If you find that your car’s battery is in frequent need of recharging, you may need to check for a car battery drain, otherwise known as a parasitic battery drain. In most modern cars, there is some power still running through the vehicle’s electrical systems even after the car has been turned off, called key-off drain. This is to prevent the loss of information stored on the car’s on board computer. A parasitic battery drain occurs when there is a problem with the key-off drain, causing excessive current drain. The following set of step-by-step instructions will give you all the information you need to diagnose a car battery drain in your vehicle.
Use a car battery charger to recharge the battery to full voltage, then check it with a voltmeter. Connect the voltmeter to the battery while the car is idling. You should get a reading of about 14 volts. Next, turn the car off and use an electronic battery tester to see if the battery holds the charge.
If the aforementioned measurements are as they should be, the problem is indeed a current drain problem. Use a digital multimeter on the ammeter setting to find the current flow between a battery post and one of the cables one hour after turning the car off. A normal current drain should be less than 50 milliamps, and anything more than that is excessive.
To determine the cause of your parasitic current drain, you will have to inspect each fuse and relay in the car’s wiring one at a time. Use your vehicle owner’s manual to identify which fuse and relay is which.