E36, 325i, 1993.
Like many out there, I have had a problem with my E36's battery, it didn't charge, and every so often I had to recharge it.
Alternator was fine, starter was working too. checked lights ,looms.
Finally, after having spent a lot of money in various garages and the fault still not being diagnosed, I started looking at it myself. Not that I'm electrician or mechanic for that matter.
At the battery I was getting 12.5v when the car was running, but at the jumper point I got from 13.5 to 15.75v. (oscillated up and down)
So I tracked the cable from the battery to the engine and I found it under the jumper point on the left side of the engine by the concentrated washer fluid tank. (See picture 1)
To do this DISCONNECT the battery first.
The access to this part is not easy but you can take the unit off the car.
The cable going to the battery showed marks of shorting all around the bolt connecting it to the jumper point bracket, were it had corroded and made bad contact. (See picture 2+4)
I took the bracket out of the unit and cleaned it with metal brush and greased all nuts&bolts and the bracket. (See picture 3)
Put the bracket back in the unit and repositioned it in it's housing. Reconnected all the cables and made sure all bolts were done up.
Started the car and checked the voltage at the battery, and got a 14.4v when running idle and 12.93v key off, constant, no oscillation or very little.
Hope this helps someone. I know the battery not charging is a common problem, and the solution is not the same for everyone.
Get the voltmeter out and look at voltage to the battery and voltage to the jumper point, if it's not the same then you most likely have this issue also. Check the cable going from the bottom of the jumper point through the firewall all the way to the battery. (On E36)