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Roger, BMW Master Tech
Category: BMW
Satisfied Customers: 5882
Experience:  10 yrs. experience as BMW Master Technician, shop foreman and dealer.
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745li: My battery keeps going..battery went dead..battery replaced

Resolved Question:

My battery keeps going dead on my 745li. In Aug. 2010 I was on vacation my battery went dead. I had the battery replaced. I put the car in storage for the winter. I put a charger on in March. I took the car out in April 2011 drove to work and put 50 or so miles on the car came home parked the car and the flashers came on battery to low to start. Jump started the car took it to the dealer. The dealer charged it and kept it for two days said it was fine. Picked up the car drove it home parked it in the driveway about 3AM the next moring the flashers went off and the car would not start. Charged the battery again and the car was fine for awhile. Flashers came on again battery dead. Know it won't hold a charge overnight and the flashers keep coming on even while it is charging. HELP
Submitted: 5 years ago.
Category: BMW
Expert:  Roger replied 5 years ago.



I have dealt with many power issues like this one. At times the draw can be sporadic or might not show on a conventional draw test. The test that works best is to have the dealer hook up the oscilloscope draw test. This writes a graph of the vehicle entering sleep mode and shows all wake ups and draw states over time. The dealer should drive the vehicle first, get it hot, use all of the electronics and then hook up the test. This will show if modules are not shutting down or going to sleep correctly.


The most common failure causing a wake up and draining the battery is the TCU or telephone control unit. This often will keep the rest of the vehicle awake. The other two items that are common are the power module and electrical fan for the engine causing high drains and bringing the battery down quickly. While it could be other components these should be unplugged first to try to eliminate the draw or wake up problem.


I hope this information is helpful. Please press the accept answer tab and good luck with your repair. Thank you.


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