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Battery or alternator

Using a volt meter on the battery, what should the readings be with the engine off and on? In other words, what’s a quick and dirty method of determining if I have a battery or alternator problem?

bout 12 volts at the battery. 14+ with the engine running. some alternators only charge over 1000 rpm or so just so you know.

The battery voltage will depend on the state of charge of the battery. A good, full charged battery should show 12.6 volts with the engine off but a voltmeter is not always a definitive test for either the battery or the alternator as neither one is loaded so to speak.

A somewhat back yard method for checking the battery with a voltmeter is to make sure the battery is fully charged, disable the engine so it will not start, and crank it over for about 10-15 seconds with the voltmeter attached.
At the end of the cranking period the voltmeter should show about 10.2 volts at a minimum. If it goes below 10 the battery is iffy.

“A good, full charged battery should show 12.6 volts with the engine off” Well, if he had a good, fully charged battery, he wouldn’t have a problem, would he??

Maybe, maybe not. A good battery should show 12.6 volts but a bad battery can show the same voltage reading. The difference between the good and the bad would depend on how quickly and how much the voltage drops when a load is applied.

At this point there hasn’t been much info provided about the exact symptoms or anything else and I have no idea if any battery readings are being taken on a rundown battery or what those readings are if the vehicle even starts and runs.

Another Elly zinger,Elly do baterys last longer if they are providing power for a V-6 or V-8? maybe it depends on how the engines are balanced or the spacing in the power impulses.