Dear Car Talk:

Is it really necessary to use distilled water in batteries? I run a store in a campground, and people are always asking for distilled water to fill their batteries. I tell them that I heard that it makes very little difference whether they use distilled or tap water, but nobody believes me. Can you answer this in your column, so I can hang it up and show them when they ask? I could just sell them distilled water, but not in good conscience.

-- Joe

Most batteries you see under the hoods of cars don't need water, Joe. Every battery we sell these days is factory-sealed, so you never have to add anything to it. The only thing you ever have to add is money: When it dies, you buy a new one.

Now, it may be that old-style batteries are more common in the RVs you see. And distilled water will never hurt, but even older batteries will do fine with tap water.

I suppose if you live in a part of the country that has excessively hard water -- so hard that it chips a tooth when you rinse your mouth -- you could consider using distilled water in the battery. Hard water contains lots of dissolved minerals, which I suppose could shorten a battery's life. But I'm not even sure that's true.

But I think you're missing the bigger picture here, Joe. There's a big business opportunity here -- but it's not in distilled water for batteries. The big money is in selling these folks distilled water with which to wash their RVs! I'll go in with you on that one, Joe.