Third party Game Gear battery packs
From Sega Retro
The Sega Game Gear had a battery problem. It soon became apparent that six AA batteries for roughly six hours of play was an expensive and wasteful way of gaming on the go. To compensate, Sega produced the Game Gear Battery Pack and later Game Gear PowerBack, but a number of third party manufacturers took on the challenge too.
Turbo Twins (Naki)
Naki produced two sets of Turbo Twins, an earlier set that could only handle three hours, and a later set which could manage six.
Power Grip (Gamester)
Power Pak (Performance)
Rechargable Battery Pack (Recoton)