Well, distilling water is probably the best way to purify it. Depending upon what your contaminants are, whether any of them form azeotropes with water, what materials your still is made out of, how many equivalent equilibrium stages, and of course what you are exposing your water to and storing it in will determine how pure you can get it. Back in the lab in my ChemE days we used glass beads in a couple glass columns for more relatively pure distilled water from a tap water source. The higher purity stuff was stored in Pyrex branded borosilicate laboratory glassware as well. Using double distillation you can remove both the less volatile and the more volatile contaminates to leave very pure water.
More to Woof's original question, in an old battery manual it said to wipe the top with a damp cloth, - use one you can toss though would be my advice. Sulfuric acid used to be called oil of vitriol. It leaves a greasy/glassy appearing coating on the surface that never seems to evaporate. A solution of battery acid will conduct, the film on the top of your batteries can contribute to the batteries self discharge and also corrosion of your terminals. Periodically wiping them off helps to minimize this.
