See Figure 1
At every fuel stop the level of the battery electrolyte should be checked. The level should be maintained between the upper and lower levels marked on the battery case or the bottom of the vent well in each cell.
Except on maintenance free batteries, if the electrolyte level is low, distilled water should be added until the proper level is reached. Each cell is completely separate from the others, so each must be filled individually. It is a good idea to add the distilled water with a squeeze bulb to avoid having electrolyte splash out. If water is frequently needed, the most likely cause is overcharging, caused by a faulty voltage regulator. If any acid solution should escape, it can be neutralized with a baking soda and water solution, but don't let the stuff get in the battery. In winter, add water only before driving to prevent the battery from freezing and cracking. When replacing a battery, it is important that the replacement have an output rating equal to or greater than original equipment. See Engine & Engine Overhaul for details on battery replacement.