Constant current load circuit? (20 amp)
Your discharge current range, 15A to 20A, doesn't require a 'constant current' sink. If you want to use it to test battery capacity, then monitor the current as you discharge the pack, and integrate.

The reason you don't want to do real 'constant current' is that it requires semiconductors dissipating a lot of power. That's expensive, and hard work in heatsinks.

One option is simply to use wire-wound resistors, dissipating to air. With '3.6v' batteries, they're obviously lithium, so will vary in voltage between 4.2 when fully charged and about 3v at end of life. Pick a total resistor value that will give you a suitable current across the range.

Another option is to use tungsten filament lamps. They have a relatively 'constant current' effect as the input voltage varies. Whereas the voltage across a resistor varies as \$IR\$, the voltage across a filament lamp varies as about \$I^{1.5}R\$, so the current will vary much less as the voltage varies. The huge advantage of a lamp in a power dissipation application is that they are designed to run very hot, and can dissipate much more power per space or per dollar than resistors.