Part of this is the charge controller on the battery, the other part is the cheap components of the battery.
When the lithium battery degrades to a certain point (or on a set schedule), the controller just stops the charge. This is not usually done in malice, though. The other components are cheaper quality (because it’s cheaper to manufacture), so controlling the charge at a certain point helps avoid a fire (and a lawsuit).
Back in the R32 and GBA days, batteries were built with better components, so the charge controller didn’t need to be so aggressive. Therefore, they still work to some capacity.
Part of this is the charge controller on the battery, the other part is the cheap components of the battery.
When the lithium battery degrades to a certain point (or on a set schedule), the controller just stops the charge. This is not usually done in malice, though. The other components are cheaper quality (because it’s cheaper to manufacture), so controlling the charge at a certain point helps avoid a fire (and a lawsuit).
Back in the R32 and GBA days, batteries were built with better components, so the charge controller didn’t need to be so aggressive. Therefore, they still work to some capacity.