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BMW – Dead Battery – Errant Electrical Current Draw Diagnosis Testing – DIY How To

June 2, 2010

I recently replaced my alternator and battery on my 540i, because my battery was constantly dying, however, the battery keeps on dying within 2 to 3 days, even when we drive the car on a daily basis.The shop that was fixing my car swear that the problem was a faulty heater control, which we replaced, but the problem continued, so, I got my money back and decided to tackle this problem and consult to a real expert.

It sounds like you have an errant current draw, that should not be happening when the ignition switch and normal systems are fully turned off. Following, is a general procedure for testing for the source of the current draw. Before performing the test, disconnect the battery (remove the negative battery cable) and see if the battery holds its charge for the typical time period that causes it to go dead, when connected. If the battery stays charged, continue with the current draw testing. If the battery still goes dead, the battery is faulty (perhaps due to the current draw problem, in the first place).

Here is a write-up on how to isolate what circuit is causing the draw. Once you know what circuit is at fault, you will then have to trace the wiring and items on that circuit.  This applies to most BMW models up through the mid to late 2000s.

Prepare to remove the positive or negative cable from the battery and connect the probes from a multi-meter to the battery terminal and the end of the cable. Remove the cable, with the probes connected to the battery terminal and the cable (you must have a meter capable of reading 10 amps DC). With the meter in place, check what the current draw is with everything turned off in the car (remove the trunk light bulb, under-hood bulb or interior light bulbs, if the trunk lid, hood or interior door must be opened to access the battery).  Now, remove fuses from the fuse box – one by one (with the meter hooked up), until you notice that the current draw has stopped when one of the fuses is pulled. This will tell you what circuits should be diagnosed next (what ever is controlled by that fuse). There will always be a very small current draw due to the items in the car that require power for memories and static running.

NOTE: Some models have more than one fuse box in the vehicle. Consult the applicable Bentley repair manual for details.

You may end up needing a wiring diagram for the car as well as charts for locations of the various fuse boxes. The Bentley repair manual for your chassis will have most of the wiring diagrams for the vehicle. We keep the Bentley manuals in stock. We can also order the BMW Electrical Troubleshooting Manual (ETM), which is all of the vehicle’s wiring.

Click below for Bentley repair manuals:

  1. Joseph castor permalink

    Had the same problem with my 85 635csi. It was the high beam switch on the steering column. When left in high beam position it would draw and leave me with a dead battery in two days. Just making sure it was in the low beam position overnight or whenever I wasn’t driving it fixed my situation.

    • In the case of the switch on the 635csi, this is obviously a faulty switch, but thank you for sharing this experience as it may help another BMW owner who has an older chassis, from the ’80s and early ’90s.

  2. Tony permalink

    I have a 2001 540i and am having the same problem. Dead battery in 2 to 3 days. Were you able to isolate your problem? I have a constant current draw of .1 amps (100 milli amps). Do you know if that’s normal? I just had the battery checked and its good (its 9 months old). Any help would be greatly appreciated.

    • 100 milliamps is really not all that much. Over 72-hrs, that would only be 7.2 amp-hours. Your battery should be able to handle this easily. Have you tried letting it sit for this same period with the battery disconnected to see if the battery holds a charge?

  3. John M permalink

    After numerous jump starts and worrying ‘ What could it be?’ my problem on my 2002 540i was also the high beam switch? Thanks for this!

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