Even maintenance free batteries need some attention every so often. Hydrometers measures the concentration of acid in the battery. This will tell you if the battery needs to be charged or replaced.
EZ Red's hydrometers, with temperature compensation, checks your battery electrolyte ensuring direct, accurate results. They are designed so anyone can see the state of your battery acid and is great for automotive, marine, and commercial batteries. They are unbreakable, easy to read, economical, and used by battery manufacturers worldwide.