Basics Equipment Batteries
Batteries
Most batteries used in solar systems are lead-acid. This is the same kind of battery used to start cars, with one important difference. Car batteries are designed to provide a high current for a short time. They are called starting batteries. Lead-acid batteries used in solar systems are designed to provide a smaller current for a longer time. They are called deep-discharge or deep-cycle batteries. Deep-discharge batteries can discharge 80% of their capacity while starting batteries can only discharge 20% of their capacity. Deep-discharge batteries provide about 4 times the energy of the same size starting battery.
The different types of construction determine the different types of lead-acid batteries. They all use the same chemistry. The ones with caps that you can remove are called flooded-cell batteries because they contain liquid sulfuric acid. The batteries vent a small amount of hydrogen and oxygen gas when they are being charged that uses a small amount of the water in the acid. The caps are there so that you can replace the water. Flooded-cell lead-acid batteries are the lowest cost and most commonly used battery in a solar system.
The so-called maintenance-free batteries are all valve regulated lead-acid (VRLA) batteries. The valves prevent discharge of the hydrogen and oxygen during normal charging. The oxygen and hydrogen then recombine to reform water. One type of VRLA battery uses the flooded-cell technology with the valves in place of the caps. Another type of VRLA battery is the absorbed glass mat (AGM) battery where the acid is contained in fiberglass mats between the plates. Another is the gel-cell or gel battery where the acid is in a gelatinous state. If overcharged the VRLA batteries can still vent, but there is no way to replenish the lost water. VRLA batteries should be charged with special voltages. Their advantage is much lower maintenance. VRLA batteries are used on battery-backup grid-connected systems and are sometime required by building departments.
Lead-acid batteries are dangerous and hazardous. Kids, pets, and casual visitors should be kept away from the battery room by using a lock. The battery room should be vented at the top to allow the escape of hydrogen gas. Lead-acid batteries should be recycled. Safety procedures should be followed.
The Solar Basics book explains more about batteries and how to size batteries is in the Solar Design book. The Solar Basics book includes safety, maintenance, and installation procedures for lead-acid batteries.