While preparing our truck and camper for the season, I thought it timely to check the truck's "maintenance-free" starter battery water level since over three years and 50,000 miles had passed. The battery had two cells with water at the top of the plates and I added a total of 2 cups of distilled water to the battery. Afterwards, the truck starter had more power and spin. We travel a lot of highway miles so the steady charging or engine heat may have lowered the water level. Know your battery before attempting this. Use goggles when cleaning the battery top before opening to avoid getting acid in your eyes and a little lite air blow worked best for me. Do not overfill the battery. Pry gently on the battery covers to open if your battery is not sealed. Use only distilled or equivalent water. If you never have added water to a battery then have someone experienced help you the first time.