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:[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/zknOI.jpg

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 ??