Batteries store electrical energy for use when Solar PV panels or other renewable sources are not producing. In off-grid and hybrid solar systems, the battery bank is the most expensive component and the one with the shortest service life. The choice of battery chemistry affects system cost, performance, maintenance, and replacement frequency.
Flooded lead-acid (FLA) batteries are the traditional off-grid storage option. They are the least expensive per kilowatt-hour but require regular maintenance: checking electrolyte levels, adding distilled water, and equalization charging. FLA batteries have a cycle life of 500 to 1,500 cycles at 50 percent depth of discharge. They are sensitive to temperature and perform poorly in cold conditions. Off-gassing during charging requires ventilation.
Sealed lead-acid batteries include AGM (absorbent glass mat) and gel types. They are maintenance-free and can be installed indoors without special ventilation. AGM batteries have a cycle life of 400 to 800 cycles at 50 percent depth and charge more quickly than flooded types. Gel batteries have similar performance but are more sensitive to charging voltage.
LiFePO4 batteries are the modern standard for off-grid solar systems. They have a cycle life of 3,000 to 6,000 cycles at 80 percent depth of discharge, 5 to 10 times that of lead-acid. They are lighter, smaller, and require no maintenance. LiFePO4 batteries charge faster and accept higher charge currents than lead-acid. The higher upfront cost is offset by the longer service life: a LiFePO4 battery may last 10 to 15 years compared to 3 to 7 years for lead-acid.
A battery management system (BMS) monitors cell voltages, temperature, and state of charge in lithium batteries. The BMS balances cells during charging and disconnects the battery if any cell exceeds safe limits. Temperature compensation adjusts the charging voltage for lead-acid batteries based on battery temperature. All batteries lose capacity as they age, and the usable capacity of a 10-year-old lithium battery may be 70 to 80 percent of its original rating.