BATTERIES
Batteries provide energy storage. They accumulate energy as it is generated by various renewable energy devices such as PV modules, wind, or hydro plants. This stored energy runs the household at night or during extended periods when there is no energy input such as a long string of cloudy days with a PV system. Batteries can be discharged rapidly to yield more current than the energy source can produce by itself, so that pumps or motors can be run intermittently.
Lead-acid batteries are the most common and by far the most cost-effective energy storage solution for remote energy systems. Here's how they work.
The lead-acid battery cell consists of positive and negative lead plates of slightly different composition suspended in a dilute sulfuric acid solution called the electrolyte contained in a chemically and electrically inert vessel. As the cell discharges, sulfur molecules from the electrolyte bond with the lead plates, releasing excess electrons. A flow of electrons is what we call electricity. As the cell charges, excess electrons bond with the sulfur compounds forcing the sulfur molecules back into the sulfuric acid solution. A lead-acid cell produces approximately two volts, regardless of size. A battery is literally a collection of cells. A typical 12-volt automotive starting battery consists of six cells, each of which produces two volts, connected in series. Larger cells provide more storage capacity, we can pull more amp-hours out and in, but the voltage output never exceeds the two-volt peak potential of the chemical reaction which drives the cell. Cells are connected in series and parallel to achieve the needed voltage and storage capacity.
Sizing of 1000 to 2000 amp-hour 12 volt (or 1/2 that in 24 volt; 1/4 that in 48 volt) battery bank is desirable because a larger bank can accept faster charging by generator and give high power discharge for inverter surge without strain.
DEEP CYCLE lead acid batteries have thicker plates and lead-antimony support grids for years of over 50% deep cycle charge and discharge. The Trojan T-105 and the L-16 batteries are the most common.
Auto batteries are shallow cycle only and will not last long in home power systems. These have thinner plates and lead-calcium grids designed for less than 20% discharge and immediate recharge.
RV/MARINE This common 12 volt battery is designed about half way between a deep cycle and a shallow cycle, and has medium length of life.