopefully) being used much less.
November 2023: Very little use over the summer, six months of 'critical' battery warnings both opening the door and at switch-of after a short trip and no Auto Stop/Start, but no hint of struggling to start. During a 200-miler I still had the warning 80 miles in but not after that. No Auto Stop/Start next day. Two days later I put the Ctek on and it complete in 8 hours. Next day Auto Stop/Start a couple of times but not since then. Less than three weeks later with just a little use the battery critical warning pops up again.