Most batteries used in solar electric systems are 6-volt, deep-cycle lead acid batteries.However, they’re not the only batteries in use today.Nickel cadmium and nickel iron batteries can also be used.These batteries can be deep discharged many more times than lead acid batteries and therefore last a lot longer. Unfortunately,they don’t store as much electricity as the standard lead acid battery and cost a heck of lot more.They’re also not widely available.

Another type of battery that is useful in certain applications is the sealed battery, also called a captive electrolyte battery.Two types of  these “sealed” batteries are currently available: absorbent glass mat (AGM) batteries and gel cell batteries.

In absorbent glass mat batteries, thin absorbent fiberglass mats are placed between the lead plates to immobilize the acid. Furthermore, the mat is a microporous meshwork that creates tiny pockets that capture hydrogen and oxygen gases given off by the battery during charging.The gases recombine in these pockets, forming water. (As a side note, AGM batteries tend to tolerate overcharging a bit more than gel cell batteries.

In gel batteries, the lead plates are separated by cavities, as they are in a standard lead acid battery. However,the electrolyte (sulfuric acid) is in a gel state, not a liquid.The electrolyte is gelled by the addition of a small amount of silica gel, which turns the electrolyte into a material much like hardened Jell-O.

Sealed batteries are maintenance-free,which means they don’t need to be filled with water or equalized.This saves lots of time and hassle and makes them a good choice for very remote locations where routine maintenance is unlikely, according to Photovoltaics: Design and Installation Manual by Solar Energy International.

Sealed batteries are also spill-proof.The gel cell batteries won’t even leak if the battery casing is broken (a rare occurrence). Because of their design, sealed batteries charge faster than standard lead acid batteries. Sealed batteries also release no explosive or toxic gases like conventional lead acid batteries. In addition, sealed batteries are much more tolerant of low temperatures than lead acid batteries.They can even tolerate occasional freezing, although this is not recommended.

Sealed batteries are commonly used for storing electricity in solar electric and wind generating systems on sailboats and RVs where the rocking motion would spill the sulfuric acid of flooded lead acid batteries, and where space is limited and batteries are frequently crammed into out-of-the way locations.They also have a lower rate of self-discharge,which means they discharge more slowly than conventional lead acid batteries.