### Soapbox on regenerative braking

Posted:

**Mon, 15 Dec 2008, 03:45**There have been a few threads lately discussing the merits of regenerative braking (regen), but I believe many people overplay it's importance.

The

Kinetic energy is the energy of movement. When you accelerate in an EV, you are basically converting electrical energy into kinetic energy. Our example vehicle at 60km/h has about 140kJ (0.04kWh) of kinetic energy - i.e the same energy as is used for 30 seconds of cruising at 60km/h ( = half a kilometre). It is this energy with regen can very effectively recapture.

But that kinetic energy is not entirely forfeit in a vehicle without regen. Much of it actually gets "used" when you take your foot off the accelerator pedal and coast up to a stop, where instead of power from the batteries being used to push the vehicle along, it's using the vehicle's kinetic energy (which slowly reduces). So how much of the kinetic energy goes to drag and RR, and how much can go to regen?

The formula is 1/2mv^2, so energy is proportional to the

The other time you use your brakes in a car (and hence regen is useful) is to maintain your speed downhill, i.e recovering your gravitational potential energy. Of course you'll be off the accelerator pedal completely while going downhill, so much of that potential energy is again being used to push the car through the air. How much of the energy is being wasted in the brakes, or would hence be recoverable with regen?

Let's take Kalamunda Hill as one of the worst-case scenarios around Perth. According to my Garmin it's 225m change in altitude over 4km, i.e average 6% grade. Coasting down a 6% grade at 75km/h

How much will this recharge your battery pack? 4km at 75km/h takes 3.2 minutes. If your regen is recovering the extra 5kW with 100% efficiency, you'll end up with 0.26kWh more energy in your battery pack. 0.26kWh of electricity costs about 5 cents (or 1.5 cents off-peak).

So is regen useful? Of course, but let's keep it in perspective.

*Regen is only useful when you would otherwise be using the brakes.*The two cases are (a) pulling up to a stop and (b) restricting speed down steep hills.The

*vast majority*of energy used in a typical driving cycle goes to pushing the vehicle through the air (aerodynamic drag) and the tires along the road (rolling resistance) - this energy can not be recovered. A typical small car uses about 4.4kW of energy continuously to maintain 60km/h for example*.Kinetic energy is the energy of movement. When you accelerate in an EV, you are basically converting electrical energy into kinetic energy. Our example vehicle at 60km/h has about 140kJ (0.04kWh) of kinetic energy - i.e the same energy as is used for 30 seconds of cruising at 60km/h ( = half a kilometre). It is this energy with regen can very effectively recapture.

But that kinetic energy is not entirely forfeit in a vehicle without regen. Much of it actually gets "used" when you take your foot off the accelerator pedal and coast up to a stop, where instead of power from the batteries being used to push the vehicle along, it's using the vehicle's kinetic energy (which slowly reduces). So how much of the kinetic energy goes to drag and RR, and how much can go to regen?

The formula is 1/2mv^2, so energy is proportional to the

*square*of the velocity. What this means is that if you wash off just 18km/h before applying the brakes (quite feasible?), you will already have put half of your kinetic energy to good use, pushing the car through the air instead of heating up your brakes.The other time you use your brakes in a car (and hence regen is useful) is to maintain your speed downhill, i.e recovering your gravitational potential energy. Of course you'll be off the accelerator pedal completely while going downhill, so much of that potential energy is again being used to push the car through the air. How much of the energy is being wasted in the brakes, or would hence be recoverable with regen?

Let's take Kalamunda Hill as one of the worst-case scenarios around Perth. According to my Garmin it's 225m change in altitude over 4km, i.e average 6% grade. Coasting down a 6% grade at 75km/h

*(Kala Rd speed limit +5)*provides about 12kW of power from potential energy! Aero drag + RR our example vehicle at 75km/h is 7kWh. The remaining 5kW could potentially be recaptured with regenerative braking.How much will this recharge your battery pack? 4km at 75km/h takes 3.2 minutes. If your regen is recovering the extra 5kW with 100% efficiency, you'll end up with 0.26kWh more energy in your battery pack. 0.26kWh of electricity costs about 5 cents (or 1.5 cents off-peak).

So is regen useful? Of course, but let's keep it in perspective.

** Sample vehicle: 1000kg mass, Cd = 0.35, 2m^2 frontal area, Crr = 0.015. Some formulas are here.*