When should you add water to a car's battery?

You should check the battery water every few months.

Take off the battery caps -- usually there are two of these caps with three cells below each. (You may need to slip a screwdriver blade under the edge to break them loose and get the removal started). Look into the holes. You will see some narrowing shelves about an inch or so down in the holes. Water should be level with the holes in these shelves. If not, add distilled water to bring the levels up to the shelf holes.

Wash your hands after removing and replacing the caps, because they may have battery acid on them.