The battery in my Pontiac was way low on water so I filled it up with tap water. Now I find out I was supposed to use distilled water. Is tap water going to hurt it?
It all depends on what was in the tap water. If it was full of lime, minerals, and treatment chemicals, notably chlorine compounds, then yes, these comtaminants will react with the acid in the battery and produce precipitants that are not good for the battery plus using up some of the acid. In other cases, tap water can be relatively free of other material and won't do much harm. It all depends.
If your battery is still working ok, consider yourself lucky and don't do it again. One dose of tap water may not kill the battery immediately, but continue the practice and it may. The contaminants build up with additional refills. If you find yourself away from home and your neutral water supply, drop in at a grocery store and buy water for steam irons.
Some folks collect the condensate from their cental air conditioner to use for neutral water, but this is not the best. This water will contain bacteria, mold spores, and common house dust like the stuff that makes dust bunnies under the bed.
Batteries usually go 3-5 years before normal cycling kills them no matter what you do. Tap water might shorten it's life. If it does, I'd bet not by much if any. Being way low on water can be an indication that it's on it's way out anyway. It can also indicate the battery is being over charged so watch out for that. Even if your battery quits tomorrow, I wouldn't conclude it was because you used tap water today.
I'd go with Gary on all of that - been using tapwater since 1953. My current battery is 9 years old.
My wife's steam iron uses tapwater. Instructions say, "Neveh use 100% distilled watah." Hmmm. :-)
I can't remember the last time I saw a battery that had the facility, even the requirement for topping up with water. I had forgotten that there was ever this need. How old is the battery concerned? Must be a miracle it is still working.
It's actually possible to add water to almost any battery even still today. (The AGM Absorbant Glass Mat, or GEL batteries are a significant exception to that) Maintanence Free, does not mean never needs service, or cannot possibly benefit from service. The battery caps come right off with minimal effort. Use ONLY distilled water if adding.