I have two battery packs for the Virtual Boy, but one appears to be broken. Unfortunately, it seems the seller I bought the broken one may not accept returns. That’s where I got the idea to make a mod to this unit for supporting rechargeable batteries. If this mod goes well, I may mod my other battery pack too.

I read somewhere about using Game Gear rechargeable batteries in a similar mod, but Game Gear batteries are rather old by this point and I would like to use new batteries.

Do any of you have a suggestion on alternative batteries that provide 9V? I could maybe solder 6×1.5 Lithium ion batteries together, but I’m not sure if that’s safe. It would likely be safer to use prepackaged batteries for this purpose.

In any case, I would also need a charging circuit (prevent overcharging) and a power connector that can fit in the top lid.

Just get a “pouch”-style 2s (7.4V) LiPo pack, made for RC models, and the proper charger. You could probably go up to a 3s, since the regulator in the VB can handle anything from 6 to 15V, IIRC.
You could also try to repair the battery box, and use your Li-ion AAs as-is…

If it’s ebay, unless it was listed as for parts and repair only AND they didn’t mislead you on why it is listed as such, ebay basically screws sellers and forces returns despite what it says.

So it’s kind of up to you if that’s where it came from if you want to fix it or forget it.

I decided to keep the battery pack and will be looking into modding it soon. Having a rechargeable solution would be nice for not needing to go through tons of AA batteries all the time.