W126 battery dies overnight

[Fast_Lane_Dane]

Hi, I have a less-than-a-year-old 100Ah VARTA battery in my 380SE. Problem is that the battery looses power overnight.
Symptom: Lights in instrument cluster etc. comes on ok, but the starter turns slowly, only a rev or two and then – dead
When recharged, it’ll hold power for a week or 2 and then the problem returns. I have suspected the auto-aerial, but that doesn’t seem to be the problem, motor in aerial stops once the antenna is retracted into the housing. The central vacuum locking pump works ok too, no leaky door cylinders, and the pump is stopped anyway approx. 30 sec by a relay.

Acc. to an electrician, I can’t use a tong amperemeter (not sure if this is what it’s called in English), since you only can use this for AC, not DC.

Where do I get started? Is there a common known problem for this, or will I need to pull every fuse out and measure if there’s consumption when the engine is turned off (hope not).

Best regards,
Hans, DK

 

[television]

Hej Hans, you need to put an ammeter in series with one of the battery leads, and check the drain, there should not be any on your car, with the meter joined up,just pull the fuses one by one and you will find it

 

[Fast_Lane_Dane]

Hi Malcolm,
I connected a digital amperemeter btw. batteri ground and the ground lead. The meter showed 0,02 A (20 mA) drain. I took out/fitted back the fuses one by one, the meter lowered to 0,01 A for one of the fuses, probably due to the instrument cluster clock. The remaining 10 mA drain is likely caused by meter deviation and (micro-)current seepage. With this drain rate, the 100 Ah-battery shouldn't be flat in > 4.000 hours(!), so the reason is not caused by faults in the wiring.
The generator charges with 14,2-14,6 V, so I guess the reason for the flat battery is, well, the battery itself, even if it's less than a year old.
The reason could also, in theory, be an intermittent failure on the generator (bad connections at the commutator coals) but this is hardly likely.
I'll check the battery codes on Varta's homepage next to see if the battery is too old, and check with my mechie who sold me the battery.

Regards, Hans

 

[television]

I have had many a new battery where it has failed within a few months, often 1 cell goes high resistance meaning that the car once started will start and run all day, left overnight and no go.

You are correct in your use of the ammeter, and as before the 126 has never had these faults, in odd cases care has to be taken when joining up to the battery so that the circuit is not broken, and the path of a short lost.

It is your battery.

 

[Fast_Lane_Dane]

Hi Malcolm,
The symptom you describe with "car once started will start and run all day, left overnight and no go" is exactly what happens. Furthermore, I checked my battery on the Varta homepage, apparently the battery was superseeded by a never model a year before I bought it, i.e. it was a year old, which might caused a cell to fail since the battery had been unused for a longer period.
I'll post the outcome of the story when I have spoken with my mechie - hopefully a new battery w.o. costs, since Varta boasts a 5-year warranty.

Cheers, Hans

 

[Fast_Lane_Dane]

Problem solved - my mechie swapped the faulty Varta battery with a 100 Ah Mercedes battery (from a W210) - no problems, car starts like a baby now

Regards, Hans

 

[tiggins]

hi malc. i have had the very same problem on a w129. even thought the electric ariel did seem to work ok. i disconected it and low and behold no problem since for four months. then out of sheer curiosity i reconected it and again a week later flat. disconected it again and three month later the problem still hasnt returned.

 

[television]

hi malc. i have had the very same problem on a w129. even thought the electric ariel did seem to work ok. i disconected it and low and behold no problem since for four months. then out of sheer curiosity i reconected it and again a week later flat. disconected it again and three month later the problem still hasnt returned.

Water plays a big part here with something that is always live, water does get into these things, but should drain out, the constant dampness eventually sets a up a short circuit path as can be seen inside the motor assembly by any part the has the green corrosion on or in it. Often it can be washed away with a good cleaner and toothbrush