How to Find a Drain on a Car Battery

Written by jason alexander
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How to Find a Drain on a Car Battery
Use a test light to quickly troubleshoot the cause of your car battery drain. (dead battery image by Katrina Miller from

If your car battery continues to die after jumping it and the battery itself is not in need of replacement, there's a good chance it may be experiencing what is known as a parasitic electrical drain. Parasitic drains can be the result of a stuck relay, module or shorted diode in your car's alternator. Before troubleshooting the cause of the battery drain, it's important to make sure your battery is functioning and you just didn't leave on the lights or another electrical device overnight.

Skill level:

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Things you need

  • Voltmeter
  • Test light
  • Owners manual

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  1. 1

    Check your car's battery using a voltmeter. Choose the 12 or 20 V scale and attach the red (+) lead to the positive (+) battery terminal. Attach the black (-) lead to the negative (-) battery terminal. If the charge is less than 12.4 V, it should be recharged.

  2. 2

    Turn off everything in your car and remove the keys from the ignition. Remove the black (-) battery cable and connect a test light to the (-) terminal. Connect the opposite end of the test light to the black (-) battery cable you just removed.

  3. 3

    Observe the glow of the test light. A bright glow indicates a significant parasitic battery drain.

  4. 4

    Remove each one of the fuses from your fuse panel. Refer to the owner's manual to find the vehicle's fuses and diagrams. Leave the test light on while removing the fuses. Remove and replace each fuse one at a time.

  5. 5

    Find the circuit responsible for the battery drain by paying attention to see if the test light goes out after you have removed a specific fuse.

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