'99 STS kills another battery :(

[keith_99_sts]

Late last summer my '99 STS was running fine with no hint of trouble. Came out one sunny morning and the car battery was totally dead, measured less than 1V. The battery green dot was black. Got a new battery at the GM dealer and all is well. Until tonight.

Opened the car door and no interior light. Oh oh. Key in ignition and turn, absolutely nothing, not even a dash light. Voltmeter shows the battery at 3V. WTF? I drove the car in the morning and parked it. 12 hours later, dead.

Called my neighbor and we jump started the beast. Ran a few errands, never shutting the engine off. At first, the voltage readout in the dash would dip to about 12V at idle, the go up to 14.4V when moving. Slowly the idle voltage crept up, and now it is sitting around 14V at idle, 14.5V moving, 13V with the engine turned off. I stopped the engine at home, and could restart the car. Not feeling too confident about tomorrow morning though.

Didn't get to check the trouble codes yet or inspect the battery (it's a pain to take out the child car seat and rear seat when it's single degrees F outside). I am 99.9% sure the wires at the battery will be fine.

Any ideas? Alternator or rectifier playing up? Are there any computers involved with the charging system? I really need a car I can trust to start in the morning. Did the car kill my battery last summer? Sure looks to be more than coincidence.

Thx,

Keith.

[pats37]

Take the car back to where you bought the battery. They can run some tests for you as it is still under warranty.

Have them check + & - connections at both ends. (Yes, the baby seat has to come out). Also, the alternator ground could be going bad. Are you in a warm or cold weather state?

[jhall]

It sounds like the alternator is working OK.

The charge regulation system compensates for idle speed to assure proper charge voltage is applied. Your comment that the "charge voltage moved up" indicates the system is working properly if it did this within a couple of minutes. If not, I'd suspect the regulator.

That said, a cable grounding problem could also be the trouble. The battery would appear to be charging if you looked at the charge voltage, but this is only an indication of what the alternator is producing, not what the battery is actually receiving. You can easily check this with a voltmeter across the the battery and compare the readings with the display on the car. A better view is to get a reading on the amperage going through the battery. The guys that sold you the battery should be able to do this for a nominal fee - if not for free.

If a ground wire is loose / dirty, the battery will not receive a proper charge - and it can be an intermittent problem. Have a good look at the battery leads - especially at the body and engine ground points. The engine ground point should be at the rear head on the driver's side just under the water pump jacket. Look for a stud with a ring terminal on it and make sure it's not corroded.

Hope this helps. Good luck!