Electrathon America Forum

Date: Nov 11, 2009

Regenerating breaking

electrathon · administrator

Kirk Swaney has his set up that way. Remember that you will loose your ability to freewheel or coast into corners, but you will get some charge back when slowing down.

Zaine Stapleton · Senior Member

Actually, that isn't true anymore with Kirk. He was having problems with the regen and took it off the car.

Date: Nov 13, 2009

Oh really. Well thats good I guess, one less thing we have to worry about...

Kirk Swaney · Veteren Racer

If you're interested, here is a short article written about the regerative braking controller I once tried on an Electrathon car. biggrin

Date: Sep 12, 2010

Nathan McCaw wrote:

Has anyone put in a regenerative breaking system in their car, and got it to work properly? If so where do you start?

For ReGen to provide a significant benefit, the efficiencies of the motor, drive electronics, and storage system must be high enough to realize a significant gain. As a rule of thumb, most lead acid batteries require 25% more energy to charge than they actually store (IE 1250 WHr in, for 1000 WHr out, is 80% efficient). The motor/drive system may only be 80% efficient as a traction motor, and only 80% efficient as a generator feeding a diode rectifier. For these conditions, ReGen will only recover 0.8 * 0.8 * 0.8 * 100 = 51.2% of the kenetic energy best case.

This poor recovery is made worse by several factors. Braking frequently requires significantly higher energies (current) than acceleration does. The electrical losses are the square of the current times the resistance. So losses go up with the square of the braking force, further dropping the efficiency. If you break to slowly, then drag losses steal the energy before you can recover it.

Many battery storage systems can only be charged at 1/10th the current they can supply current. Charging too fast will damage the battery. This requires oversizing the batteries for the peak ReGen current, or using a large capacitor bank to store all the joules of the fastest speed the car can travel. This is fequently more expensive than the batteries. For stop and go commercial applications like a transit bus or trash truck, this can be significantly cheaper than the fuel used.

The back EMF of the motor will quickly drop below battery voltage in most cases, requiring a boost circuit to increase the back EMF up to storage charge voltage. This boost circuit is likely to be a PWM boost mode switching supply, that will also only operate at about 80% efficiency.

The net result is that real system efficiencies can easily be as low as 20-30% usable recovered energy using the common off the shelf components and solutions that most EA teams have access to. All the rest of the energy turns into heat in the motor, drive electronics, wiring, and batteries.

There are ways to significantly improve this using designs that push efficiencies for each component well above 95%, for a system level recovery of better than 90% of the available KE. Nothing that is affordable for EA teams that I'm aware of.

John Bass, Sr. Engineer, DMS Design