Replenishes Energy up to the current maximum. It's alright.
That will recharge your energy one turn. Pretty good.
The Extra Battery is an Item in Act III of Inscryption. When used, it will refill your Energy to its current maximum.
The Extra Battery can be found alongside the tutorial path on the way to Central Botopia. Only one can be obtained, and if used a replacement copy will be given at a Waypoint.
Strategy[]
The Extra Battery is generally useful in any situation you need to play more cards. Don't be afraid to use this item when needed instead of waiting longer to get more "value" out of its effect.