How do you test if an alternator is charging the battery?

Simple just disconnect the negative wire while your car is runing if car dies then alternator is bad and not doing any good to battery so alternator needs to be replaced.

Answer 1

If you have a major chain auto parts store nearby, it would be easier to go there as most of them have the proper test equipment and will simultaneously test the alternator, battery and related circuits for free.

If you want to test it yourself, you will need a volt/ohmmeter [also called a VOM].

With the VOM selector switch set to the 50 volt DC Scale, and the engine running, touch the negative [black] probe of the meter to the negative post of the battery [black cable attached], and the postitive probe [red] of the meter to the positive terminal [red cable attached] of the battery.

Depending on the type of vehicle, charging system, and some other factors, the meter voltage reading will vary. However, the meter should read at least 13.5 DC volts minimum to about 16 volts maximum. If you have set the meter selector and connected the probes properly, and the indicated voltage is within this range, then the alternator is working properly.