New battery runs completely flat.

[Paul Winnett]

I have a C-Class 270 W203. The battery required replacement and a new battery purchased and fitted by myself. After running the car for about 8 miles I parked up. Returned an hour later and the battery was completely flat. Assuming the new battery was at fault, I purchased a second, fitted it and after a short run in the car, the battery went completely dead.

It seems that although the car's electrics seem fine when running and is charging, after half-an-hour the battery dies.

Any ideas?

 

[Blobcat]

Hello and welcome

Faulty diode pack on the alternator can cause high battery

 

[Paul Winnett]

Even while it's parked?

 

[Blobcat]

Even while it's parked?

Especially when parked

 

[davemercedes]

Funny timing for your post. I bought a Mercedes battery for my S203 from MB Avonmouth 3 - 4 years ago but the car has been standing outside in the cold and wet for weeks while we're locked down... I had to take swmbo out (we had run out of milk!) and turned the key... the charging light went out and "yeeuch" it was dead as a dodo...

Cutting it short, I started the car with my jump starter then afterwards. I left on charge pn my 12v automatic charger. Five hours later when I checked, it was fully charged (well, at least it started on the first turn as usual. We've had to go out twice since and the cold start result has been the same - i.e.: one turn and it starts.

MB batteries seem to justify their price (and I know... other brands are available!)...

 

[Tony Dyson]

Especially when parked

If one or more diodes fail in the alternator the failed units will allow a back feed from the battery to the alternator and it will discharge the battery overnight, you may have flashing lights while the car is running indicating failed alternator diodes.

 

[Blobcat]

If one or more diodes fail in the alternator the failed units will allow a back feed from the battery to the alternator and it will discharge the battery overnight, you may have flashing lights while the car is running indicating failed alternator diodes.

I had it on the 911, it was a random fault as it depended where the alternator stopped as to whether it would discharge or not.

 

[Tony Dyson]

I had it on the 911, it was a random fault as it depended where the alternator stopped as to whether it would discharge or not.

It's one of the pitfalls of using an alternator to provide a DC Voltage, it wouldn't have done any of the consumers on the car any good either providing them a DC Voltage peppered with an AC Component.

 

[star]

That’s a big battery drain, I would suspect the block/aux heater.

 

[sonic]

For a battery to go flat in an hour or so requires a massive drain, possible 10 to 20 amps.
Did the battery connections spark when you connected them to the battery?
You need to meter the battery on & off charge. You either have a short somewhere, or the alternator is not charging.

 

[Paul Winnett]

Thanks to those that have made suggestions.

SONIC, yes the battery sparked when I reconnected it which would suggest a current drain, so possibly a short somewhere. A fully-charged 90-amp battery draining down in less than an hour seems odd considering there is no indication of a major short circuit. Could it be the regulator?

 

[Flyinspanner]

So it has a 90+ amp drain!
Hasn’t blown fuses so suspect battery feed.
The battery ‘controller’ (if fitted to your model)?
That’s massive, it will be a short, check you haven’t pinched a cable.
Check for chaffed cables, loose cables, any that run near rotating parts. (Belts/pulley etc)

that will also generate a lot of heat! Look for witness heat/scorch/marks

 

[Paul Winnett]

Thanks, I'll check for scorch marks.