Not to be confused with the passive item, 9 Volt.
A silver battery with a lightning bolt on it.
Gives Isaac an extra charge for his Activated Item every 15 seconds per room, up to a maximum of three charges. Using an item will reset the counter and let Isaac charge a maximum of six bars per room. The charging only happens as long as there are enemies in a room.
In Rebirth, it also allows you to charge fully charged item once more (overlaying common green charge indicator with yellow-red one), resulting in possibility to use twice charged item twice in row.
If used in conjunction with X-Ray Vision, this allows an infinite use of the activated item by running between incomplete rooms and the secret room. This can also be done with Dr. Fetus, Epic Fetus or Ipecac by blowing up the door of an uncompleted room and going in and out again. Using the Book of Revelations or The Nail for example can provide unlimited Soul Hearts. However, neither of these exploits will work in The Chest.
- The Battery synergizes very poorly with items such as the Dead Bird and the Cube of Meat because they will prevent the player from waiting for items to charge.
- 9 Volt - Allows for rapid use of held items.
Typically, the final (third) charge of the battery will be silent.
Additional item charges can be farmed at a rate of three item charges per Bomb using the following method:
- Enter a room with enemies and wait for the battery to give you three charges. Enemies can be killed, but leave at least one enemy alive so that the room is not completed
- Exit the room using a Bomb, being careful not to kill the remaining enemy(ies).
- Now re-enter the room and repeat the process.
When possessed, Isaac has the bolts of the monster from Frankenstein in his neck.
Occasionally when you purchase the battery from the shop, your character can die randomly. I do not know if the character or other previously acquired items used are important to trigger the bug - it has occurred when playing as the lost on the depths 2 floor but may occur under different circumstances for other people.