Dear Car Talk:
I just purchased a new Hyundai Ioniq 5 electric car and love it. I use the highest level of regenerative braking, so the car slows down quite a bit every time I lift my foot off the accelerator, and I rarely have to use the brake pedal.
My question is: when this happens, I don’t think the brake lights go on. So, the drivers behind me can’t tell if I’m slowing down. Seems like it is dangerous. What do you think? — August
Great question, August. Actually, your brake lights probably are coming on.
All electric vehicles have a technology called regenerative braking. Traditional cars use friction to reduce speed. A couple of metallic pads squeeze a metal rotor attached to each wheel, and as the car stops, that creates heat and dust, neither of which is terribly useful.
EVs, on the other hand, use the motion of the wheels to make electricity. When you first step on the brake pedal, the wheels power a motor/generator that makes electricity and sends it back into your battery. And because it takes effort to turn that generator, that slows the wheels.
If you need more stopping power than the generator can provide, the traditional friction brakes take over, and it’s all managed by computer.
But what about the brake lights, you ask? Well, the Department of Transportation sets standards for when brake lights must come on. And it has to do with the rate of the car’s deceleration.
So, your Hyundai has an accelerometer that measures deceleration in meters per seconds squared. And before the car’s deceleration rate hits the DOT limit, it turns on your brake lights — whether you touch the brake pedal or not.
The same type of device, by the way, is often used in airbag deployment. If deceleration occurs almost instantly, the computer knows you’ve hit something, and you’re saying hello to your airbag.
If you want to confirm that your accelerometer works, August, have a friend follow behind you and get him on phone. Then try using the regenerative braking at different settings, and ask him to let you know when he sees your brake lights come on.
And if he doesn’t see them and rear-ends you, well, you already know him so you won’t have to exchange information.
Got a question about cars? Write to Car Talk write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at www.cartalk.com.
