Battery, alternator or something else

[forestb]

Originally Posted by RustyPile

You didn't say whether the engine is running or not. 12 volts at the battery when the engine is running usually means the alternator is not charging. When checking a battery without the engine running, a fully charged battery will read 12.50 volts. Depending on the load being applied at the time of the test (electrical items turned on), a functioning alternator will produce 13.7 - 14.2 volts at the battery. The big wire on the back of the alternator is simply the other end of the positive battery cable, so the 2 points will always read very nearly the same voltage (+/- .1 - .2 volts), engine running or not. The 11 volt reading at the in dash voltmeter is probably correct and just represents a 1 volt drop over the length of the wire.

Based on what's happening now, I doubt the first battery was bad - just improperly tested.. A 30 minute "quick charge" will not build enough charge in a dead battery to give an accurate test.. 90% of the time, the battery will always test "bad".. A dead battery needs a slow charge for a few hours to bring it to full charge.. Never use the alternator to charge up a dead battery. Alternators aren't built to work like that.

The truck was running yesterday when I just tested it and it read 12.01 volts.
Don't worry I didn't try and use the alternators to charge a dead battery.
It was the auto parts store that tried to charge the battery and then tested it on Sunday.

So at 12.01 volts we are definitely thinking that it is a bad alternators?