http://www.ti.com/lit/ds/symlink/ina138.pdf?ts=1591116835944

there are good things -- Fig.-s 7 , 8 , 10 , 11 , 14 , 19 (last indicates a possible relaxed PCB layout / input noise immunity)
-- but also bad things -- Fig. "Typical Application Circuit" (? steeling current from the input) , +missing "Fig. of common mode voltage step"

it expects your LOAD to be reference-grounded (although you can adjust output a bit . . .)

 

your MOSFET-s look like bottom-right enh (no bulk) --e.g.-- an n-channel ones
if you use both p-ch. -- your design goes that much more simple that you need no supply over the 18V
but that also requires checking and re-vising all specific situations as no battery , evt. insert , evt. remove ... re-biasing the op-amps ...

everything (topology) can be made to work but some require loads of more control circuitry than the others

i'm not attempting to change your design . . . just mentioning the possible alternate options . . . (there's no much point to design your own charge controller from multiple parts ... when theres a commercially avail. and tested in singe iC . . . unless it doesnot do exactly what you intend ... or want to learn to make one yourself ...)