Written By Michael Howard

Since most vehicles on the road today have built in computer programming as well as any number of electronic devices that suck power from your battery, parasitic drain can be a real problem for vehicle owners.  Parasitic drain occurs when there is a load placed on your battery even when the vehicle and its built in devices are turned off.  Unfortunately, although turned off, the devices may still be pulling power from the battery for such things as the memory on your clock or radio.  If this drain is excessive, it can easily lead to a dead battery that can seem quite a mystery.

If you wake up in the morning and your battery is suddenly dead, and there is no evidence of having left the lights on or leaving a door partially open with the interior light on, then it is very likely that parasitic drain is the culprit.  It is possible to test for parasitic drain, and to manually disconnect the devices that are primarily contributing to the problem.  If you are not familiar with working with your vehicle’s electrical wiring system, you might wish to leave this job to your mechanic.

To test for parasitic drain, you’ll need an ammeter, which measures the amperage running on the battery.  Disconnect the negative battery cable from the terminal and attach the ammeter.  You’ll want to make sure that your vehicle is turned off with the key out of the ignition when you perform this test.  Remember, you are looking for something that is drawing electrical power while the car is turned off.  Make sure you have all doors closed, as well as the trunk, otherwise the lights that come on will create a false reading.  Once the car has been off for a few minutes, check the ammeter.  You want to find a reading that is between 25-50 milliamps.  If it is higher, then you definitely have a parasitic drain somewhere.

To find the drain you’ll need to start pulling fuses, starting with the lowest amp fuses first.  As you pull each one, watch the ammeter.  When it drops down to an acceptable level, you’ve found the drain, and you’ll want to check each electric device on that fuse to see what you’ll need to disconnect.  This should ultimately take care of the problem.  As always, take all safety precautions when dealing with the battery and electrical currents flowing through your vehicle.

Parasitic drain can sometimes be caused by the battery itself.  If there is water or corrosion between the battery terminals, it could cause a current to drain between them.  Battery terminals can, and should, be cleaned regularly with a solution of baking soda and water.

RVs also often have difficulties fighting parasitic drain.  Because they offer a home-like environment, it can be easy to forget that much of the power for the electric devices comes from the batteries in your rig.  To prevent unnecessary battery failure, you might want to disconnect such things as the microwave and TV when they are not being used, to prevent them from drawing unneeded power from your batteries.

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