These are from my childhood they store a battery which clips onto the back of the Nintendo game boy advance and powers was rechargeable so could be used while charging.

They perfectly match the glacier clear pink/purple game boy systems and i thought we bought them officially but they do say electronics boutique on so I'm not sure if they are official Nintendo.

The batteries were corroded so have been removed but left as much as possible internally and the cases are in excellent condition so they should be bake to be fixed up with new rechargeable battery and used. I've never seen these sold anywhere so it's a cool vintage retro addition for game boy fan

Picture to show how they attach but console not included.