Battery voltage: how much is enough?

I'm trying to fix my GF's Subaru Legacy (doesn't everyone's GF have a Subaru wagon???). I've been jump-starting her car for her and disconnecting it every night for the past three weeks. Last time I drove it, I got it home and turned it off and immediately tried to restart it...no luck. Like it didn't charge at all. I checked the voltage as it was running to see if it was pulling a charge from the alternator, yep, it was, with 14.3 volts charging. The battery by itself shows 12.08 volts at rest. For comparison's sake, my battery in my truck at rest is more than 13 volts.

Ideally, I'd like to have 12.5 volts, but I have no idea what these numbers actually mean. Why won't this thing crank?

It's under warranty, and I'm going to take it back. Are they going to tell me everything is "just fine"?

All help is appreciated.

 

sounds like the battery is bad. 12.6 is minimum. You can check with a volt meter when you crank or try to crank. see what the voltage drops to. Bet it drops a lot.

 

When my alternator died on the trail, my truck still started at 9.7 volts. The gauges weren't working correctly, but it ran fine. I think it's more so that you don't have the amperage stored in the battery to crank. The starter is high amp low voltage. Bad battery.

 

It is about amps but unless he has a load tester, the way I explained is OK. Go to an Auto Zone for a load tester but you need 12.6 Volts.V=IR. R is constant

 

Thanks, you guys. I took it back to the battery merchant and he tested it: 12.09 volts, 105 Cold Cranking Amps. Really really discharged, he said. He put it on the charger overnight and said it now shows over 600 CCAs. I'll reinstall tonight and go looking for the parasitic draw... What he said made sense. We'll see if it holds up.

Thanks again.

 

12.08 is about 25% of capacity. Start the car and make sure you are getting 12.8V or higher at the battery from the charging system. If not you need a new alternator and battery (battery is ruined anyway). If it's charging you need just a new battery. They don't last that long because the environment in an engine bay a the worst thing for a lead acid battery.

 

Bad battery. Voltage is fine, you don't have enough amps

 

Amps are your problem not the voltage. As a battery ages it may maintain the voltage but the number of cold cranking amps will deteriorate. Sounds like it's time for a new battery

 

Free. Take it to a parts store and they can run a Load test of the charging system and see what's bad. Free

 

The thread is 1.5 years old. I hope @james found a solution by now.

 

That's funny. I never even looked.