BATTERY DRAIN

[houghman]

Hi guys, got a problem with my S3, it has all of a sudden started draining the battery, the alternator was dead, so I replaced it and tightened the belt and also put in a new battery. It ran ok for a day then overnight the battery was dead again! Any ideas please??????

[tvrgit]

First step is to diagnose if it's not getting charged enough, or if it's going flat.

1. Voltmeter over battery terminals after a run should show about 13 to 13.5 Volts. If it's significantly lower then there is a fault in the charging circuit.

2. If it's ok, leave the car for several hours (eg overnight) and check again next morning - it might be a bit lower but should still be 12 to 12.5 volts. if significantly lower then either battery is not holding its charge, or there is a current drain.

To find out which, repeat step 1, but disconnect battery earth before leaving the car.

then repeat step 2 - if battery is still lower volts, then it's not holding a charge.

If battery is now ok, then there is a current drain in the car somewhere - diagnosing that is in art in itself, involving an ammeter in the battery circuit, and removal of fuses until the drain stops - that cuts it down to a particular fused circuit to check.

[HarryW]

Check what the current drain is (you've already replaced the alternator and battery). If you are handy with a digital meter you should be able to do it. If not then I suggest you pop fuses and or relays unitl the drain stops, my money would be on the heated mirror relay, which is on a timer every time you turn the igniton on, perhaps its stuck on .

[tvrgit]

HarryW said:

Check what the current drain is (you've already replaced the alternator and battery).

In my time I have had to take back a new alternator that didn't charge properly, and a new battery that was flatter than the one it replaced. Never take anything for granted!

[houghman]

Thanks guys! I'm no engineer or spark but my neighbour got a meter on it and we've found that the amp for the CD changer was still drawing 13 volts when the ignition was off, happily sucking juice from my battery. Hopefully now sorted! Thanks for your help!

[HarryW]

YHM