Many things in your car continue to work even after your vehicle is turned off – radio presets, security alarms, emissions computers and clocks are just a few. They continue to draw power from your car battery, and the cumulative load that these devices produce is called key-off car battery drain, or parasitic drain. Some drain is perfectly normal, but if the load goes above 150 milliamps, that’s about twice what it should be, and you could end up with a dead battery. Loads below 75 milliamps are normal.

What causes excessive parasitic drain?

If you find your battery dead in the morning, chances are it’s because something has been left on. Common offenders include under hood lights, glove compartment lights, or trunk lights that fail to turn off. Other problems, like shorted alternator diodes, can also cause excessive key off car battery drain. And of course if you forget to turn off your headlights, your battery can discharge in a matter of a few hours.

Whether the problem is key off drain, or a faulty battery, the last thing you want is to discover that your car won’t start, especially on a cold winter morning. If it does happen, though, our mobile mechanics can help. We come to you, so you don’t have to worry about having your vehicle towed. We can diagnose your car battery problem, and determine if the problem is key-off battery drain or something else within your vehicle’s charging system.

This article originally appeared on YourMechanic.com as What Is Key-Off Battery Drain? and was authored by Valerie Johnston.


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