AC, Car Battery and More - Some batteries need water
If your battery has vent caps, remove them to check the level of the electrolyte. It should rise 1/2 inch (13 mm) above the battery's top plates. If it doesn't, use distilled water to raise the level to 1/4 or 3/8 inch (6 or 10 mm) below the bottom of the vent cap. Don't use tap water, as it may contain minerals that can damage your battery. Mechanics should check your battery as a part of your regularly scheduled maintenance, but they often skip the procedure. Be sure to ask to have it done.