Gregory Milligan

What is regenerative braking, often called “Regen”?

Simply put, it is using the electric motor as a generator when you are slowing down or going down a hill so you put some power back into your batteries.

Wow! Free energy!

Well, no…..not quite, but more on that later.

Most electric motors can also be used to generate electricity. By spinning the motor you get electric power out. Some motors do this better than others, and some can be set up in a different way to work better as a generator.

Broadly speaking, there are two different types of motors used in electric cars, DC (Direct Current) and AC (Alternating Current). Which one you have has a big impact on whether you will get regen or not.

DC motors can be used as generators, but in EV applications they are set up in a way that gives more power, but stops you from using them for regenerative braking. This is because of brush timing. Just like the ignition timing in your old dinosaur burner, brush timing has a big effect on the power and efficiency of a DC motor. By advancing the brush timing much more power can be extracted from the motor, as well as better efficiency. The downside is that if the motor is run in reverse, or asked to generate power, the brushes are now seriously retarded in their timing and will burn out very quickly. For reverse this is not such an issue as generally a car is used in reverse only for short periods and at low power levels, not really a problem. When you are regenerating at highway speeds and asking for a lot of power, this is a big problem and your motor life will be short. Hence, DC powered cars don’t have regenerative braking.

AC motors operate on the induction principle, with no brushes to transfer electricity from the stationary part of the motor to the rotating part. With no brushes to burn out, there is no issue using them for regenerative braking, and all AC powered cars on the market as well as almost all home conversions using AC have regen implemented.

There is a third type of motor, rarely seen but not to be ignored, called “Brushless DC” which can give regenerative braking.  No production electric car uses DC or Brushless DC though. These are only seen on conversions.

So……how useful is regen? This has been a controversial subject in the conversion community, with proponents of DC motors maintaining that regen isn’t worth the added expense, and AC proponents espousing the efficiency of regen…..

In reality, regen is of limited use (but still useful) where the terrain is flat, and really REALLY awesome where the terrain is hilly.

Regen feels like engine braking in a petrol powered car. That feeling when you lift off the accelerator and the engine slows you down. How much it slows you down depends on what level of regen is set on the controller. Some cars allow you to dial up the amount of regen you want, and most give you some degree of control. On the Mitsubishi i MiEV that I drive, the regen is set using the gear selector. Now an i MiEV actually has no gears…reverse is the motor turning backwards and drive is the motor turning forwards…but the other positions (Eco, Coast, Brake depending on which year model you have) select different amounts of regen. On the Mitsubishi Outlander PHEV there are paddles behind the steering wheel which you can use to dial regen up or down. It is lovely to be able to set the regen on a long downhill section to hold the speed you want without touching the brake or accelerator and watch the fuel gauge go up! See a red traffic light up ahead? Dial regen right up and you won’t have to touch your brakes until the last moment…one reason why EV brakes last so long!

So…this regen stuff is awesome, it means I can go up hills and get all my energy back coming down the other side right?

Not quite…every time energy is transformed from one state to another there are losses. Some of the power you took from the battery was lost in electrical resistance, some was used to run the controller, and some was lost as resistance and heat driving the motor…and all of those losses happen again on the way from the wheels back to the battery. Generally speaking, if you drive up a hill then coast back down again using regen you will get back around a third of the energy you used to climb the hill. So, regen doesn’t make hills go away, but it makes them a lot more bearable than if you didn’t have regen….the most efficient way to drive is still to coast as much as possible, and use any form of braking only when you actually need to slow down, but if you do have to slow down regen is a much better way of doing that than using the foot brake to turn all of that energy into heat!