I have a '05 Mini S with a sunroof. I have 28,300 miles on the original battery.
When took it to the dealer to replace the original battery, a significant amount of water and corrosion was noted in the battery box located in the boot. I now have to have the BST cables replaced as well as the fuse box located in the battery box. Of course the rust must be neutralized. I hand wash my car only. No automatic car wash allowed.
It has been suggested from other post dated March '09 that there could be many different possible causes for the water accumulation in the box.
These range from damaged seals around the brake light assemblies, poor design of the hatch door, molding issues and plugged drain channels from the rear sunroof. My car is currently been at the dealer for 1 week. Does anyone who experienced this problem have any definitive cause for this problem? Please share.

 

1- boot handle gasket. Water enters the actual hatch via gasket. Replace gasket smeared with black silocone sealer, remount.

2- Rubber plugs in hatch. There are two long hatch gromets on either side of the inside bottom of the hatch. They are knurled so they sort of screw in. In other tests it was found that with age, they would also let water into the hatch area.

In both cases the water enters the actual hatch lid then finds it's way to the locking mechanism and then into the battery box.

You can perform your own tests with a friend. Remove plastic molded panels in the hatch area and hatch. While sitting in back of car have friend spray water around known areas prone to issues.

Hope this helps.

 

The other thing that can cause this is when your boot hatch drain gets plugged causing the water to run over and out the latch for the boot and then directly into the battery box. Take a piece of wire and with the hatch open look in the center bottom for the drain hole. insert the wire and make sure that it is able to move freely. Once this is done the water will not back up and flood the battery box.