NOTE: Before topping up battery electrolyte the correct PPE should be worn.
Check the level of the electrolyte in the cells. It should just cover the vertical lead plates. Top it up ONLY with either distilled or demineralised water using a plastic funnel or similar. Do not overfill and do not use metal implements to aid the topping up. Each cell should be checked.
As a point of interest each cell in a lead/acid battery generates 2.2 volts regardless of it's size. You can ascertain the battery voltage by counting the cells and multiplying by two.