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Regeneration on electric bikes: Is it worth it?

1. Main

2. Conclusion

by Jérôme Daoust. Revised 2008/8/13

Main (Top of Page)

Assumptions:

Regeneration efficiency is 40% at best towards recharging the battery.
We do 75% of our braking with the front wheel (or 3:1 ratio of front/rear). Reference: Bicycle Source.

Gathered knowledge:

While riding downhill, you only recover the potential energy (mgh: mass x gravity x height) minus the lost due to aerodynamic drag. So it is better to go slower (more regenerative braking) while going downhill for efficiency purposes. This is a tradeoff by reduce the fun of going downhill fast.
High rates of regeneration can harm the battery life compared to charging it at a normal rate.
It is not recommended to use regenerative braking for extended periods of time while going downhill. Battery will heat up and this can be a problem particularly on warm days. That's too bad because it goes against easy-to-use scenarios.
The best application would be for heavy vehicles in stop-go traffic, operating at low (compared to their max) speeds so aerodynamic drag is not prevalent. This is hardly the case with bicycles which are light and most often operate near their cruising speed (unless on hilly terrain) where most energy is lost against aerodynamic drag.

Short-time braking regeneration

Based on the assumptions listed above, the following shows the expected recovery rate:

Front braking	Rear braking	Recovered energy	Comment
All regeneration	All regeneration	40%	Theoretical best.
All regeneration	All friction	30%
½ regen, ½ friction	½ regen, ½ friction	20%
All friction	All regeneration	10%	An ebike with rear hub, which could modulate regenerative braking based on brake lever pressure. None exist yet.
All friction	½ regen, ½ friction	5%	Like most ebikes with a rear hub going to a pre-set regenerative braking level.

So for most current ebikes, we can only expect 5% energy recovery from those short stops.

Downhill regeneration

This is different than the previous scenario of short-time braking, as while going downhill we can have any one of the two wheels do the full regeneration and avoid using mechanical braking.

Also, this is where we have the highest hope to recover some potential energy (mgh: mass x gravity x height), yet we have to be weary of using intense regenerative braking for extended periods (see gathered knowledge above). Also to optimize efficiency we should be riding slow downhill to improve the energy recovered by regenerative braking, instead of lost to aerodynamic drag, which is kind of a killjoy. Having said this, lets look at some theoretical scenarios...

Symbolic course elevation profile start ¾¾¾® end	Comment
	No advantage. Full battery cannot be charged further by always going downhill. You don't even need a motor.
	No advantage. No regeneration possible.
	No advantage. No regeneration possible.
	No advantage, except that the rider will have more energy left in his battery after the ride.
	No advantage. Full battery cannot be charged further.
	Will allow going further on the final flat portion. One may worry about damaging the battery from the intense long-duration charging.
	No advantage. Full battery cannot be charged further.
	No advantage, except that the rider will have more energy left in his battery after the ride.
	Energy lost on 1^st flat is replenished, giving extra energy for last hill. One may worry about damaging the battery from the intense long-duration charging.
	When the number of up and downhill portions becomes significant, the advantage of regeneration will be maximized. This minimizes possible damage from intense long-duration charging of the battery. One example where rider claims 40% efficiency: (11.9 – 8.5 ) / 8.5 Ah = 40%.

Conclusion (Top of Page)

It is worth having regeneration capability if:

1. You plan riding courses with multiple downhill sections, not just a big one at the start or end.

2. You accept the risk of reducing the life of your battery due to fast recharge rates.

3. The system does not add significant weight and it's easy for controlling your downhill speed without mechanical braking.

Alternatives to regeneration:

1. Replace your battery for a higher capacity one. If none available, wait since there is significant progress every few years.

2. Install/carry an auxiliary battery pack.

3. Loose weight (you or from your bike).

4. If you ride recreationally (not for commuting), pick an easier/shorter route. Every system has its limits.

5. Reduce losses due to aerodynamic drag:

a. Go slower when motor-assistance is needed. Is this really an option?

b. Install aero-bars to reduce your frontal area.

6. Upgrade your motor (or the whole bike) for a more efficient one.

Related discussion(s):

· Tidal Force: Regeneration: Is it worth it?