Originally Posted by graniteguy
Put a voltmeter on the battery while the truck is off. Do it again with the motor started. If the voltage is higher while the engine is running, then your alternator is probably fine.
Good idea but a little more to it than that.
1. Battery alone, no load should be about 12.6 or above. If it's sat awhile, maybe 12.4 or so.
2. Alternator should raise the voltage to AT LEAST 13.8 volts with the motor running, no load. Loaded voltage should not drop below 12.8 volts or you stop charging the battery and start draining it. Really, with a loaded voltage that low, you'd be in the situation of balance -- really neither charging nor draining and that alone is a bad sign. Anything less than about 13.2 volts and the battery isn't charging much if at all.
3. Maximum voltage from the alternator shouldn't be much more than 14.2 to 14.5 volts -- more than that for any length of time fries a battery by overcharging.
Optima's are great batteries but they have a VERY high failure rate, FYI.