Dear Tom and Ray:

I am always irritated by people who have their accelerators pressed right up until the moment they apply the brakes. For example, I might be a half a block from a red light and will start coasting in anticipation of the stop. Someone behind me will swerve into the left lane, accelerate past me, and then I will pull up next to him at the light, having lost the race. This, it seems to me, is a great way to use extra gas. But with the new regenerative brakes on electric and hybrid cars, it may no longer be such a stupid maneuver. What percentage of the energy a car uses to accelerate is gained back via regenerative braking? I'm guessing about half, but if it's 90 percent, it might not make much difference anymore if you drive stupidly, at least from a cost standpoint.

-- John 

TOM: Yeah, it's still a stupid way to drive, John.

RAY: Cars that use regenerative braking can capture half, or even a little more than half, of the energy that would otherwise have been lost to heat during braking. That's a wonderful thing, no doubt about it.

TOM: But if you keep spending a dollar and getting back 50 cents, you still will go broke eventually. It'll just take longer.

RAY: "Regenerative braking" is kind of a misleading term, because it doesn't really apply to the brakes, as we think of them.

TOM: What it does is use your car's wheels, which are already turning, to generate electricity. That electricity can then be sent to a battery, where it can be stored for later use.

RAY: When the wheels are powering the generator, the generator provides resistance, so the wheels naturally slow down. That's the "braking" part of all this.

TOM: And what's so clever is how hybrid- and electric-vehicle makers use both that resistance and the traditional brakes to slow and stop the car.

RAY: When you step on the brake pedal, the car's electronic braking controller determines how much braking is needed, how quickly, and how much electricity the battery can accept and store at the moment. Then it figures out whether to get the braking from regeneration, the mechanical braking system or some combination of the two. And if it's done well, with well-designed software, you, as the driver, don't know the difference.

TOM: So, when you race ahead to a stoplight and then hit the brakes at the last minute in a car with regenerative braking, you do recoup some of that energy that would previously have disappeared as heat from the friction of the brakes. But you don't get all of it.

RAY: In fact, the more urgently you need to stop, the more likely the mechanical brakes will have to be called into action, which means you'll get even less recouped through regeneration.

TOM: So we don't recommend this style of driving, even if you have a hybrid or electric car, John.

RAY: Here's the final reason why: Even if you don't waste as much energy as you appear to be wasting, you still feel like a jerk when the guy you annoyingly raced past pulls up next to you at the light with a smug look on his face and smiles at you.