Here are some general guidelines below. For additional information please download and read the owners / installation manual for your specific battery:
At regular intervals this water loss must be replaced with distilled, deionized or approved water, so as to maintain the electrolyte level at the mid-point between the high and low level lines marked on the jar while on float.
Cells are equipped with flame arrestors with a filling funnel. Add water through the filling funnel by removing the dust cap, but without removing the flame arrestors from the cell covers.
The best time to add water to the stationary lead-acid battery is when the recharge or equalizing charge is about two-thirds completed. In this condition the electrolyte should be brought up to the high line. Water tends to float on top of the electrolyte for awhile, but the gassing action of the latter part of the charging period will mix the water into the electrolyte. If temperatures may possibly drop below freezing, water should be added at the start of the recharge or equalizing charge to ensure thorough mixing with the acid solution.
Take care to keep the solution level below the top mark of the cell jar’s solution level markings while on equalize. Overflow of solution can occur during gassing if too much water is added to the electrolyte.