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Battery check & alternator frying

to see if my alternator is charging, i to see if alternator is putting out, take a cable off a battery post while engine is running and watch lights to see if they dim or brighten as engine revs.



question - does this do bad type things to my alternator? this used to work with older cars, does it still have a place in the mechanic’s tool head?

You should NEVER disconnect the battery while the engine is running. Older cars didn’t have computers. You can make a mess of things doing this to a modern car.

If you want to check the alternator take the car, or the alternator, to a mechanic or a parts store and have them tested. Most parts places will check the alternator and the battery free.

Just in case you are not familiar with McParadise’s high competence, and if wonder if you should believe him, I’ll second his advice. Do NOT try to run the car on the alternator with the battery disconnected. Doing that might zap the computer(s), and that would make you wish you had only fried the alternator.

You can see if the alternator is working by checking the voltage across the battery while the engine is running around 2000 RPM. You should see between 13.3 and 14.8 volts DC there. For a total check have a load test done on the charging system. That will check your battery and alternator.

Removing a battery cable was a safe thing to do prior to the 1962 timeframe, when cars had generators.

With alternators, the battery acts as a big capacitor, which is needed to absorb the normal voltage spikes that an alternator produces. When you disconnect the battery cable, those voltage spikes can do lots of harm.

If you disconnect your battery cable and you fry your computer and alternator, then you will know your alternator WAS good before you disconnected the cable. But as others have noted, there are less painful ways to determine this.