can a faulty alternator draw voltage from a battery
But a battery power drain can also be the work of a bad alternator diode or even a faulty battery. Depending on the amount of current being drained, a parasitic draw will suck all the juice from your battery in a few hours or minutes. Here, you have four tests to help you find the culprit: The amperage battery drain test. The voltage drop battery drain test. The bad alternator diode test.