Modmatic has just released a DIY rechargeable battery pack for GBA systems called the PowerUp Advance. The kit comes in four combinations ranging from $20 to $33 (plus shipping), depending on accessories. Unfortunately, no pre-assembles kits are available, only board + components that must be manually soldered together: https://modmatic.io/products/powerup-advance?item=APU-001-FL
The kit requires that you cut the battery door, but cheap 3rd party replacements are available (link below), so please use one of those, rather than cut the original plastic! Here’s some listed features:
- 850mAh battery provides 6‑7 hours of play time per charge*
- Native GBA low-battery warning light compatibility (20‑40 min warning time)*
- Play and charge at the same time
- Detailed assembly instructions
- Optional light pipe add-on, for charging status LEDs (+ $2.49)
*tested with a regular GBA cart (not an Everdrive), AGS‑101 backlight screen mod, average volume, and an 850mAh lithium-ion rechargeable battery. Your mileage may vary.