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I know there are several questions on here about finding the source of a battery drain (like Battery drains by itself when vehicle is parked (parasitic draw) ). However I don't think the answers address my problem.

Occasionally when I go to use my car in the morning, it will have a dead battery. Because it is an intermittent problem (on the order of once every couple of months), it's no surprise that mechanics find nothing when doing an overnight draw test. That is to say that the problem has never occurred when the car is in the shop.

FWIW, I have not left the lights on on the nights that the battery drains. I can't rule out any similarly stupid mistake, but at least its not the lights. Also, we tried the simple step of replacing the battery about 6 months ago. The load tests done by various mechanics never implicated the battery, but we did it anyway.

So my question is, what strategies should I use to track down an intermittent battery draining draw?

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  • Important missing data: how old is the battery? Also, what is the temperature like there?
    – Bob Cross ♦
    Aug 23, 2012 at 21:59
  • It's a new battery, I added this info to the question above. It's in New York state, and has been a relatively mild summer.
    – brooks94
    Aug 24, 2012 at 13:09

1 Answer 1

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You could set up a low voltage relay in-line on the battery ground cable (obviously only for when parked at night). Even a small current from an activated circuit (like the door light) should cause the relay to trip. Use the relay to turn on an alarm so you can go see what's causing the draining.

If you don't have a relay, a flashlight lightbulb wired in series with the ground cable will do the same thing, show when current is flowing. A multimeter is more sensitive, but you have to watch it carefully. It will always register a few milliamps to keep the radio stations set but if it goes up, you will know.

It's going to take some testing with a multimeter to tell how much voltage will develop during small current draws, but it's probably at least 10 watts to drain a healthy battery overnight during the summer. Use the dome light to calibrate.

I had to use this method last year and the culprit was the remote locking system picking up my wife's keys somehow and activating the lock solenoid all night long.

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  • 2
    So before I got a chance to try this out, I found a penny that someone--who shall remain nameless--wedged a penny into the cigarette lighter outlet. Removing that seems to have solved the problem.
    – brooks94
    Sep 5, 2012 at 12:41
  • Now that is truly evil!
    – Chris Chubb
    Sep 17, 2012 at 19:42
  • Ah, yes. The old penny in the cigarette lighter to drain the battery trick. It's so obvious!
    – ojait
    Sep 16, 2015 at 17:02
  • Mmm, something not ringing true as a penny there would blow the fuse not cause a drain.
    – user17447
    May 19, 2016 at 14:33
  • It depends. If there is enough corrosion to make the contact ohmic, then it might not draw enough to trip the fuse. That circuit is fused for something like 10 amps, which means at 12V you would only need 1.2 ohms of resistance. If the contact is intermittent, or corroded, that would be enough. It actually is kind of diabolical.
    – cdunn
    May 19, 2016 at 17:16

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