Solution for water in battery compartment
Im not sure if anyone has found a good solution but I was experiencing the same water in the battery compartment in my 2007 5 series.. what I found is: open the truck of your vehicle and pour water down the back windshield on the right side nearest the battery. The water drains down the window then it follows the track around the trunk, down between an opening behind the taillight and out of opening in the bottom of the bumper.. The problem is as it drains through the opening near the taillight is splashes and when that happens the droplets find there way to the vent that located on the inner wall where the battery is located and it leaks through the open spaces around the vent.
1. REMOVE THE VENT
2. USE A SOME TYPE OF SILICONE AND RUN A BEAD OF SILICONE AROUND THE TRIM OF THE VENT .
3. REINSTALL THE VENT
IT WORKED FOR ME!!!!