I can't imagine anywhere in the US that the tap water worth drinking has enough minerals to cause a problem - I'm serious - if your water has over 500 ppm of disolved goodies, well it probably doesn't taste like water...so yeah use DI, distilled etc...your idea may be fine...

I remember there used to be an "old mechanics/wifes tale - urban rumor" about never using DI H2O for the 50/50 coolant blend because the "minerals were too low", it was "too low pH" ...utter poppycock.

I fortunately live in an area that has relative unmineralled water and have used tap water for some changes and DI other changes when I remember to buy it - and have never had an issue in any of my cars in 29 years of driving.

Now for a battery I would never use tap water - only Evian (just kidding - DI)
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