AUTOMOTIVE BATTERIES

10. WHAT CAUSES MY BATTERY TO DRAIN OVERNIGHT?

Parasitic (or key off) drain is the cumulative load produced by electrical devices, for example, clocks, computers, alarms, etc., that operate after the engine is stopped and the ignition key has been switched off. Parasitic loads typically run 20 to 120 milliamps. Glove box, trunk, and under hood lights that do not automatically turn off when the door is closed and shorted diodes in alternators are the most common offenders. Cooling fans, power seat belt retractors, radios and dome lights left on, alarm systems, and electric car antennas have also caused batteries to drain. Leaving your headlights on will generally discharge a fully charged battery, with 90 minutes of Reserve Capacity [36 ampere hours], within a couple of hours.

There are two methods that are commonly used to test the parasitic load without the engine running, underhood light disconnected and the car doors closed:

10.1. Connect a 12-volt bulb in series between the negative battery cable terminal clamp and the negative battery terminal. If the bulb glows brightly, then start removing fuses one-at-a-time until the offending electrical component is identified.

10.2 . A better approach is to use a DC amp meter inserted in series with the negative battery terminal. Starting with the highest scale, determine the current load. If the load is above 120 milliamps, then start removing fuses one-at-a-time until the offending electrical component is identified.

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