WonderSwan:Rechargeable Battery Pack
The WonderSwan is typically powered by a single AA mounted in a carriage. While this lasts quite a long time when using stock parts, once you add an IPS screen and flash cart, the battery life gets cut quite a bit. To cut down on waste, there are a few rechargeable battery options.
While an OEM NiMH carriage replacement exists, most have expired batteries at this point and need the cell to be replaced.
Rechargeable AAs
Rechargeable AAs are probably the easiest way to do so. No replacement parts are needed and they can be found pretty much anywhere. LiIon rechargeables are slightly better due to their stable voltage curve, but NiMH can work just as well in most cases. Recommended Brands:
Ikea LADDA 2450mAh - Rechargeable NiMH AAs (Ikea Link) These are some of the highest capacity NiMH batteries around. Also sold as Eneloop Pros and under other names, these work well for the most part. The main issues is that the voltage curve is considerably lower than alkaline AA batteries, which means the low battery warning light will not be accurate.
Jugees - Rechargeable LiIon AAs (RGRS Link) While these have slightly less battery life than LADDA 2450mAh, they have an advantage of a constant discharge curve. These batteries output a constant 1.5v for 90% of their capacity. When they get below their last 10%, voltage drops to 1.1v in order to trigger the low battery light on most devices.
Wonderpack
WonderPack - USB C Battery Mod - Battery mod created by FlashMasta (Can be purchased bundled with the FlashMasta and a headphone adapter for a small discount). Contains a 2,000 mAh LiPo, and charges over USB C. Battery life is estimated at 10 hours when using with an IPS modded WonderSwan Color on max brightness. Supports charge and play. This battery replaces the stock AA carriage completely, and sticks out a little bit more.
RetroOnyx WonderSwan Battery Pack
RetroOnyx WonderSwan Battery Pack - Battery pack by RetroOnyx (the guy behind the E-Ink Virtual Boy Flashcart). Contains a 2,300 mAh lithium cell and also charges over USB C. Battery life is estimated at 11 hours when using with an IPS modded WonderSwan Color on max brightness. Supports charge and play. This battery replaces the stock AA carriage completely, and sticks out quite a bit covering the entire backside of the console. While quite comfortable to use when playing the WonderSwan in horizontal mode, it can be a little uncomfortable in portrait mode.