Virtually all power generation systems require some form of energy storage. For grid-tied systems, the utility accepts surplus power and gives it back when needed. A battery bank is required for systems that need to function without the grid, either all of the time or during an outage. In these systems, the solar array or other charging source charges the batteries whenever they are producing power, and the batteries supply power whenever it is needed.
The most common battery technology used is Lead-acid, in which lead plates are used with a sulfuric acid electrolyte. The electrolyte can be fluid or absorbed in fiberglass mats (AGM), or gelled. AGM and gel batteries are together known as VRLA (Valve Regulated Lead Acid) and are sealed, do not require water addition, and do not emit gases when operated within specifications. Lead-acid batteries are relatively inexpensive and readily available compared to other battery types. New advanced lead-acid batteries have carbon additives in the negative plate to prevent sulfation at partial states of charge (PSoC), while remaining less expensive than high-technology batteries. Lithium-ion batteries are lighter weight and compact for their power and energy capacity. One advantage of Li-Ion batteries is their long life even when cycled heavily, and without needing to be brought to a full state of charge each cycle. This makes them particularly suitable for short to long-duration use in self-consumption systems where net-metering is unavailable or utility rate structures otherwise discourage energy exports during peak solar production hours.
Standby or Cycling Batteries
Batteries come in a wide variety of sizes and types, but the most important designation is whether they are made for daily cycle service or standby service. Automobile starting batteries should not be used for renewable energy systems.
Standby power batteries are designed to supply power to loads for occasional use, and are preferred for grid-tied solar systems with battery backup. They are optimized to supply moderate to large amounts of power only during utility power outages, and float at full charge most of the time. They are designed to use a minimal amount of energy to stay fully charged. They are not made for frequent deep discharges and have a limited cycle life but often very long calendar life when kept in float conditions. AGM batteries are most common for standby power applications as they are less expensive, have low self-discharge and require little to no manual maintenance. Deep cycle flooded batteries are not desirable for standby applications. They do not have longer life than AGM batteries when kept in float charge for long periods of time. They also have a high standby loss (often as much load as a refrigerator or more), need isolation, ventilation, and much more maintenance.
Deep cycle batteries are designed to be repeatedly discharged by as much as 80% of their capacity and are therefore a better choice for off-grid PV systems. Even when designed to withstand deep cycling, most batteries will have a longer life if the cycles are kept shallower. Deep cycle batteries can be either flooded or sealed lead-acid variants or, increasingly, newer chemistries like lithium-ion or aqueous hybrid ion.