Wireless charging for a HP-25 calculator

[jwr]

> I have thorougly tested the small battery protection board. It reliably turns off the charging current when the battery reaches 4,2V.

Max — I really strongly suggest using a LiPo charger circuit. Yes, the battery protection board is supposed to intervene in extreme conditions (such as under- and over-voltage), but it does not control charging. With small LiPos you might be able to get away with things, but I would not rely on it, because mistakes with LiPo batteries cause serious fires. I wouldn't risk it.

Lithium polymer batteries should only be charged by a specialized IC. You can easily get little modules which will do the job, such as this one from AdaFruit: https://www.adafruit.com/product/1904

Your D24V5F2 synchronous buck is a much better choice than the 1117 linear regulator, because a linear regulator wastes lots of power.