While replacement batteries are easily found on Amazon and AliExpress, capacity tests by community members such as Makho figured out their true capacities are so low that sometimes they last less than an old, deteriorated, original battery. With the increased usage of battery draining cartridges such as flashcards, the situation is even more dire.
However, one can use an old Nintendo battery and a new modern lithium ion pack to substantially increase the battery duration by making a custom one. In fact, this one advertised itself as a 900mAh battery and actually delivers 920mAh - making it substantially better than the real Nintendo provided batteries from back in the day, lasting surprisingly long.
If you guys want a tutorial for how to make your own, do let me know!
