What is Regenerative Braking?

Since there is a very small cache of electrons in the battery, we have to squeeze out every possible efficiency. There are several ways we can do this:

  • Better aerodynamics — wind resistance of the car moving through the air is energy lost. There is no way to retrieve this energy. What’s worse, the amount of power required to move a car through the air goes up as a square (power of 2) of the speed!
  • Lighter weight — the less mass we have to get moving, the less power it will take. This is somewhat of a battle, since batteries used in EVs are relatively heavy.
  • Regenerative braking

The last point, regenerative braking, is one of the easiest ways to get back a portion of the energy you’ve expended getting your vehicle to move. How does this work?

In regular vehicles, friction brakes are used to slow the car down. The energy of the car moving (inertia) is disbursed as heat in the friction brakes. That heat cannot be captured and therefore is lost.

In electric vehicles, we have an electric motor that gets us moving. What some people don’t realize, is that the motor can also be used as an electrical generator. If there is an electrical load on a generator, it takes more effort to turn the generator. With regenerative braking, a load is put on the motor/generator to charge the batteries. This creates more effort to turn the motor/generator, creating a braking effect. This would be similar to us being able to capture the heat energy in friction brakes and using that to power the car.

batavia-regen

Tests with regenerative braking put the maximum braking power at around 1G of stopping power. That’s very good. However, most regenerative braking systems aren’t that aggressive with the recharge. It would make the car very difficult to drive smoothly. Instead, the vehicle manufacturers add friction brakes to supplant the regenerative capabilities of the motor/generator. Additionally, regenerative braking is not nearly as effective as friction brakes at low speeds.

Many electric vehicles now have different levels of regenerative braking that can be user-set. For example in my Mitsubishi, the shifter has “D”, “Eco”, and “B” settings.

2012-Mitubishi-i-MiEV-shifter

  • D is for drive, where the car behaves very much like a regular, gas-powered car. This mode has the least amount of regenerative braking.
  • Eco slows the accelerator to help reduce power to get the car moving, and slightly increases the regenerative braking effect.
  • B is for braking, which gives you full power like D mode, but gives aggressive regenerative braking effect. This is great for stop-and-go traffic, because you can virtually drive the vehicle using only the accelerator, rarely touching the brake.

This graph shows the regenerative braking strength for the three modes:

Regenerative braking modes in the i-MiEV.

Regenerative braking modes in the i-MiEV.

You can clearly see that D has the least amount of regenerative braking, Eco has a bit more, and B has the most amount of regenerative braking.

Why wouldn’t you want to drive in B mode all the time? Well, it’s difficult to control the car. In a gas-powered car, when you release the accelerator, the car coasts for quite a distance. In B mode in the Mitsubishi, it’s like the brake is being pressed with light pressure. It takes some practice to drive in B mode all the time and be smooth about it.

That being said, I drive my i-MiEV in B mode all the time. I love a challenge.

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