Originally Posted by BimmerFan52

+1

A low battery can do incredibly strange things to an E60. Its whole existence is built around a steady voltage and plenty of current.

Usually the easiest place to start is by measuring the battery before you try to start it (voltage is measured under the hood at the battery jump terminal and ground terminal near the drivers side strut tower. A healthy battery should be 12.6V fully charged. Once it is below 12.1V starting is very iffy. Jump only under the hood.

Once started, give it about 15 seconds for the alternator to stabilize and then measure the system voltage. If it is not 14.1V - 14.6V then your alternator may be dying.

If the alternator is good then as pcy says, drive it for 1/2 hour to fully charge the battery. Then turn the car off and let it sit a couple of hours for the surface current to leave the battery. Then measure it again. If it is below 12.5V then chances are you need a new battery.

If the battery measures 12.5V to 12.6V after a few hours then measure it again the next day. If it has run down overnight then you may have something staying awake in the car which is running down the battery. For early model E60s if the IBS is damaged by moisture it is famous for partying all night and keeping the ECM awake which runs down the battery fast.

Originally Posted by FaintReality

When you say early models, what years are we referring to? I have an 07 with issues... How do I diagnose if it's the IBS or not? Just re-charge, unplug and check voltage next day?

Originally Posted by BimmerFan52

Not sure exactly when the change was made. Improved encapsulation for better water proofing was implemented in the 2007-2009 range, but I'm not sure exactly when, and may even vary from model to model. Realoem lists a series of obsolete part numbers for the IBS for the 2004 545i, suggesting the part went through a number of iterations.

Yes, if you start with a known fully-charged battery, disconnect the BSD line (blue plastic connectors) and wait a day or two you will know.

Originally Posted by FaintReality

I have an 07 but will give it a try tomorrow. Seems every time I wash car I have issues with battery after, yet I can't find any water in boot, wheel well, etc. Now I am thinking maybe the moisture is causing the IBS to act up.

If so, I should just leave unplugged and be good to go, right?