The kindle battery circuit expects Lithium Polymer (LiPo). Not sure how other lithium battery technologies would work. But, the battery cover I peeled off lists BOTH 3.7v (standard LiPo voltage) AND 4.2v (common for other lithium batteries, such as older kindles used). I suspect anything below 5v is probably okay, and perhaps anything up to what could cause overheating of voltage regulators in the kindle (probably lower than IC max specs, because no active cooling). The battery thickness is typical of a single LiPo cell. The length and width are limited by the battery storage frame, but there are backplane support ribs that could be ground away to make room for a larger battery cell (at least in the open PW2 I just examined while typing this, and my PW3 was identical in all ways to my PW2, from a visual inspection).
