Why does my car battery drain overnight even though everything is off?

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Short Answer: You have a parasitic draw — something is staying on when it shouldn't.

Detailed: Modern cars have many modules (ECU, alarm, radio memory, clock) that draw 20–50 mA when "off." A parasitic draw above 100 mA will kill a battery in a few days. Common culprits:

  • Glove box or trunk light stuck on

  • Aftermarket stereo or alarm wired incorrectly

  • Faulty alternator diode (leaks current backward)

  • Stuck relay (e.g., fuel pump, cooling fan)

Test: With the car off, disconnect the negative battery terminal and connect a multimeter in DC amps mode between the terminal and the cable. Wait 20 minutes for modules to sleep. Normal = 20–50 mA. Above 100 mA = problem.

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