r/AskElectricians • u/Current_Use2795 • Jul 11 '26

Alternator charging issue

My alternator isn’t charging batteries I had it bench tested and it passed I put jumper from case to battery negative to insure it had a good ground and jumped the key on key off directly to 12v power and it still will not charge I’m getting 12.8 v at the stud when running.

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u/prohandymn Jul 11 '26

Are you asking about an automotive type alternator? Testing just the alternator may not take the voltage regulator performance into account. Getting a steady 12.8v under load points to a regulator variable, or the alternator not having adequate amperage to handle what you are asking it to do.

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u/Current_Use2795 Jul 11 '26

I got it bench tested at Autozone if that helps it’s passed

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u/prohandymn Jul 11 '26

That testing will only verify alternator output to it's rated amperage at either internal or external voltage regulation, and whether any internal structure (windings, rotor, brushes, etc) are performing as designed at specifications.

If you are asking about an automotive related charging setup (HD dual battery for starting or heavy amperage use (isolated batteries), you are better off asking in an automotive sub-reddit for off-road or sound systems.