Then, what's the function of this board, on battery?
Ok, let's look at the circuit:
Tools for handling firmwares of DJI products, with focus on quadcopters. - o-gs/dji-firmware-tools
github.com
First you'll notice 4 identical chips. They have 6 legs, but chips often merge legs together - so hard to draw conclusions from that alone. But there are 4 of them, which suggests they might be connected in parallel to allow higher amperage through the circuit - these are probably MOS transistors for cutting off power.
Now there's a diode on the left. Why would you need a diode in such circuit? It could be for reverse current protection, but then it would have to allow the large current as well, and there's only one, small diode. So what else it could do? There are Zener diodes, used as voltage reference sometimes - is that it? Do we need a voltage reference?
If we have cell undervoltage detection - then yes, we need a way of detecting if cells are below 3V, and Zener diode would help with that.
So if we have power switch + zener diode, one thing which is missing for a complete CUV circuit is a voltage comparator - and I'm pretty sure one of the remaining chips is just that.
So this leaves us with one unknown chip, plus few discrere components. Caps are most likely for stabilization of power supply to the chips, so can be ignored. One resistor is signed RT, which suggests it's a temperature sensor? This is a bit wierd, as BQ24259 datasheet suggests a different type of sensor. But I haven't read the datasheet really, only watches pictures. so not sure. The chip might be a buck converter for the comparator, or part of the temperature measuring circuit.
Not that hard, right?