All About Regen on an Ebike
There are few topics that are more misunderstood and mis-represented than regenerative braking and we'll try to set the record straight here.
What is Regen?
Regenerative braking means using the electric motor as a brake to slow down a vehicle, and is one of the very unique features of electric propulsion. It not only provides a zero maintance and perfectly consistent braking force in all weather conditions, it also captures this braking energy and puts it back into the battery pack, rather than burning it all off as heat. This not only increases your range, it also encourages safer riding habits as you know energy isn't being wasted every time you use the brakes.
In principle regen is available on any system with a direct drive hub motor or a geared hub motor that doesn't freewheel (like the GMAC). However, it does require a motor controller that supports regen, and many budget controllers either don't or don't support it very well. That may be one of the reasons that regen isn't widely appreciated in the ebike scene and is often even dismissed outright.
For us, regen has been one of the most alluring aspects of ebikes from our earliest days building prototypes out at UBC, and understanding regen was one of our main motivations for making what later became the Cycle Analyst.
How is Regen Activated?
In order engage regen, the motor controller needs some type of signal telling it that the rider wants to brake. Depending on the signal type, this can result either in a fixed braking force, or a proportional force that can be modulated in intensity.
- 1) Ebrake Lever. Most regen-enabled motor controllers have a digital ebrake input plug, and when the brake lever is pressed this results in a fixed regen intensity. That works better than nothing, but the inability to adjust the braking force limits the amount of gain from regen and the mechanical brakes still come into play.
- 2) Ebrake Lever + Throttle: In order have some way to modulate the regenerative braking force using existing ebike hardware, we had the idea back in 2007 to have the throttle switch behavior from controlling motor torque to braking torque when the levers are active. This takes advantage of existing hardware already on the handlebars to provide variable braking force. While it's not immediately intuitive to use the throttle for brake modulation but you get used to it quickly, and was featured both in the cross canada ebike and the 1st generation Grinfineon controllers.
- 3) Separate Regen Throttle: Many EV controllers support an analog brake input signal that can be mapped like a throttle, so in a pinch a rider can install two throttles on their ebike. One of them controls acceleration, and the other throttle controls braking force allowing for a smooth modulation from zero to full regen. In theory this regen throttle could be in the form of an ebrake lever that has proportional signal instead of a simple on/off switch, but such levers are not readily available on the mass-market.
- 4) Bidirectional Throttle Signal: In all of the Grin motor controllers (Grinfineon, Phaserunner, Baserunner), we have mapped the otherwise unused portion of the throttle signal below 1.0V to activate proportional regen control without any additional wires. Throttle signal voltages from 1.2V to 3.6V provide proportionally more power, while if that same signal drops down to 0.8V it starts doing regen, increasing to a maximum regen intensity at 0.0V. This would in theory allow for bidirectional throttles that can be twisted one direction for power and in the other direction for regen. We've heard of such throttles existing, but have yet to see one.
- 5) Speed Limit: Some controllers allow for regen to engage at a certain speed and vary in intensity to keep the bike from exceeding a speed limit. This behavior is especially nice on hilly terrain and allows for proportional regen to kick in automatically on the downhill sections with no user input.
- 6) Back pedaling on a PAS sensor: Quadrature based PAS encoders have the ability to tell when the cranks are being spun backwards, and in principle this can be used by the controller to activate regen . This is a feature we played with years ago and plan to include it in future CA3.1X firmware releases.
Regen with a V3 Cycle Analyst
In the V3 Cycle Analyst, we take advantage of the 0.0-0.8V throttle regen map in order to support modes 2, 5, and (eventually) 6.
The most common approach is mode 2, with an ebrake lever on the opposite side of the handlebar as the throttle. The ebrake lever is squeezed to activate a baseline regen intensity as set by the Brake_Out voltage, and then the throttle is used to increase this intensity. There are built in delays in the event that the brake is released while the throttle is still held down. Another approach is to enabled regen speed limiting and have one of the digital or analog auxiliary inputs control the speed of the vehicle. This is like mode 5, but with the added benefit that you can freely adjust this regen speed while you are riding, and you can come to a complete stop by gradually dialing the speed to zero.
Regen at High Speeds from Back-emf Voltage
You will also get braking force from the motor and current flowing into the battery pack whenever the bike is moving faster than the unloaded speed of the motor. This happens whether you want it to or not because the mosfets in the motor controller have body diodes can't help but conduct current when the back-emf voltage of the motor exceeds the battery voltage.
This can be used as a deliberate feature to create an ebike that naturally governs its top speed. It can also be annoying to people who want the ability to coast super fast on a downhill stretch without the motor hindering their speed, but who don't need the motor to power them very fast. That scenario can be addressed with our Phaserunner or Baserunner controllers via field weakening to increase the effective unloaded motor rpm.
Is it Worth It?
- Plug Braking
- Regen Efficiency
- Maximum Power Capture
- Super Capacitors