How To Check For Bad Alternator By Disconnecting The Battery. To test whether an alternator is draining the battery with the cars engine switched off disconnect the cars electrical system leaving the battery only. Disconnect the positive terminal of the car battery while it is still running.
Disconnect the battery while the car is running. Finally you only need to re-install everything in reverse. Also you can avoid the most common mistake made when testing an alternator of disconnecting the negative terminal of the battery.
DaveJM on October 23 2021.
If your car keeps running upon unplugging the battery it means the alternator is in good condition. So in the old days your test WOULD work. The instant you disconnect the battery cables to test the alternator the alternator goes into full field mode where it can generate a voltage spike as high as 150 volts. Disconnect the negative battery cable before testing an alternator.
