Regenerative braking efficiency versus speed (Part ONE)

This article is focused on this subject: “regenerative braking efficiency versus speed”.

Quite often we hear many discussions on “regenerative braking” efficiency.

Typically we have people who are convinced that we can recuperate a good amount of energy we have used by applying “regenerative braking” in the descentes.

This is really a much broader discussion, and here we are NOT going to address following generic question:

“Is it worth to have a hub motor in order to recuperate energy by regenerative braking?”.

Instead here we would like to bring some concrete results of real life tests on a more specific point:

Is there any relationship between:

  • amount of energy recuperated by regenerative braking

  • speed of the bike in the descente

In order to discuss above relationship I have performed the following test:

  • maintain the maximum speed in a given descente below a given limit in km/h

  • at the end, measure the amount of energy (Ah) that has been regenerated in above section

To perform this test I have selected  a descent not too far from my place and I have set up following speed levels:

  • Run n. 1) max speed = 25km/h

  • Run n. 2) max speed = 30km/h

  • Run n. 3) max speed = 35km/h

It is possible that sometimes maximum speed may have exceeded relevant limit during the test: but since this is the case for each of the three speed levels we think that the result should be considered a valid one.

For completeness I would have liked to perform also a 4th run, but it was really cold and I decided to go back home…

Anyway the results are pretty clear so I think that 3 runs were enough to produce consistent results.

regen graking by speed

1) At 25 km/h in the first descente I have been able to regenerate 0,43 Ah, while the consumption to complete the circuit was = 0,64 Ah

25km/h regen
25km/h consumption

2) At 30 km/h in the second descente I have been able to regenerate 0,39 Ah, while the consumption to complete the circuit was  0,73 Ah

30 km/h regen
30 km/h cons

3) At 35 km/h in the third descente I have been able to regenerate 0,30 Ah, while the consumption to complete the circuit was  0,93 Ah

35km/ regen
35km/ cons

Interpretation of above results

These results show that there is a strong relationship between “regenerative braking efficiency” and ” speed”.
This is explained by the fact that:

  • increasing the speed creates much higher areodynamic resistance
  • this additional kinetic energy is wasted and is not usable to recover energy by regenerative braking
Therefore if we want to MAXIMIZE the recovery of energy through Regenerative Braking we should not exceed a speed of roughly 25km/h. 

 

In above graph we can see:

  • with the BLUE line the trend of regenerative braking: this shows that at 25km/h we had the HIGHEST amount of regenerative braking while at 35 km/h we had the LOWEST.
  • with the RED line the percentage of regenerative braking loss:
    • from 25 to 30 km/h we ‘waste’  9% of energy
    • from 25 to 35 km/h we ‘waste’ 30% of energy

It’s also quite interesting to note that the ‘Regen/Consumption’ ratio of the first run (max speed = 25km/h) is 67%.

This ratio in this example is very high: it means that roughly 2/3 of the total energy required to complete this circuit has been recovered thanks to Regenerative Braking!

This is really considerable and it contributes to explain WHY it is generally beneficial to use regenerative braking.

Just as a reference, here it is the circuit where the test did take place:

  • altitude gain: 137m.
  • descente: 4km
  • avg. grade: 7%
  • total distance: 7,4 km
loop Maule

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