Battery drain troubleshoot

[Charles Morgan]

My BMW M535i has a brand new battery but is going flat about a week after charging (2v last time left). Checking all the fuses I found a 0.3v drain on the cabin fuses which doesn't seem enough to kill the battery so quickly.

To establish whether it's a battery fault or a real drain (or both) I need to test the draw - how can I do this with a standard multimeter?

 

[Charles Morgan]

Anyone?

 

[Stratman]

Disconnect the battery positive connector.

Set your multimeter to its maximum DC amps range, typically 10 or 20 Amps

Connect the negative meter lead to the battery positive terminal (the one you've just disconnected) and the positive meter lead to the positive battery lead

Read the current. It will be displayed in amps. If it's lower than the next lowest range on your meter (typically 200 milliamps or 0.2 amps) you can select it for a more accurate reading.

Remember, ammeters go in series with the circuit voltmeters go across it. What you must never do is put an ammeter across a circuit as they're almost a short circuit. The results may be spectacular but short lived.

 

[dozypillock]

Disconnect the battery positive connector.

Set your multimeter to its maximum DC amps range, typically 10 or 20 Amps

Connect the negative meter lead to the battery positive terminal (the one you've just disconnected) and the positive meter lead to the positive battery lead

Read the current. It will be displayed in amps. If it's lower than the next lowest range on your meter (typically 200 milliamps or 0.2 amps) you can select it for a more accurate reading.

Remember, ammeters go in series with the circuit voltmeters go across it. What you must never do is put an ammeter across a circuit as they're almost a short circuit. The results may be spectacular but short lived.

As above, but I would suggest disconnect the Negative connector. Far less chance of doing any damage if anything accidentally touches anything metal.

Neil

 

[markjay]

Does this help?:

http://www.mbclub.co.uk/forums/electronics/197164-battery-draining-2.html

 

[Charles Morgan]

Using the methods above the drain is .5 A. That's rather a lot! Now to find the actual source.

 

[grober]

See if you can isolate the drain by taking single fuses out of line one at a time.
The classic is the bootlight switch faulty meaning the bootlight is on all the time but invisible of course. Other common areas are where the wiring enters the doors at the door hinges [ lots of flexing and presence of moisture]

 

[grober]

Outside possibility is a faulty diode array in the alternator- this of course cannot be tested except by physically disconnection of the main charging lead- which has to be done with care as its attached directly to the main battery terminal! Not sure how old your car is Charles but be aware modern cars go through a programmed shut down of their control units meaning you need to wait a certain time before measuring the quiescent current drain

 

[Stratman]

If you have a wiring diagram you can isolate/eliminate individual circuits by removing a fuse and putting the ammeter in its place. Effectively the ammeter replaces the fuse.