I would never use tap water in a battery. I always have gotten at least 5 - 6 years plus out of my 8-D's on just about every boat that I have owned, which included 4 Hatteras'. Distilled water is so cheap at the supermarket that it's a no brainer, especially since they shouldn't use much to begin with. Now for mixing the 50-50 mix of "Power-Cool", Detroit anti freeze, I save the condensate water from a dehumidifier which should be free or close to free of minerals and costs nothing. I change my Power Cool every 4 or 5 years. The way I figure it, with all the horror stories about pin holes in the cylinder sleeves caused by bad coolant, I don't want to take any chances, especially since my engines are 92's which have wet sleeves. If anyone has any reason to believe that I should not use the condensate water, please respond. I only use it because I have a reputation as an extremely frugal boat owner and I need to protect it. lol Bobk.........
Walt