Waytoslow is right with what he said. Being an ex-batterery tech, we only ever used tap water on fork truck batteries. That's the 3,000 pound jobs. The tops were scrubbed with soap and water, then hosed down. The plates absorb the acid during discharge (that leaves water behind, allowing a discharged battery to freeze in the winter) which drops the level. Charging drives the acid back out, raising the level to overflow point if filled when discharged. All batteries (good or poor) give of gas bubbles when at 80% charged (leave caps on) and should not stink. At 80% a good charger will cut back on the current rate till 100% is reached. They also should not boil or get real hot....that's a sign of a sick battery.