Measuring Discharge Rate of LiPo battery
What you need is a &quot;constant current sink&quot;.  It's a fairly straightforward circuit to build out of a transistor, an op-amp, and a couple of resistors:
https://circuitdigest.com/electronic-circuits/voltage-controlled-current-source-circuit-using-op-amp
https://circuitdigest.com/electronic-circuits/design-a-simple-constant-current-sink-circuit-using-op-amp
Good luck.