How to Diagnose Car Battery Drain
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.
Step 1: Recharge Battery And Check Voltage
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.
Step 2: Measure Current Drain
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.
Step 3: Find the Cause
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.