The term battery usually refers to a necromancer who uses the skill Blood is Power to help party members by recharging their energy. Blood Ritual and Well of Power are also commonly used by a battery necromancer.
Generally, low health and a regen skill are preferred, to reduce the health taken away by Blood is Power. The battery is fragile, and stays at the back end of the party. Other tasks can involve additional healing or removal of hexes.
It may also refer to a paragon who uses Aria of Zeal and/or Energizing Finale to keep caster energy up.