Why you put distilled water in a battery?

Any foreign objects or minerals in tap water will produce an "Eddy Current" which lowers your batteries efficiency by lowering its current capabilities.

Without getting too technical (hole flow theory etc..) Lets say the current inside your battery goes from Pos. + to Neg. -

If one anode is pos. then it will see debris in the water as something less pos., or actually a neg. So it will send current to the debris instead of the neg. anode. This is called "Eddy Current".

Just like a stream with a flat bottom flows faster than one with a lot of rocks which produce eddyies.