MELISSA BLOCK, host:

So, what exactly led to the latest recall of the 2010 Prius and some Lexus hybrid sedans? This time it's not the accelerator, it's the brakes. In particular, a system called regenerative braking.

And David Champion joins us to explain how that braking system works. He is senior director of automotive testing for Consumer Reports. Welcome to the program.

Mr. DAVID CHAMPION (Senior Director, Consumer Reports Auto Test Division): Hello.

BLOCK: And what is the idea behind regenerative braking? What's it supposed to do?

Mr. CHAMPION: Well, one of the advantages of hybrid vehicles is that rather than wasting energy when you put your foot on the brakes, it actually collects the energy and stores it in the battery so you can use it in the electric motor to get going. So, when you put your foot on the brake pedal it actually turns the electric motor that drives the vehicle into a generator and absorbs that energy, storing it in the battery so it can be used later.

BLOCK: So, what would be lost as friction is going back in and charging the battery?

Mr. CHAMPION: Yes, and that's why most hybrid vehicles do a lot better in city driving than they do on the highway.

BLOCK: Now, do all hybrid cars use this system?

Mr. CHAMPION: Yes, they do. That's the main advantage of a hybrid in that you are gaining the energy out of braking and to be able to use it again later to accelerate.

BLOCK: And this would only work, regenerative braking would only work with only electric motors, I'm assuming, not with an internal combustion engine.

Mr. CHAMPION: Correct.

BLOCK: I've seen this referred to as a brake-by-wire system, and if I understand this right, these cars would also have conventional friction brakes and a system that would decide which one of those is needed to stop the car.

Mr. CHAMPION: That's correct. When you first put your foot on the brake pedal to slow down, it goes into regenerative braking first so to absorb some of that energy. But the size of the motors and the amount of energy that they could absorb is not enough to slow the car fast enough for, you know, everyday driving. So, you push through the regenerative braking side of it and then go into conventional friction brakes after that.

BLOCK: Okay, so that's how it's supposed to work. Now what's going on with these models that Toyota recalled today, what's happening?

Mr. CHAMPION: Well, it seems that when you're driving along and you put your foot on the brake pedal, if the car happens to go over a pothole or bump or maybe a railway crossing, that sort of pulsing through the car momentarily turns the brakes off. So, you put your foot on the brake pedal, you're slowing down, you hit the bump and all of a sudden the brakes go away and then they come back again. So, it's more of an issue of, oh, the brakes have gone rather than the car not being able to stop because the brakes after that momentary second do come back into full function.

BLOCK: Now, why would this not be a problem with the older Prius, because it's just the 2010 model that's being recalled here?

Mr. CHAMPION: Well, each time a car is redesigned they do make changes to the software to try and improve it. In this case, they seemed to have improved it in one way but then caused a problem in another.

BLOCK: Hmm. And it's a software fix that Toyota's implementing here, they say it will take about 40 minutes. What's involved with that fix?

Mr. CHAMPION: Basically the technician will go into the braking system, download a software patch or a new program that will eradicate this problem from these Toyota Priuses.

BLOCK: Hmm. And can you tell from where you stand - Consumer Reports - of how confident drivers should be in that fix?

Mr. CHAMPION: I think they've done - it's a lot easier to fix something if you've found what causes the problem and with this Prius issue it seems to be bumps and, you know, potholes and things like that that seemed to be causing it. So, it's a lot easier to actually fix it if you know what's causing it because then you can identify where the problem is. It's some software problems that you don't know what's causing it that are a lot more difficult fix. So I'd be, you know, pretty confident that this is fixing the problem.

BLOCK: Well, David Champion, thanks for talking with us.

Mr. CHAMPION: Thank you very much.

BLOCK: David Champion is senior director of automotive testing for Consumer Reports. Transcript provided by NPR, Copyright NPR.

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