battery going flat over night

[colin machin]

hi 2007 c5 hdi 2.2 battery goes down occasionally over night seems to happen when its cold and the car stands for a day or two i put a new battery on last winter it seems i changed the battery unnecessarily seems ok in the summer it dose not happen all the time any ideas colin.

[spider]

Is the new battery of the correct capacity ?

Too small = will not really be up to it.
Too large = alternator will not be able to keep it fully charged so you're running with a semi discharged battery for short trips.

You can use a multimeter to measure constant current drain, allowing for a bit for ECU/BSi/Radio and Clock memories etc it should not really exceed (at a guess) 100ma. Any more than that indicates a drain which could be a number of things, boot light stuck on or perhaps dying alternator diodes.

[Old-Guy]

Check auxiliary drive ('fan') belt tightness.
Dirty/loose battery, engine earth, alternator connections.

[wheeler]

If I remember right the max current drain after the BSI has shutdown should be 40 milliamps (or 60 milliamps if it has the factory alarm fitted)

[Peter.N.]

I had this problem with my 2.2 Mk1 and it turned out to be caused by an aftermarket radio that someone had fitted which was switching the BSI on after the car had stopped. I connected a meter in series with the battery and found the current drawn was going from about 60ma to 1 amp and back again in about 1 minute cycles, disconnected the radio and it cured it. This will only happen with radios that have the same connectors as the original, most aftermarket sets have adaptor leads which don't cause the problem as the BUS lines are not connected.

Peter