Hub Motors and Regenerative Braking

Why regenerative braking is useless—or essential to have

The topic of regenerative braking is a contentious one. Adding regenerative braking to slow electric vehicles down—to recharge the battery and increase range, is trivial to implement on non-geared hub motors. Doing so costs but a few dollars more, and those who make, sell, and promote such motors are tempted to make extravagant claims about the benefits of having regen capability. They may avoid overtly false claims (in writing), but somehow they manage to give prospective buyers the impression that regen is a must-have wonder.

Those who make, sell, and promote geared hub motors or mid-drive (through the gears) e-motors, tend to minimize regen or even denigrate it as a manifestly useless marketing gimmick to be avoided. The alleged benefits of regenerative braking are indeed limited. Ecospeed, a maker of mid-drive motors, has done an excellent job of pointing out the limitations of regen. See: Is Regenerative Braking Useful on an Electric Bicycle?

For a bit of gentle debunking, consider: The self-charging electric bike: fact or fiction? But before contemptuously rejecting or enthusiastically embracing regen, read on.

On flat land you could race up to the stop sign and hit regen to slow down. Friction brakes would still be needed to come to a complete stop, but much ware on the brakes would be avoided. The assumption would have to be made that no harm is done to the battery by charging it too quickly. A brief surge could do harm unless dissipated as heat. But the absolute most inefficient way to operate an e-cycle is to go 100% throttle from a standstill until fear forces you to back off, then slamming on maximum regenerative braking to come to a stop before repeating the pattern.

This raises the question: how to hyper-mile an e-cycle? Let's say you're an average sort who can only putout 40 watts over the long haul. Still, humans can crank out a lot of power for a short period of time, such as is needed to go from a stop to get up to speed. Hub motors are inefficient at low speed. Pouring on the amps will get you up to speed. The problem is you won't hear the giant guzzling sound as the battery is being drained. If you want to guzzle, drive a car. On low-power vehicles the battery is your friend. Don't abuse it just because it feels good to speed away from a standstill. So be nice. Peddle up to a moderate speed before asking the motor for assistance.

Stopping is best done at the very end of coasting most of the way to the stop using only the friction brakes. The energy saved will far exceed what might be generated by a hard stop. Hard braking is for emergency stopping which should not happen all that often. So on flat land, or when going uphill, regen is not needed/used. But if you go uphill, you'll find it difficult to avoid going downhill. Up to a point, going downhill is just free gravity assist, and you'll just want to sit there and enjoy the ride. The 'point' alluded to can be calculated. Assuming 20 mph (32 km) as the maximum (legal) speed, any downhill slope less than 1.6% is a joy ride. But there are terror slopes too, so be afraid, be very afraid.

Assume a moderate downhill grade. Assume you're an average male American (so on the heavy side) and are carrying a few things in addition to the battery/motor (say 300 lbs, 136 km total), The more weight, the harder it is to go uphill and the faster you go downhill. On an upright bike at 25 mph (40 km), about 80% of energy is used to stir the air. When coasting downhill, wind resistance is what determines your terminal speed. Let's assume a recumbent position making for higher potential speed. Assuming no friction or regenerative braking, what will your terminal velocity be? That too can be calculated. Ain't math fun?

Slope Speed (mph) Speed (km)
1% 8 14
2% 27 43
3% 38 61
4% 47 75
5% 53 87
6% 60 97
7% 66 106
8% 71 114
9% 76 122
10% 80 130

Bicycle Downhill Terminal Velocity Calculator

There are much steeper slopes out there in the real world, but without brakes you were terminated at about 4% to 5%. Still, nothing to worry about assuming your smoking brakes didn't fail. But with regenerative braking, its a different story. At the bottom of the hill, the motor may be warm, your battery more charged, your friction brakes untouched, and you will not have been terminated.

So when going downhill, use regen instead of wind resistance (which uselessly desipated your precious potential energy stored up while climbing uphill). You could go over 20 mph on a >2% slope, but speed really does kill your energy efficiency if nothing else. The hyper-miling way to go downhill is to continue pedaling with all due moderation on moderate slopes and use regen to keep the speed down to about 15 mph. On steep slopes don't bother pedaling as batteries can only be charged so fast.

If you live in Florida with no hills in sight, then forget regen as you may only get 1% energy recovery. It won't hurt to have it, but you'll barely benefit from having it. But if you don't want hills to be a deal breaker (or neck breaker), then regenerative BRAKING is your friend. Consider any recharging (figure 12 to 18% energy recovery downhills) to be a pleasant plus.




Note: Okay, so it will hurt to have regen just a little. All non-geared hub motors are part of the wheel. If the wheel moves due to human-power alone, the motor moves and there is some resistance known as cogging. It may not be noticable, but if the wheel is supported off the ground and spun by hand, it will slow to a stop faster than a wheel without a motor in it. Geared hub motors freewheel. The motor inside doesn't move with the wheel, so no cogging, and since the motor, when it is used, spins faster and therefore more efficiently, geared hub motors are better hill climbers. The downside is the gears always whine when in use. Not as much as a mid-drive system, since the gears are enclosed within the hub, but still they whine noticeably. To coexist with bicyclists pedaling under their own power, whining is a bad thing even if the owner gets used to it. And geared hub motors and mid-drive systems don't do regen BRAKING.