The electrolyte level should be 5–10 mm above the upper edge of the separators and not rise above the lower edge of the filler neck.
During operation of the battery, the electrolyte level gradually decreases, as the water that is part of it evaporates. To restore the electrolyte level, add only distilled water.
If it is determined that sloshing is the cause of the low level, then add electrolyte of the same density as that left in the battery cell.
If the level is above normal, then suck out the electrolyte with a rubber bulb with an ebonite tip.