How to Check for Battery Drain with a Multimeter?

Perform a Simple Battery Drain Test to Find a Current Leak in Your Car Using a Voltmeter

Although forgetting to turn off the headlights is one of the main causes of battery drain, other common but less obvious incidents not only make your car battery slowly die but also make them hard to find. A door left slightly open can fail to disengage a light switch and a short can keep electrical current flowing through a circuit for hours, leaving you with a vehicle unable to start.

When a visual inspection is of little use, you can use a simple digital voltmeter to help you find the culprit behind your battery drain in a matter of minutes, even if you do not know much about electrical circuits.

How to Use a Voltmeter to Find Your Battery Drain

1. Locate the fuse box in your vehicle–usually under the dashboard, on the driver’s side–and pull off the fuse for the dash clock or radio. Your car owner’s manual will help you locate this box and the clock fuse, if necessary.

2. Verify that your glove box door is properly closed. Then walk around your vehicle and make sure that every door, including the back door, is closed. If you find one of these doors slightly open, probably one of the courtesy lights stayed on all night. Still, go ahead with the following test to make sure you have solved the problem.

3. Open the hood of you car and unscrew the light bulb under the hood, using a clean rag, just enough to turn it off. This will minimize current draw on the battery as you troubleshoot your vehicle electrical circuits.

4. Now, disconnect the negative (black) battery cable from your battery using a wrench.

5. Connect your meter leads to your voltmeter. The black lead should connect to the common (COM) socket and the red lead to amps (A) on your meter.

Connecting Your Digital Voltmeter

6. When ready, connect your digital multimeter into the car battery circuit. Attach car battery the other end of your meter’s red lead to the battery terminal and the black lead to the negative battery post. You might want to use alligator clips on your meter leads for convenience.

7. Set your meter to direct current (DC) amps and turn it on. If the display on your meter reads 10 milliamps (mA) or less, most likely you are not dealing with a current leak but possibly a failed battery or a malfunctioning charging (alternator) system. If your meter reads over 10 mA, an electrical circuit is draining your battery charge and you need to find it.

8. Make sure all doors, including the glove box, are closed and that any other accessory remains off during this test.

9. Ask an assistant to pull one of the fuses from the fuse box using a fuse puller and watch the display on your meter. If the reading on your meter goes below 10 mA or to zero, you have found the problem. This means that the fuse you just pulled is protecting the circuit with the problem. For example, if this fuse protects the circuit for the tail lights, probably the switch for these lamps was left on or there is a short in this circuit. If there is no change in your meter reading, ask your assistant to reinstall the fuse.

10. Repeat the previous step, pulling one fuse at a time, until you find the electrical circuit causing the current to leak.

Once you found the culprit, use a battery charger to charge your battery or get some jumper cables and jump it!

In most cases, you will find out that forgetting to turn off the headlights or parking lights, or close a door properly are behind a battery drain. Perhaps a short in the electrical circuit to your ignition system is causing one or more devices to remain on. Whatever the cause, learning how to use a voltmeter as described in this simple troubleshooting strategy can help you solve your problem.