Last Updated on March 12, 2003
Written and compiled By Bill Darden (freely reproducible public domain)
Re-produced here by www.optimabattery.co.uk for your information, and it will be updated regularly.
Parasitic (or key off) drain is the cumulative load produced by electrical devices, for example, emmissions computers, clocks, security alarms, etc., that operate continuously after the engine is stopped and the ignition key has been switched off. Parasitic loads typically are 20 to 120 milliamps. When the parasitic load is great than 120 milliamps, batteries will drain more quickly. Glove box, trunk, and under hood lights that do not automatically turn off when the door is closed or 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 overnight. 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.
It is highly recommended, especially if you are using a sealed, maintenance free battery, like a ACDelco for example, to fully recharge it, remove the surface charge, and load tested both the battery and the charging system for latent damage from the deep discharge(s).
Here are two methods that are commonly used to test the parasitic load without the engine running, under hood light disconnected and the vehicle 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 ammeter 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.