Don't Put a Stop to Regenerative Braking

Dear Car Talk:

I love your column -- it is the first thing I turn to every week, much to my spouse's consternation! You are able to explain mechanical things in a way I understand.

I have a question about regenerative braking. I have a 2019 Subaru Crosstrek plug-in hybrid, which I love. It has a regenerative braking option, which I generally use around town but have to turn off in order to use cruise control.

My question is: How does regenerative braking work? Am I doing more harm than good by using it most of the time? Am I wearing out my brakes faster, or some other part, and how much does it really charge the battery? Is it worth using? I appreciate hearing your erudite answer! -- Kelly

The great thing about electric motors, Kelly, is that they work in two directions. Every electric motor is also a generator.

So, you can put electricity in one end and have motion come out the other end. Or you can put motion in one end and have electricity come out the other. And that's what regenerative braking does.

All hybrids have two kinds of brakes. They have traditional "friction" brakes, where two pads at each wheel squeeze against a spinning disc. That slows down the car and produces heat and brake dust -- neither of which are particularly useful.

Then you have regenerative brakes. They use the rolling motion of the wheels to spin the electric motor/generator and produce electricity to recharge your battery. Essentially, they use your already-spinning wheels as a power plant.

And the reason that slows your car is because the more electricity you ask the wheels to create, the harder it is to turn the motor/generator. So, by adjusting the amount of electricity the wheels are producing at any moment, you can adjust the stopping power of the "regenerative brakes."

Most hybrids work this way: When you step on your brake pedal, the car engages your regenerative braking first. So even though you think you're using traditional brakes by stepping on the brake pedal, most of the time, you're using the regenerative braking. At some point, if the car needs more stopping power than the regenerative brakes can provide, the car switches over to -- or adds in -- your friction brakes. Software is supposed to make the switch undetectable to you.

And unlike brake pads, which are supposed to work by wearing themselves out and turning themselves into dust, regenerative braking doesn't wear out anything. You're just recapturing the extra kinetic energy of the car and making electricity with it. How much? Some cars can recoup 90% or more of rolling energy and turn it back into electricity. In fact, it's not unusual for us to see hybrid cars in the shop with 80,000 miles with brakes that look brand-new.

So, actually, now that I think about it, your regenerative brakes are putting a real dent in my IRA, Kelly. So please feel free to use them less.

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Todays Car-o-Scope

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Potential roadkill looms on the horizon, but the immutable laws of physics will provide a simple solution.
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