Regenerative braking efficiency

 

We have seen some amazing advertising claims about the efficiency (>70%) of hydraulic, air, and flywheel regenerative braking and so began to wonder why we were not able to get anywhere near their results with electric motors  
Our data is showing that regen is worth a 10-15% improvement in range in Miami city traffic and a 15%-20% gain at the track.  

In Miami traffic, we were able to use the regenerative braking exclusively.
Due to the extreme braking requirements of the race track, we blend friction braking with regenerative braking.

The type of driving is a big influence on how much energy is reused through regenerative braking. Any driving that uses the brakes more, will offer more opportunity to recover energy.

To get an idea of the optimum effectiveness we could expect from the regenerative braking system of the Electric Imp, we did a series of test runs where the car would:
1) accelerate from a stop [at a fixed current limit of 50 amps] to around 30 MPH, then
2) immediately regenerative brake [at a fixed current also 50 amps] back to a dead stop.

The low speed was chosen to minimize the effect of aerodynamic drag. The test was done on a flat windless road and repeated in both directions.

When we totaled the Watt-hours out, from the battery pack accelerating and the Watt-hours that went back into the battery pack, from the regenerative braking, we recovered around 25% of the energy.

In 696 feet, 80.5 W-hrs out, 22.5 W-hrs back from regen, so 58 W-hrs total used. Since there are 5,280 feet in a mile, the distance was about 0.128788 of a mile. That means 625 W-hrs per mile, 450 W-hrs per mile with regen.

We might conclude erroneously that regenerative braking is only able to capture 25% of available energy.

 
 
   
When the data is graphed out using changes in kinetic energy it reveals a different story.  
   
Graph: Changes in kinetic energy -Power from Batteries and Kinetic Energy gain

Changes in Kinetic energy

   

The purple line is Watt hours being drawn from the batteries. The green line is Watt hours of Kinetic energy that the car now holds. The difference between the two is power lost in translation from electrical energy to motion. This power is either lost in the inverter and motor or used up overcome drag.

By the time we reach 30 MPH (at around 450 feet), we have managed to translate just over 50% of our electrical energy into kinetic energy. Our cumulative kinetic energy peaks at 40 Watt hours and we have pulled a total of 77 Watt Hours from the battery pack.

We regenerative brake and recapture 22 Watt hours, 55% of what was available.

 
 
What happened to the missing energy?  
   

 

 
Fig 1: Efficiency Low RPM  

Efficiency Low RPM

 

We do not have an efficiency graph for our specific motors.

Fig. 1 above comes from Meticmind.com and is for the same inverter, but a different model of the same motor brand running at a much lower voltage.

 

Our RPM peaked at 1735.
Our Wattage was less than 19kW.
The motor is operating in an area where its' cousin would be less than 70% efficient.

   

Conclusions?

For one, we can document that electric regenerative brake systems are able to recover 55% of the vehicle's kinetic energy. I suspect it would be easy to get over 70% if we changed the gearing to improve efficiency.

The real lesson is that if you want to recover more total energy, start by being more efficient in translating the electric energy into kinetic energy.
That is where the biggest losses are.

 
 

 

Tracking efficiency
back to Lessons Learned menu
Improvement #1 Gearing

 


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