Check the battery fluid level (except in maintenance free batteries) in each cell at least once a month, more often during extreme weather (mid-summer and mid-winter) and extended periods of travel. The electrolyte (water) level should be about 3 / 8 in. (9.5mm) above the plates as you look down into each cell. Filling each cell to the bottom of the cell ring is satisfactory.
Some battery makes are equipped with an eye in the cap of one cell. If this eye glows or has an amber color, the level is low and only distilled water should be added. If the eye has a dark appearance, the battery electrolyte level is high enough. It is also wise to check each cell individually on these eye type batteries.
Distilled water is the only fluid you should add to your vehicle's battery. It is widely available in supermarkets and auto stores. Tap water in most areas of the U.S. contains chemicals and minerals that are harmful in the long run to battery plates.
When adding water to a battery in freezing weather, the vehicle should be driven immediately for a few miles so that the water and electrolyte mix. Otherwise, the battery may freeze.