Battery: Parasitic Drain
Excessive parasitic drain eventually results in a no start condition. It’s only a matter of time. This is because excessive power is being drained from the battery during a key off situation. Most vehicle manufacturers specify a parasitic drain no greater than 30-50 milliamps. If a customer’s complaint involves a battery that dies overnight, look for something like a small light or device that has been accidentally left on. This is the most common reason for excessive battery drain. Find out if there has been any work done to the vehicle recently, like the installation of a new stereo/amplifier, alarm system, or such.
Interior lighting is often on a delay timer that when faulty leaves different interior lights on. Faulty alternator diodes can fail, draining the battery anytime it sits for a long time, like overnight. Anything that can be left on or may drain more current than it’s supposed to in a key off situation will result in excessive parasitic drain and a dead battery.
It may be necessary to perform a battery drain test. This test is necessary any time there is 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. Older vehicles can be tested with a test light between the negative battery post and the negative battery terminal. Check with manufacturer's recommended procedures before testing.
Testing: Check for current only after the vehicle has been at rest for a recommended period of time. This is because some computers must enter sleep mode and certain components are supposed to shut down only after a preset period of time. Most manufacturers recommend using a special switch that’s connected in series between the negative post and the negative battery terminal. An ammeter/multimeter is connected in parallel with the switch and registers the parasitic drain when the switch is opened. The fuse box is located and fuses are removed and replaced to find the problem circuit. Use a diagram to find the component causing the drain. Use a multimeter, ammeter, or a test light with the proper impedance and look for a drop in current as each fuse is removed.