The Battery Burger is a Gadget hidden in Oasis, which, when enabled, restores 0.5% health for each Powercell spent. To unlock it, the player has to give the Swim Trunks to Johnny Waterfall in the Hidden Sanctuary in Oasis. Once it is unlocked, its effect can be turned off and on at the Gadget Rack.
Johnny Waterfall asking for Swim Trunks
Location of Johnny Waterfall on the map
Unlock pop-up for the Battery Burger
Happy Johnny Waterfall