by MtnDon » Thu Jan 02, 2020 9:20 pm
The bottom of that lip, at the point of the lower end of the slot, indicates the absolute maximum level of the electrolyte.
As long as the plates are covered by electrolyte, and not above that split, you are fine.
It depends on how obsessive-compulsive one is about being all-the-way-full, of fluid.
Note that as long as the plates have some fluid covering them, charge the battery fully before adding water. If you add water when the battery is only partially charged there is a danger of the cell(s) being overfull after charging. That is because the electrolyte expands when going from low charge to fully charged. ...so, be sure plates have some coverage; then charge fully, add water if necessary and give another charge to cause bubbles which will mix the fluid. If you just add water and do not recharge the fluid will remain stratified and in winter there is some danger of freezing in that upper area which will be mostly water.