This article documents the charge cycle of a NanoVNA-H4 from fully discharged to charged.
The DUT is probably a ‘standard’ H4, but with Chinese sourced produce, you never, never know.
The original battery fitted to the NanoVNA-H v4.3 is a 804050 (8.0x40x50mm) 2000mAh LiPo pouch cell (1S) with protection board.
The charger chip is a TP4056, and it would appear to be limited by Rprog to about 0.75A (which includes the current drawn by the working NanoVNA-H4) (though the circuit employed would appear to tweak that limit between VNA on and off conditions with R44). The TP4056 is simply a charger chip, it will not prevent over-discharge of the cell so it is wise to use a cell with protection board (as originally supplied on the DUT).
Above is a plot of the calibrated battery voltage reported by the NanoVNA-H4.
It has taken about 11h for a to reach constant voltage phase (probably about 80% of full charge) with the NanoVNA powered on. There is not much change to the voltage plot from here on as the current simply tapers off as the battery reaches full charge.
Average input current of the NanoVNA-H4 with fully charged battery is 0.35A, so the maximum current available for charging the battery would appear to be around 0.4A, hence the long charge time with the NanoVNA powered on. It should charge a little quicker if powered off.