The Battery

Effects

Activated items can store an extra level of charge, allowing them to be used twice in one room.
- Extra charges appear in yellow over the normal charge meter.

The Battery does not affect self-recharging activated items.
- (Added in Afterbirth †) Self-recharging active items can charge the yellow meter, allowing Isaac to use them twice immediately.
(Added in Repentance) If the recharge time of Isaac's held item is less than 6 and he picks up a Lil' Battery, it'll overcharge the item for the remaining charges left in the Lil' Battery.

48 Hour Energy pill: A fully discharged item will be doubly charged, along with spawning 1 or 2 Lil' Batteries.
(Added in Repentance) Friendly Ball: Walking over a captured enemy will double charge the ball, allowing the enemy to be released, and allowing to catch a new one in the same room, and the process repeats. This allows the Friendly Ball to be used much, much more often, and gathering many more friends. Note: trying to catch a new enemy while the old one is in the process of spawning will result in the ball phasing through, catching nothing, thus wasting charge.
(Added in Repentance) Notched Axe: Causes remaining charge from the previous floor to carry over to the next one.