Shield Batteries are used by pilots to restore their shields when they are damaged or knocked out. Shields generally will stay down for ten seconds before coming back up unless Shield Batteries are used to restore them. Pilots therefore usually save shield batteries until their shields are completely down to minimize the amount of direct hits taken in combat.
Each ship has a maximum number of shield batteries it can carry; additional batteries can be acquired in combat from destroyed enemy ships by using the Tractor Beam. Shield batteries only restore shield points; they do not restore hull points. To restore hull points, one must use Nanobots or repair the damage in a repair bay on a base or planet.
Shield batteries always can be bought or sold at any base for $200 credits. If one expects to defeat enemies without needing shield batteries, it might not be a bad idea to venture forth with less than a full load of batteries, to leave some room for shield batteries dropped by enemies. This is a useful technique if one is camping an enemy base or jump hole, or otherwise expects lots of easy kills. Oftentimes, especially in the beginning of the game, selling shield batteries and Nanobots can be more profitable than selling certain Commodities or equipment dropped by enemy ships.