Need car help, non-Toyota, non-truck

Hey, all -

The "too long version":
While in the car the fully charged battery quickly drains to about about 7.5 volts. While the car is running I read about 14 volts. Autozone says both battery and alternator are good.

The "full version":
I think I've either got a bad battery or a short in the car.

My daughter's 2004 Civic has just started crushing its battery. It wouldn't start new year's eve and my volt meter said the battery was making about 7.5 volts. So I jumped it and sent her on her way knowing I'd have to jump it to get it home the next day . . . which I did. (While running I'm reading a little north of 14 volts)

On her way home she stopped at autozone who tested the battery and the alternator and declared both "good", with the battery at 89% charged. I put it on a trickle charge while still in the car until the morning of the 2nd. I took it off the charge and the volt meter said about 12.5 volts. 4 hours later she called me and said it wouldn't start again. When I got home, total time about 6 hours off the charger, it was again at about 7.5 volts.

So, I took the battery out of the car and put it on a charger for about 15 hours. This morning I took it off the charger and sitting on the floor it read 13.5 volts. 2.5 hours later it reads 13.1 volts. I'm not convinced the battery is bad, yet. Yes, it's reading almost 1/2 volt lower 2.5 hours later, but the voltage you read immediately after taking it off the charger isn't entirely accurate.

I clipped onto the positive and negative leads in the car and tested resistance, hoping to find infinite. Instead, I'm reading about 200 ohms. This makes me think there is current flow through that car. But, there should be *some*, right? There are parts of the car that are always drawing current.

How do I diagnose this problem?

Thanks,

Matt

 

I found it. The AC Compressor relay was stuck on (or off, whatever a bad relay does) and was causing the compressor to be always engaged, drawing amps continuously.

 

How did you diagnose the problem? Did you use a multimeter in ammeter mode and pull fuses one at a time?

 

My intent was to do that, and I had the multimeter setup to do it. But, when I completed the circuit by putting the multimeter probe on the battery terminal I heard a snap. After doing it a couple of times I recognized the noise as the AC compressor engaging. Also, it was pulling 3 amps. So, I pulled the relay for the AC Compressor and completed the circuit - all quiet and about a tenth of an amp. So, I put a new relay in and we're back in business.

 

I like happy endings...

 

I was going to say check the voltage regulator but it looks like you got it figured out already.

 

Useful information! So the AC compressor's magnetic clutch draws 3 amps to hold it in place? Interesting.

 

I was very surprised by that number. I should do some research and see if that's normal.

Also, I have 3 multimeters with only a rudimentary knowledge of how to use them. I didn't get the same reading from any 2 of them. So, I'm sort of guessing at the draw. But, the one that said "3" with the bad relay in said ".12" with it out.