Regenerative Braking: Enhancing Control and Comfort in Electric Vehicles

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