'04 545i battery drain issue
 

Hey guys! New to the forum, brought here by a troublesome 545i I purchased not too long ago.

Anyways, when I purchased the vehicle, if not driven every day it would be dead. Silly me thinking it would just be a battery replaced the old battery and registered the new one.

Helped the issue for a bit but over time it's gotten worse. I even bought a trickle charger to try and put a band aid over the whole issue and that did no help whatsoever.

I hear this is a common issue? Wondering what a common fix is for this of there is one. At this point if the car sits more than a few hours after a drive it won't turn on.

I would check the following:

1. A new battery may have in fact been on the shelf for a long time. This was the case when I bought a new one. I had to have a charger on it for many hours, then drive it to get an error message to turn off. Also, although unlikely, new does not mean that it is good.

2. Perhaps your alternator is not charging the battery when running. Check the voltage of the battery when running. I believe that the correct voltage is 14.5 V, give or take.

3. If the above check out, I would look at the parasitic drain of the battery after the car has been shut off. Over time, modules will go to sleep. I hesitate to give you an exact number, but after a period of time (certainly within an hour), the current draw should be minimal. May have to check around for the accepted value. My guess is in the low milliamp range. A google search should give you an exact number.

Yep, I would be measuring the current draw on the battery and if it's above a few hundred milliamps I would start pulling fuses until the current drops. The fuse that does it should be the culprit. I'll admit, this is much easier done with a helper pulling the fuses or holding the ammeter on the battery for you. I would also leave the trunk open until the interior lights and everything go off so that the fuel pump doesn't try to run or anything, and so that all the lights which draw current will be off.

I had this happen recently. I ignored it, then a few weeks later my trunk stopped working. Found a bunch of broken wires inside the fat trunk hinge where the harness runs and that fixed both problems.