Odd Battery Behaviour

[E55BOF]

It was on 12.86v after I disconnected the Ctek at seven yesterday, so it had had four hours to settle to that 12.69v.

I've never had a car with a drain before, so this is all new to me. For all I know there may not be a problem; all I can say for sure is that after the car had been standing, not run, for a week, with occasional use of a 12v charger while I was playing with the audio, the AGM battery was discharged to the point where nothing electrical worked until it was jump-started. If I'd thought of it when the battery was flat I'd have checked the actual battery voltage, but I was in a hurry, and didn't think of that. I would now...

Then, after about thirty-five minutes/twenty miles' driving over to Terry Gates for the tailgate to be sorted, and after about six hours being worked on over there, and three days there altogether, it started for a test drive with no difficulty.

I'll keep checking, but if there is a drain, on present indications it's no more than an annoyance until I can get it sorted. I won't sell the other ML until it is, though.

More anon...

 

[ALFAitalia]

Put an amp meter in line it and see what's being drawn.....at least you will know for sure. 50 to 85mAH is considered normal on newer cars....less on older ones. Anything more than that and you need to start pulling fuses to see where the juice is going!!

 

[Petrol Pete]

Does this car not have 2 batteries ?

 

[Bobby Dazzler]

Does this car not have 2 batteries ?

If it’s like mine then it doesn’t, as there’s no stop start.

 

[E55BOF]

Put an amp meter in line it and see what's being drawn.....

Easier said than done on a W164; the battery is tucked away under the driver's seat, and it's a pig to get at if your knees are knackered...

 

[ALFAitalia]

If it’s like mine then it doesn’t, as there’s no stop start.

Mine has two....and no stop start....I think it's because has the electronic column gearshift and the aux battery would still enable you to put it in neutral if the main one failed.

 

[E55BOF]

I think mine is the same.

 

[Bobby Dazzler]

Mine has two....and no stop start....I think it's because has the electronic column gearshift and the aux battery would still enable you to put it in neutral if the main one failed.

If there are two batteries in the W164, then in my car one of them 19 years old!

 

[E55BOF]

You have to remove the main battery to get at the auxiliary. Maybe many people just don't bother...

 

[BTB 500]

If there are two batteries in the W164, then in my car one of them 19 years old!

The (only) battery in my R129 is 21 years old!

 

[ALFAitalia]

Impressive...I changed the original battery on our W169 after 18 years.... even then it started fine and it was just precautionary as it was dropping to about 11.5v over night!

 

[BTB 500]

I keep my R129 on a maintenance charger, and I'm sure it's down to that.

I changed the 17 year old battery in our Vito (which had never seen a charger in its life) last year as we had a weird issue with it occasionally not cranking when the engine was hot (cold starting was fine). Changing the battery fixed it, but I still have no idea why (it's inside under the passenger seat so not exposed to temperature changes)!

 

[Petrol Pete]

The aux battery might be one of those tiny ones , some are under the centre console , some are buried in the dashboard on some cars ......

 

[E55BOF]

Took it over to Neil Braybrook this morning; an acceptable parasitic drain is 30-50 milliamps, and it's only doing a steady 15.

It definitely went dead in four or five days before, though, so it can't really be anything but the battery itself; he doesn't have the kit to check it out with the modern equivalent of the (brutal) old load test, but a specialist will, and that I can arrange myself.

 

[E55BOF]

It gets odder... The car has been stood, unused (and unopened except to check the battery voltage) since Friday last. Voltage readings 12:56, 12.47, 12.42. 12.42, 12.42 volts for Saturday through to today, Wednesday.

That's either about 75% charge (AGM battery) or 80% charge (wet battery), holding steady for three days. Not 100%, but not dropping. ISTR that on some Mercedes with AGM batteries, the alternator does not charge the battery to 100% anyway. Anybody know if that applies to a 2007 ML?

Even more puzzling, how could it have been as dead as a dodo after a week unused a month ago, yet not discharging now? I'm baffled.

 

[E55BOF]

The battery is a Numax (I'd have fitted a Varta...), manufactured in June 2020, so it was near the end of its likely five-year shelf life when fitted late last year. I suspect it's just a bit tired, and may improve a little with use. I'll keep an eye on it, and if I have to replace it in the autumn, so be it. I've let the selling import dealer off the hook; he's suffered enough...

I haven't a clue how it could have gone completely flat in a few days a while ago, though. The earth terminal connection was slightly loose, but the car's electrics were completely unresponsive, not just breaking contact on trying to start it, and it has never failed to start at any other time.

 

[E55BOF]

Update: The Ctek is charging the battery, in AGM Recondition mode, and it is now at stage 7 (Float), when it should be putting out 13.7 volts. The battery is at 13.37 volts, checked at the recharging posts under the bonnet, and there's no voltage drop between the battery terminals and the posts. Does anybody know, does that confirm my suspicion that the battery is past its best?