In EM's original system, regen braking is accomplished by a push button on the left handlebar. It only has one level but is also affected by the throttle position and the “clutch” lever.
The Kelly controller's factory setting for the regen braking was 20% (of maximum motor current). The allowable range is 5 to 50%. My first experiment with the controller was to reconfigure the regeneration. Although on my concrete driveway 20% seemed reasonable, it proved to be too much for loose downhill dirt and it locked the rear wheel. I tried 10% which seemed as though it might be reasonable on soft dirt. I also tried 50% to see what it did. That was very enlightening as the braking was quite strong for about one second on concrete and then it freewheeled with no regen braking whatsoever. I had to get back on the throttle in order to reestablish regen braking.
The Kelly controller user manual explains this behavior as follows, “Regeneration has braking effect but does not replace the function of a mechanical brake. A mechanical brake is required to stop your vehicle. Regen IS NOT a safety feature! The controller may stop regen, without warning, to protect itself or the battery (it won’t protect you!).” My guess is that 50% regen was causing a battery over-voltage condition. I did not experiment with the maximum level of regeneration that did not cause the protection to occur as 20% was already too much for dirt.
Later, I also ticked a checkbox to enable “ABS” in the configuration software. There was no description of what it is or how it might work other than, “A novel ABS technique provides powerful and smooth regen.” This seemed to be a big benefit as locking on dirt was no longer a problem. Of course, the mechanical rear brake could still lock the rear if you wanted it to slide.