My project "RPN calculator" needs to be powered from 3.3V battery. I assume to use a LiON battery. So I tested the LTC1733 battery charger circuit. I have designed small universal PCB.

The PCB has size 1.33" x 2.11" (34 x 54mm). If you like it, I am ready to send you sample by post. Here is circuit diagram:

and how to connect it:

The charger offers different charging currents that can be selected using a rotary switch or potentiometer, with 4.1 V or 4.2 V battery selectable, optionally also NTC to monitor the battery temperature. The charger has three LEDs that indicate charging.