Parasitic Draw Test

A slow battery drain when everything is off is called a parasitic power loss. This happens when something – a glovebox light, an amplifier, any electrical component – is pulling power from the battery when it shouldn't. Since the alternator isn't generating vehicle electricity, the problem component pulls from the battery, slowly draining it overnight until your vehicle won't start.

This probably sounds familiar for some 2004–2008 Ford F-150 owners, as that generation had parasitic battery drain issues caused by a failed park sensor not allowing the body control module to turn off interior components.

Slow power drains are inconvenient and annoying, and result in a shorter battery life.

Step one, after getting a jump or charging your battery, is to test your battery or visit your favorite auto parts store for a free battery test. Batteries need to be replaced every few years as part of regular maintenance. Your battery could be near the end of its service life and it should be recycled and replaced with a new one. Fortunately, shopping for a car battery isn't a hassle with expert advice from Advance Auto Parts Team Members. We're always happy to point customers in the right direction when it comes to choosing the right type of battery as well as choosing battery accessories such as chargers that will keep your vehicle charged and ready to go at all times. 

If the battery test shows that it is still good, then it's time to start looking at a parasitic power draw. While you're under the hood (or at Advance Auto Parts), test the alternator. A failed diode in the alternator creates a closed circuit that drains your battery even when the engine is off. If you're too busy during the workweek and can't get to it until you're off, some short term fixes are to disconnect the battery at night, and carry a portable jump starter in your vehicle.

Keep in mind that you'll lose the radio, power seat, and other presets stored in memory, and the engine could be difficult to start due to a lack of recent fuel trims.

Any electronic device with memory settings, like a clock and radio station presets, draws power from the battery while the vehicle ignition is off. This drain is so small, you can leave a modern truck sitting for a week or more and it will still start up and run fine.

For some perspective, 0.05 amps should be the usual power draw when a vehicle is off. A clock in the radio draws as little as 0.01 amps, whereas the combined interior lights can exceed 1 amp. That seems like a tiny amount, but a single incandescent dome light pulls enough power to drain a battery in one day. Remember that the drain doesn't have to take your battery to zero overnight, just low enough for it to not start. If the battery drains when the vehicle sits for three or four days, that's a lower amp draw than one that drains the battery overnight.

Discovering the source of an electrical draw is a process of elimination. First, check the easy stuff. Make sure the dome light or other accessory lighting isn't staying on in a closed and locked vehicle, which would mean you just need to flip the switch to off. New aftermarket modifications or changed equipment may also be the culprit. If you have recently upgraded the stereo, installed a new alarm system or aftermarket fog lights, you should start with a hard look there.

Note that the multimeter listed below should be able to measure the same amperage of the fuses, up to 20 amps, and as low as the hundredths range. The repair manual is a must for this one, as it will list amps pulled by each component, how to test them, and the wiring diagrams showing everything on a fuse.