Battery Or Alternator

The battery puts out a steady (but low 12.50 or so) volts while idling
Put on the headlights and/or blower and the voltage drops 'till the engine dies

I can't remember which that means, bad battery or bad alternator?

Thanks In Advance

 

I don't know about the newer vehicles but you used to could run a vehicle without the battery - so I'd guess the alternator.

 

alternator
should be putting out 13.5-14.5

 

Take a voltage reading from the battery posts while running and while off.
While running you should be 13 to 15 volts.
While off you should be 12 or 12.5

If thier is not a noticable difference in voltage between running and off then your alt is bad or failing.

Try cleaning the battery connections and make sure they're tight.

 

Thanks Guys!

 

Did someone say battery?!? (Waking self from sleep)

When idling, the voltage at the posts should be MUCH higher than 12.50. Definitly a bad alternator IF your test connections were ok. The fact that your putting on the blower and lights eventually causes everything to die further proves that it's the alternator at fault.

Without any load whatsoever, a car battery should read about 12.65V or higher. The 12.0V mentioned in the previous post would indicate that the battery is dead for all intents and purposes.

slickshift: Your battery has been seriously depleted during each one of these tests. Unless you have a decent charger with pulse capabilities, you should consider your battery unreliable now. CERTAINLY charge it fully with an external charger as soon as possible. Replace the battery altogether if it is mission critical at all.