Car battery dies - trying to diagnose problem?

Question

I replaced this battery maybe 2 yrs ago. Here's what I tried:

1. With car running, the voltage across the battery is 13.8. That seems fine I think. I measured voltage drop across the ground strap; the drop there is not excessive, perhaps 10 mV.

2. With car not running and no load, the battery voltage is 12.8. Turning the headlamps on drops the voltage to 11.8. I left the headlights on for about a minute, then off. Now the battery voltage is 12.25.

3. I disconnect the negative terminal and put the current meter across the terminal, and what happens completely baffles me. When I first touch the meter to the terminal, it shows just over 1 amp current; but when I hold the meter on the terminals, within 10 seconds the leakage current is down to 6 mA. I did it time and again and it did this consistently

Because of the voltage drop under load I'd guess the battery is bad; but that current thing baffles me. Any ideas?

Thanks for the comments. I've pretty much come to the conclusion that it must be the battery. After less than a minute of the headlights being on, the battery dropped to 12.25 volts (which I read is 50% charge level), and the car wouldn't even crank. I went out and checked again, and it was below 12 volts.

So, off to the parts store I go. There must be something about that car (heat, vibration, etc.) that causes batteries to die after only 2 years - it's happened before.

Answer 1

Sounds like the battery has a weak cell in it.
In step 3 if you are testing between the negative post and the battery cable for a drain, what you see is normal. It is caused by the capacitors charging in the various modules, as in radio, abs module, pcm, air bag montior,etc. The drain then drops to negligible after the modules charge.

Answer 2

The mechanic is wrong. If your battery is draining when the vehicle is turned off, then something is pulling current to make that happen. You MIGHT have gotten a bad battery.. but I doubt it. I suspect there is something that is drawing current with the ignition off. This could be a light inside the glove box, under the hood or some other place that you can see when it's all closed up. You could also have a short somewhere that is drawing the current at a slow enough rate that it doesn't burn the wire or blow a fuse.. it just runs the batter down. Here's how you can tell. First. fully charge the battery and use the truck. Then, take the negative connection off the battery and let her sit for the 2 or 3 days that it normally runs the battery down. Hook the battery back up and see if it starts. If so.. then obviously there is an electrical drain in the truck, if not you have a bad battery. Good luck and I hope this helps!

Answer 3

your battery may have an internal short to it. have it checked out and have it replaced if that's the case.