Were you cycling a 1S battery when achieving 20 amps? (guess based on picture). Per the 80 watts internal discharge power rating 20A should be doable on a 1S. With a 2S pack it's limited to essentially half of the current it can do on a 1S. Need an external resistor setup as mentioned, or a regenerative setup with extra batteries, basically discharge on one port of the charger while charging on the other port.