OK the other day my car just all of a sudden wouldn't crank. The headlights came on but were dim and eventually wouldn't come on at all. I got a jump but once I cut the car off it would not start again. I put a voltmeter on the battery and get 12V. Which one is bad, the alternator or the battery? Help Me REAL FAST!!!

 

you could have a problem with the charging system, if it was exactly 12v the battery is at 25% charged, a full chargerd battery will be 12.6v, most likely its a weak/bad battery

most auto stores have something they can use to check the charging system, have them check it out so you dont waste money

 

I would say battery. Did you just get it jumped, and then kill it real quick? Just curious if you saw how the voltage gauge was reading while it was running.

 

A fast answer since you do not want to test anything...Replace both!

 

Could still be either one. I've had a battery before that showed the volts, but no longer had the amps to crank the engine. To really know, you need to test the voltage output at the alternator while the engine is running. Or take the battery and alternator in for a load test.

 

Take the alternator and get it tested. As Camaro 69 said, you can have a battery that outputs the correct voltage, but it can't output enough current to crank the motor. You can't get a good picture of what condition a battery is in just by testing the voltage, all that will tell you is if it has a bad cell. The voltage curve for a battery is almost flat and then it drops off extremely quickly at the end of it's life, you would have to pull a reasonable amount of current from the battery and watch to see if there was a voltage drop once you put a load across the battery. Or you could go for the easy solution and get stuff tested...

 

I like MadMikeZ28 answer best. The hell with testing, just throw parts at the car. lol

 

yeah money is no option