6F22 9V Ni-MH Charger on DC-DC step UP module.

Inside the battery(6F22), there are 7 cells connected in series, each with 1.2V, totaling 8.4V. As is known, Ni-MH batteries should be charged to about 1.5V, hence the calculation: ( 1.5 x 7 = 10.5V ).
For charging a 6F22 Ni-MH battery, it must be charged to a voltage of 10.5V and a current not exceeding 45mA. To do this, we will make a converter from 5V to 11.5V because 1V drops on the LM317 and there will be exactly 10.5V left. The charging current will be limited to 35mA; at the end of charging, the current will be about 2mA. The end of charging will be indicated by an LED indicator when the voltage on the battery reaches about 9.85V.

This project was based on Multi-function Mini Boost Module Step Up Board 5V/8V/9V/12V 1.5A LED Indicator.

It's a small module, very popular in many markets. We need to set up a resistor divider on this board to output 11.5V(-1V lose on LM317). From the datasheet (SDB628),

the divider must be 100K and 5.4K~5.6K for 11.6V or (200k and 11k). With this, we got a perfect small charger for 6F22 Ni-MH battery, working from any 5V USB charger, with an LED indicator, indicating when charging is complete. We don't need to implement charge termination, as at the end of the charge cycle, the current to the battery will be very low, around 1mA, and in such a mode, the battery can remain connected for quite a long time, up to several days.