Rechargeable Gameboy Cartridge shell

For awhile I had been wanting to design a gameboy cartridge shell around a rechargeable battery so I would not have to worry about losing save data or having to check every so often what the coin battery voltage is at. The clear shell was a proof of concept I made last year. Thanks to PizzaHandhelds for their FDM friendly cartridge, I modified their design to hold a 1000mAh from a disposable vape and a TP 4056 charging board. I also added a hole to easily probe the battery contacts with a multimeter without disassembling the shell. It is not pretty but it gets the job done.

The game I tested with is a Japanese copy of Pokemon Red, at full charge is 4.22v, and over a year after charging and sitting in my closet dropped to only 4.08v. The front half of the shell is designed for an 18350 sized battery as that is what came out of a vape I was given. The voltage of a coin battery meant for these cartridges is around 3v but the cartridges run at 5v when powered on in a console, so the higher voltage is not a concern. The charge board can also be wired in so it charges while powered on in console, but I would not recommend unless strictly played in a gameboy player or retron console. Doing so with a mobile console will quickly decrease the console's battery and causes audio issues

I would recommend printing in clear to better see the charging indicator on the TP 4056, but the light is still quite visible with opaque filament. Double sided tape should be placed on the underside of the charge board to prevent shorts. I used rolled up masking tape to hold the battery in place as I have long since lost the foam the was originally on it. I recommend holding on to the foam if your battery has it.

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Voltage readings:

4.22v on Nov 8, 2023
4.14v Jan 6, 2024
3.92v Aug 25, 2024 - I accidentally broke the positive wire from the battery to the charge board and repaired it within 10 minutes and the save was still intact, probably residual power in the charge circuit
3.89v Oct 1, 2024 - swapped out the Japanese Red cartridge for a AM29F016 converted cartridge running English Pokemon Crystal to see how the battery performs with a RTC causing extra load
3.84v Mar 12, 2025