Friday, November 26, 2010

Regenerative Braking: How Much is Enough?

For the most part, electric cars like the MINI-E drive the same as their pollution spewing  ICE counterparts. Sure they are much quieter and the instant torque of their electric motors cannot be matched by traditional gasoline powered motors. However overall they are still cars, much the same as others just with a different energy source. The biggest difference in driving an EV as compared to a traditional car is the brakes.

While traditional cars use regular mechanical brakes, who's function is solely to slow the vehicle down, electric cars employ regenerative brakes which have two functions; slow the car down, and capture energy.
When you use your traditional brakes, you are creating energy, you just have no way to capture and use it so it is simply converted into heat by way of friction and wasted. Regenerative brakes are a mechanism which slows a vehicle by converting its kinetic energy into electricity which is then put into the cars battery to increase it's range.

The MINI-E has become notorious for it's extremely aggressive regenerative brakes. When people drive the car for the first time it can be startling how strong the regen is. Journalists have written that the MINI-E's regenerative brakes are like deploying a parachute at highway speeds. I think that's a bit over the top, but I have to admit, they are very strong and you need to drive the car for a day or two before you get fully comfortable with them.

However once you do get used to them, everybody that I have spoken with absolutely loves it and missed it terribly when they drive other cars that do not have regenerative brakes. A friend of mine, Michael Thwaite drives a MINI-E in New Jersey as I do. Michael also owns a Tesla Roadster(lucky bastard!) and he and I were talking about the two cars recently and he told me that he likes the regenerative braking of the MINI-E better than that of the Tesla because it's stronger in the MINI-E. He even told me that after getting his MINI-E he wrote Tesla and asked them why they didn't make the regenerative braking stronger on the Tesla like it is on the MINI-E!

I have come to realize that although it may be disconcerting when you first drive a car with regenerative braking, once you have a little time with it you love it. This is true for all the MINI-E drivers that I know. You learn to drive with only your right foot and use the traditional brakes only in emergency situations. I'm certain I could drive the MINI-E more than 100,000 miles before I needed brake pad replacements for how little I use them.

However recently when I took #250 in for the regular 5000 mile service, I was told that they were making a "software tweak" and to see if I notice any differences. I do and I'm not really happy with it. The regenerative braking is definitely dialed back a bit, requiring me to ease up on the accelerator earlier than previously needed to slow down in time for a turn or a complete stop. During the eighteen months I have had the car, BMW has done these software tweaks a few times and almost always it seems the regen is dialed back a bit. I guess they are trying to get the most efficient balance between performance, range and regen. I just want BMW and the other auto manufacturers working on EV's to know this one thing: The stronger the regen the better! If you are afraid that some people won't like it so strong (believe me, don't worry about that) then allow the regen to be set by the driver and make it adjustable. Just don't take away my aggressive regenerative braking, the stronger the better!


  1. Thank you. This confirms what I thought would be true. Stronger regenerative braking is preferable.

  2. How much energy is created by the regenerative braking? I have heard it doesn't do much at all to recharge the batteries

  3. Shael:

    I'm not an engineer, but I have spoken to the engineers that developed the MINI-E and are working on future BMW electrics and I have been told that effective use of regenerative braking can increase the cars range by as much as 20%. That is certainly not what you would consider negligible.

  4. Tom,
    The Tesla can't have as much regenerative braking strength because it is rear wheel drive. Any more rear-wheel braking force and the rear-end could start sliding.

  5. Thanks for that information! I'm working on the development of a sports EV and we had the idea of putting in a know with which the driver can choose between 5 regen modes. I think this is a good approach since some drivers won't be comfortable with a too strong regen