Door Stop - Provides infinite charges by clearing some enemies then leaving the room, and repeating.

Mercurius - Provides infinite charges by clearing some enemies then leaving the room, and repeating.

Schoolbag - The item in the second slot gets charged when the first one is full. Overcharges on the first item also jump to the second one.

Causes Isaac's active item to charge when Isaac deals damage to enemies, rather than when he clears rooms.

Charges are gained at a rate of 60 damage dealt per charge.

Damage dealt by active items works as well.

Active items don't charge when clearing rooms. Instead, they're charged by dealing damage to enemies.

The Binding of Isaac Wiki

4.5 Volt