One test is not enough if you are diagnosing a charging system problem. All too often, if only battery voltage is present with the engine running the alternator is condemned as the source of the problem. If this is the case, it means that the alternator is not charging, but does not reveal why. For some alternators, it’s normal to see as low as 13 volts, while a constant 13.6 volts can be an indication of a problem. For others, it’s normal to see as high as 16 volts for long stretches.