The battery gauge fills up when striking an enemy. Certain amounts of battery, or discrete cells, are needed to heal or use special attacks.
In Hollow Knight, striking an enemy fills up Soul. The player can use Soul to heal or cast spells to damage enemies. Along with collecting Soul from enemies, Soul Totems will grant Soul on strike, but only a certain amount until the player rests at a bench.
Outside of combat, the Soul level stays constant.
The player can equip implants that grant abilities once the battery has been filled to a set amount. The player can launch a drone or use alternate healing methods. Each implant has a specific amount that needs to be charged, and the implant's symbol marks the amount needed on the battery gauge.
Outside of combat, the battery will slowly drain.
The Surge 2
Implants now cost a "cell" to use. The player starts with three cells in their battery, which can be upgraded to five. Out of combat, the battery will slowly discharge to the next lowest cell, followed by each cell discharging altogether.
Outside of combat, the battery will slow drain to the next lowest cell, then after ample warning, the cell will discharge altogether. The player can bank these cells in "injectables" to be used later. The battery gauge is required to heal in the game.