Sunday, July 19, 2009

Regenerative Braking

The AC induction motor and controller give us the option of employing "regenerative braking". This concept derives from the fact that there really isn't much difference between a motor and a generator. If you apply voltage to the motor, the shaft turns. On the other hand, if you turn the shaft, voltage comes out.

The BMW Mini-E does employ regenerative braking. And by all reports, it is a nightmare. It kicks in when you remove your foot from the accelerator. And it pretty much throws you forward into the seatbelts when it does so. Nobody likes it.

The problem is twofold. First, when you quit using the motor as a motor, and use it to convert the forward motion of the vehicle to electricity to recharge the batteries, it is actually quite controllable. By altering the excitation to the motor, you change how much power it will put out, and that is a function of how much power is available at the shaft. What this means is that you set it up so that it slowly decreases the speed of the vehicle, or really QUICKLY decreases speed, electronically. They provided NO adjustment for the driver. Assuming that would be too techie, they just hardwired it full on. So as soon as you take your foot off the accelerator, it goes into full power regen, and the car speed plunges. It's just LIKE putting on a brake hard.

The solution is pretty simple. We'll tie our regen to the brake light signal. In this way, you don't get regen until you put your foot on the brake. This makes sense, braking should come from the BRAKE not the accelerator. The brake lights come on at the lightest touch, so by just feathering the brake pedal, you'll be able to kick in regen without putting much brake into the game.

The second solution is to give the driver the control. We'll have a small knob/potentiometer where you can dial in just the amount of regen you want. On a long steep downhill, you might want to crank it up and get some juice out of it. In normal flat city driving, you might want to dial it all the way down and not deal with it. And at all times, you should quickly learn where it feels most comfortable for you.

This opens another pretty touchy opportunity. The Mini has power assisted brakes that work off of engine vacuum. We don't have engine vacuum. So we are going to have to install a small vacuum pump. The problem is not installing it, and they aren't terribly expensive. But they are annoyingly noisy.

We have opted for a unit from EVcomponents that features an oil filled muffler and is supposed to be ultraquiet at $375. Any $100 automotive vacuum assist pump will actually do but they are noisy.

But we have a target of opportunity here. It may well be that we won't want to use this pump at all. With regen on the brake light switch, we may find that regen offers sufficient braking, and we actually WANT the real brakes to only come in with dramatic pressure on the brake pedal. I suspect we don't need the vac boost AT ALL. The problem is, we won't know until the car is pretty much completed.

So we are going to install it, but have a switch to turn it off or on. You may not need one at all. We'll see.

Jack Rickard


  1. Jack; Truckers have been using the equivalent of regenerative braking for years in the form of a 'Jake Brake' or engine brake which through a dash switch, feeds engine oil instantly through solenoids to close the exhaust valves during deceleration, greatly increasing the engines braking torque to the drive line. Electric retarders used to be seen on occasion years ago, as I mentioned in another comment.
    Trucks have a second brake controller, variously referred to as the hand valve or trailer brake, which is a lever on the right side of the steering column intended (solely) to actuate the trailer brakes to straighten the rig in the event of an impending 'jack-knife'.
    Perhaps a hand brake controller, wired in conjunction with the brake light switch, could be used to dial in the amount of regen desired with out ever touching the brake peddle... Jim T.

  2. That's a pretty good idea. I've been kind of pondering something similiar. Years ago, on what we called "dirt bikes" then we had a very small lever near the throttle that acted as a "compression release." It actually opened a valve into the cylinder head and caused the engine to misfire as long as it was opened. It provided a great deal of decel going down steep hills although it sounded horrendous.

    I'm thinking a little lever like that might look a little goofy on a mini steering wheel. The advantage is that I could likely both trigger regen, and tie the regen adjustment into lever travel. Light squeeze - a little regen. Strong squeeze, much more regen.

    We've been giving some further thought to all of this. We have a vacuum pump planned for the Mini to support the power assist brakes. As we are triggering regen with the brake light signal, we're considering omitting the vacuum assist. In this way, it would take quite a bit of brake pressure to actually put on the brakes. But a little pressure would kick in the regen which might actually feel a little like power assisted braking. We might not need the vacuum pump at all.


  3. Sorry for reading this late. But I understand your comments on regen braking. Its either on or off. If it was that easy all cars would simply have an on/off switch for the brakes. They don't because it doesn't work. I've never tried the following as I never had the right vehicle to try it on, but I wondered if a pressure sensor could be inserted into the brake line and modulate the regen level as a function of pedal pressure. This way the regen braking assists the mechanical brakes and is variable as a fucntion of brake pressure. Again no experience here, but it sure seems like a possible way to seamlessly introduce controllable regen.