In version 1 of the board I used a comparator to disable the discharge, but now I have simplified the design. It uses a cheap battery protection circuit as the basis. You may find these Li-ion protection modules aliexpress for $0.15 each.
They have DW01 protection IC and the FS8205A dual N-channel mosfet both from Fortune Semiconductor. I have designed this circuit in components so I could replace DW01 which has 2.40V undervoltage level with another IC with higher undervoltage level like the FS312 which is at 2.90V.
I have ordered the PCBs from Elecrow and I expect they arrive 22 May 2020. Components were ordered from Aliexpress.
I have just received the boards and assembled two of them. One is equiped with the DW01+G and another with the DW01A-G.
It seems to work. The LED turns on and the resistors get hot, too hot to touch, so in the next design I reduce the discharge current from 1C (~3A) to 0.5C (~1.5A).
DW01A-G discharged until 2.40V and when voltage exceeded 3.00V it started discharging again in an endless loop, because it has "standby function release". When discharging is terminated this IC continued to use 1.5uA current from the battery.
DW01+G discharged until 2.40V and then it went into 0.1uA power down mode. It never reconnected the load even when battery voltage exceeded the re-enable voltage of 3V. This IC does not have "standy function release", it has a sort of one-shot protection. It can be restored to normal operation by shorting the CS pin to GND (or by connecting a charger), so in next design I need to add a momentary switch there. After the load was disconnected the battery regained voltage to slightly above 3.00V, which is perfect because most battery chargers will start the fast charging above 3.0V.
I also learned the batteries are not easy to remove from the holders and you need a string to remove them.
In near future I wan to test with FS312F-G that disconnects the load at 2.9V, and it will recover to a voltage much higher than 3.00V, like 3.3V or something.