As long we use "pure" water, it shall do the job in the battery or radiator.. distilled water is a pure 100% water, without any other content inside, esp minerals.. the other water we can use is RO (=reverse osmosis) which is also 100% pure water, only difference is the method they use to get the pure water, distilled is by boiling the water and collect the water vapours back by cooling, RO is by using membrane to filter out those minerals.. sort of.. :)
So u can actually go to supermarket and buy Distilled or RO water, just make sure it is 100% pure, no other minerals added inside from the ingredients stated on the label.. of cos, easiest to get Battery water fr petrol kiosk, which is consider cheap also la, usually $1/bottle.