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1995 Buick LeSabre kills battery unexpectedly

My 1995 Buick Lesabre will intermittently drain the battery after sitting for about 12 hours. It is unpredictable. The battery was newly installed in December, 2010 and the alternator was tested at that time and about three weeks ago and proved to be recharging properly. This battery kill happens even when there is no visible sign of current usage, such as a dome light being left on. Please provide any information on what I should look for in order to to find the source of the current drain. Thanks.

When a battery is drained in 12 hours the current draw is very high. I would check 2 things, make sure the fuel pump is not staying on when the engine is shut off.

A bad diode in an alternator will kill a battery in short time. I had a bad diode in my alternator, and with engine off was pulling about 4-5 amps but would still charge ok.

Diodes in an alternator when good will only pass current one way. When one goes bad it will allow the battery to drain back into the alternator. So 12 hours x 4.5 amps would be 54 amp hours which is close to a batteries capacity give or take.

You need to have the vehicle checked for a current draw. If found you will need to start pulling one fuse at a time and see which circuit it is on. If not found then I would put my money on a bad alternator.

If this has happened a lot there is a good chance the battery may be ruined as they do not like to be fully discharged and causes it to sulfate.

59thunderbird, Thanks for your advice and the math. I will start pulling fuses to see if that might be the culprit. Otherwise, a new alternator my be the next investment.