Update on this. While the water did help the battery did die. One morning it would barley crank a half revolution. When I took it out, I saw the min/max lines. The water line, before I added, was well above the max. So adding more may not have helped much.

Either way for $84 I have a brand new battery with a 900 cold crank amp rating, twice as much as the Mercedes OEM battery I had. The car has NEVER cranked this well and after a week continues to work great.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car