Game Gear Battery Pack
From Sega Retro
The official Sega Game Gear Battery Pack (バッテリーパック) is a rechargeable battery pack for the Sega Game Gear, and the first released by Sega. It provides an alternative to spending a fortune on AA batteries by having something which can be charged and reused.
The Battery Pack contains several Nickel-Cadmium rechargeable battery cells and plugs into the Game Gear's AC adaptor port. The Battery Pack is an odd device which hangs out of the unit, adding to the questions of whether the Game Gear is a truly portable console. It was designed to clip onto a user's belt, though the obvious assumption it makes is that Game Gear users were wearing belts, else it would dangle around and be damaged.
Though in retrospect it appears to have been poorly designed, Nintendo originally had the same idea with its Game Boy equivalent battery pack. What separates the two is that the Game Gear's battery pack is substantially bigger and inconvienient.
The Battery Pack can be recharged with the widely used Sega MK-2103 (or equivalent) AC adapter. The unit would be replaced in North America and Japan with the PowerBack Rechargeable Battery Pack, possibly inspired by better designed third party alternatives.