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Rather interesting week. It started with a longish drive in the new Speedster design. We reached about 107 miles when it seemed politic to bring it into the barn and check some batteries.

One notable thing about this was that 107 miles was the max drive we ever actually did with the original Speedster. Over the winter, we took out all the toe-in on the front end, went to a Redline MTF fluid in the transaxle, and made a couple of other minor tweaks that might perhaps get us another mile or so. But I never did get around to actually doing the drive.

But the new Speedster design features an AC induction motor and 3 phase Curtis controller that has more than adequate provisions for regenerative braking. As we are using a very similar battery pack between the two cars, we should have gotten SOME increase in range from the regenerative braking.

Regenerative braking, of course, is the concept of using the motor as a generator to slow down and stop a vehicle, instead of using the “friction” brakes that basically convert forward motion into heat in order to stop the vehicle.

It is fairly obvious to anyone that this must offer some efficiencies for an electric car. And indeed, the idea is not only not new, but has been actively discussed and deployed for some thirty years.

I have for some time suspected all is not as it seems in this. The thing that bothered me was the law of conservation of energy. Broadly applied this indicates that a mass in motion will tend to stay in motion, and the energy of interest is usually referred to as kinetic energy or “stored” energy held in the forward inertia of the mass. But its best and highest use is of course to continue in motion. If we convert it to electricity and try to put it back in a battery, we naturally fall prey to all the inefficiencies inherent in that.

In a real drive of a real car, how much of this regenerative braking would actually be useful? While the concept of driving a car down the street doesn’t sound like rocket science, there are really a LOT of different things going on more or less simultaneously and in various sequential time frames that is in reality quite complex. Yes, we do accelerate, and yes we do brake, but we do a lot of other things as well, and so does the car.

So I had always said it is probably a little overhyped, and my sense was regenerative braking could provide 6-8% efficiencty gain, not the 15-25% I’ve heard bandied about.

More recently I fell under the spell of a fascinating speaker, Dr. Illah R. Nourbakhsh. Dr. Nourbakhsh is Associate Professor of Robotics, The Robotics Institute, Carnegie Mellon University. He was the former Robotics Group Lead,
Ames Research Center, National Aeronautics and Space Administration.

Dr. Nourbakhsh drives a RAV-4 EV, and has noted some startling efficiencies in real world driving. Unfortunately, on reviewing his data, it becomes apparent that he actually measures AH consumed going forward, AH produced during regen, and assigns efficiency values based on those measurements. Bu the hasn’t actually done anything with actually measuring how much energy is required to move the car. This tends to ignore driving dynamics.

He has started a web site and is aware of the concept that regen will do various things on various driving patterns, and indeed his web site at purports to collect actual drive data from various people across the country. But he appears to be focused on variations in the two measured quantities – AH out and AH in.
They are actually working on a design for an ultracapacitor implementation that will make capture of this regenerative braking more efficient.

Unable to resolve all this, we set up three tests.

In the first test, we drove the car 76.6 kilometers with full maximum current regenerative braking tied to pressure applied on the brake pedal. We drove the route almost entirely in 2nd gear. And we ignored how much was coming out and going back in, and looked for the TOTAL AH consumed on the 76.6 mile drive. We drove through very urban area mostly with dozens of traffic stop lights and signs, lots of vehicular traffic, and plenty of hills and curves that should have made regenerative braking quite attractive.

In test two, we drove the same route at the same temperature and SOC and again used 2nd gear all the way. But this time we added neutral braking. Neutral braking is braking applied by using the motor as generator but driving that with the accelerator pedal. The car accelerates of course when you press the brake pedal. But when you start to release the throttle pressure, regenerative braking kicks in. I missed a turn on this lap and so we totaled 75.2 km.

In our third test, we drove the same lap under the same conditions but with ALL regenerative braking disabled.

Obviously, we expected maximum efficiency from the more aggressive regen derived from brake and throttle both.

We would expect the 2nd greatest efficiency from the car with regen deployed only on the brake signal.

And NO regen should bring up the rear.

Surprisingly, here’s what we found.

Drive 1 76.6km 78.2 AH or 1.02088 AH per kilometer
Drive 2 75.2km 77.7 AH or 1.03324 AH per kilometer
Drive 3 76.6 km 78.0 AH or 1.01827 AH per kilometer

This shows the LEAST amount of energy used to accomplish the 76.6 km drive to be when using NO regenerative braking at all. The differences were pretty close – probably in the noise level. But what IS apparent immediately is that regenerative braking in ANY form would appear to have no efficiency additions AT ALL.

Yes there were plenty of AH going out and coming back in during regenerative braking, but over the course of the drive, it appears to have nulled out to zero as a benefit.

I found this a bit astounding. Chevrolet’s Volt, the Tesla Roadster, almost ANY serious attempt at an electric drive train pays homage to regerative braking. Has NO one ever done an end to end test to see if it actually worked?

I’d repeat the test, but I haven’t a clue what I would do different.

Jack Rickard