Electronic Load Build, Issues

[trilerian]

I haven't read all the other replies...  But I am in the process of building an electronic load for discharging 1s and 2s lipo batteries at up to 45A.  Also using the 328P mcu.  We differ in that I am truly using the pwm and an external load instead of sinking the current to the fets.  But I am using a low pass on the output of my current sensor, which is the big brother of what you are using.

So in you design you are using the lowpass on the gate to get a dc line, but I use the same cutoff lowpass on my current sensor out.  I use a 5k1 resistor and a 1uF ceramic cap. I also have implemented a PID (really just PI) controller in my code to ramp up to the discharge rate faster and help with the oscillation.  Also I am averaging the output of my current sensor.  For me this keeps the oscillation to within +/- 0.2A and the oscillation has slowed down quite a bit.  Before implementing the PID and averaging it was pretty bad, would oscillate a couple of amps and rapidly. 

Tuning the PID was fun, and I could probably get it a bit better, but it was neat seeing what the different settings did to the response.