As a matter of periodic planned
maintenance, I’m not usually very concerned with the bare minimum, that I can (or might) “get away with”.
Given the ready availability & low cost of Distilled/Demineralized
water;
I see no routine reason to plan on using anything else in flooded batteries.
MidLandOne is correct, that a
water analysis (or TDS reading), along with Table 3, can be used to determine the acceptability of alternate sources,
when unusual conditions require.
BTW: I’ve owned several TDS meters, ranging in cost from $14 to $125; and never noted a significant difference in their readings. I recommend that every cruiser (using a
watermaker, or taking water from unknown shore supplies), obtain & use one.
I’m certain that Alan (redcobra) did far less harm (if any) to his
batteries, in replenishing them with bottled water; than would have occurred with exposed plates had he not replenished at all. I'm just as certain that he would have been wiser to obtain distilled water, if possible.