Battery Eggs are a special type of egg found in Banjo-Kazooie: Grunty's Revenge.
Bozzeye teaches Banjo and Kazooie how to use them in the Monster Manor of Bad Magic Bayou. The shock that these eggs deliver when fired deals a bit more damage to enemies than normal Blue Eggs, but aside from that, they are used to open special egg tolls within the manor which can only be activated by Battery Eggs.
Unfortunately, these egg tolls are seen nowhere else in the game, and since there are no enemies which are particularly weak against them, Battery Eggs typically expend their usefulness once Bad Magic Bayou has been completed.
Eggs in the Banjo-Kazooie Series 