I am looking for circuits used to measure or monitor the current charging of a battery (Li-Ion or NiMH), microchip has proposed the following circuit:

My concerne is about the selected circuit in RED

I did not understand how does this ciruict works! *Why have we used R24=1.2M  ? Especially in case we remove R21 (As it is noted by DNP)

*What is the contribution of R16, C12 and R18 in measuring the current flowing through a parallel resistors R4 and R5

*What about C15  , is it used as a filter or just for optimazation ??

I am looking for circuits used to measure or monitor the current charging of a battery (Li-Ion or NiMH), microchip has proposed the following circuit:

My concerne is about the selected circuit in RED

I did not understand how does this ciruict works! *Why have we used R24=1.2M? Especially in case we remove R21 (As it is noted by DNP)

*What is the contribution of R16, C12 and R18 in measuring the current flowing through a parallel resistors R4 and R5

*What about C15, is it used as a filter or just for optimization?