Helllloooooo NiLuJe.
I find different information. According to ifixit the battery of PW1 should have a I2C interface with four contacts (+, -, sda, scl). The PW2 has only one thermistor with three contacts (+, -, T).
At I2C there is a (maybe complex) conversation between Kindle and battery. At the thermistor it is only a temperature monitoring. It's probably only charged according to the voltage, and even smaller batteries will fit. The charging current is regulated by the temperature.
Do you know where to find pictures? Do you know if somebody has disassembled a PW2 in this forum and posted the pictures?
https://de.ifixit.com/Store/Amazon-K.../IF295-003?o=1
https://de.ifixit.com/Store/Amazon-K.../IF295-004?o=1