Many solar electric systems either require batteries to function or can use batteries to add a backup power component to its power harvesting and conversion functions.  Other systems are purely power harvesting and conversion factories and won't have any use for information about battery storage.  These systems typically are grid-interconnected and sell back any excess power back to the grid, essentially using the grid as its storage medium.

Batteries for solar power typically fall into two major technology categories of lead-acid type batteries  - flooded lead acid (FLA) and absorbent glass mat (AGM).  There are other types of battery technologies - including alkaline, nickel-cadmium (Nicad), lithium ion, and a whole host of much more exotic types - but these other types of batteries typically either have chemical or electrical properties that don't mate well with the needs of a solar power system or are cost prohibitive.  There is also another type of lead-acid battery, termed the gel battery, that has a combination of characteristics of the other types of lead-acid batteries but for a couple of reasons is not going to be used as much as the flooded or AGM types.

In lead-acid batteries, the combination of lead and sulfuric acid allows for a rechargeable energy storage source.  Within these batteries, when connected to a load, the lead and sulfuric acid generate a voltage which is used to power the load.  A by-product of this reaction is lead sulfate which ends up coating the lead plates and thereby reducing the voltage available.