Parasitic Battery Drain Test
Excessive parasitic drain eventually results in a no-start condition. It's only a matter of time. Excessive power (greater than 30-50 milliamps) is draining from the battery during a key off situation.
Run a parasitic battery drain test when there's an unnecessary or excessive key-off drain. Many manufacturers recommend installing a test on/off switch in series between the negative post and the negative battery terminal. A test light can be used on older vehicles connected between the negative battery post and the negative battery terminal. Check with the manufacturer's recommended procedures before testing.
Testing for Battery Drain
Check for current only after the vehicle has been at rest for a recommended period. This wait is because some computers must enter sleep mode and individual components shut down only after a preset time.
An ammeter/multimeter is connected between the terminal and post and registers the parasitic drain. Fuses are removed and replaced to find the problem circuit. Use a diagram to find the component causing the drain.
The most common reason is a small light or device unintentionally left on overnight. Interior lighting can be on a delay timer that when faulty leaves different interior lights on. Faulty alternator diodes can fail, draining the battery while it sits.