There are a few different types of batteries that are used with solar electric systems, and must all be evaluated on the capacity per dollar idea. The most common type is the lead-acid battery, and those can be flooded (distilled water must be added occasionally to refill water lost during the charge cycle) or sealed absorbent glass mat (AGM) batteries that are completely sealed, maintenance free, and designed to be kept at or near full charge, such as in a grid-tied system. There are also gel-cell batteries that are freeze-proof, and therefore can be placed in unheated areas.