If the holder is hit by a Electric-type damaging move, it consumes the Cell Battery and its Attack is increased by one stage.
It does not activate if the holder already has +6 Attack stat stages (unless its Ability is Contrary, in which case it will not activate if the holder has -6 Attack stat stages). It does not activate if the holder is protected from or unaffected by the Electric-type move (e.g. due to Motor Drive or being Ground-type).
If multiple Pokémon each holding a Cell Battery are hit by an Electric-type move at once, each hit Pokémon will consume its Cell Battery before Symbiosis transfers any item (including Cell Batteries).