How To Check If Battery Or Alternator Is Bad. Note the reading on the meter you should have at least 12 to 13 volts. With the engine off battery voltage should be between 125 and 128 volts.
That is the output of the alternator at the battery and that range is needed to keep the battery properly-maintained. If the battery is below 122 volts then you will need to charge the battery and re-test or try a different method. Alternatively you can test both the alternator and the battery using the TOPDON 12V load Car Battery and Alternator tester.
Place the black test lead on the negative battery post.
Then start the engine and check for increased voltage readings. Battery voltage measured at the terminals with the engine running should be about 137-147 volts. This easy-to-use tester can test the aging and resistance of your battery and help you figure out whether you have an alternator or a battery problem on your hands. However a very simple way to check if the alternator is working is to run the car and disconnect the positive terminal of the battery.
