How It Works: Regenerative Braking

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In the past, brakes were just brakes — you pressed the pedal and your vehicle slowed down. But as more cars come equipped with electric motors – hybrids or completely battery-powered – the big deal is regenerative braking.

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This system captures kinetic energy during deceleration, storing it in the battery so it can be used as electricity to power the electric motor.

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This is why conventional hybrids don’t need to be plugged in: they use regenerative braking to recharge their batteries as they’re being driven. Electric vehicles (EVs) run primarily off the charge they stored when plugged into an outlet, but use regenerative braking to help top up the battery.

In addition to the regenerative system, all electrified vehicles have conventional braking systems as regular vehicles do. These use metal discs, called rotors, that are located behind the wheels and which turn with them. When you press the brake pedal, the pressure of hydraulic fluid squeezes metallic brake pads tightly against the rotors, and the resulting friction slows the car.

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That friction converts kinetic energy to thermal energy, and the brakes get hot. The heat dissipates – automakers design everything to cool very quickly, because hot brakes don’t work as well – and that energy is lost. The idea behind regenerative braking is to capture that otherwise-wasted kinetic energy and put it to use, converting it to electricity.

Capturing kinetic energy from braking

With an electrified vehicle, the electric motor drives the wheels, either in conjunction with the gasoline engine as in a hybrid, or on its own in a battery-electric vehicle. As you drive forward, the motor runs in that direction, supplying electric power to the wheels.

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But when you decelerate by taking your foot off the throttle, the electric motor stops supplying power so the vehicle will slow down. When the motor stops, it immediately disengages, and then starts running backwards. The transmission is still in Drive, so it doesn’t reverse the wheels; instead, it acts like a generator. It captures the kinetic energy from the wheels as they slow down, and converts it into electricity. It’s then stored in the battery, to be sent back to the electric motor when it’s needed to drive the vehicle’s wheels.

“One-pedal driving”

How much energy is captured can depend on how the system is configured. With some, the driver can decide how much regenerative braking to use, usually by moving the gearshift lever while slowing down. When more aggressive braking is selected, the system will capture and store more energy, but the vehicle will also slow down sooner, and in some cases, may come to a complete stop.

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Automakers often refer to this as “one-pedal” driving. With practice, drivers may be able to get through heavy traffic by only using the throttle, accelerating enough to move ahead as needed, and then letting off the pedal and letting the regenerative braking make the stop.

But a regenerative system isn’t enough to stop a vehicle in every situation, especially when driving at higher speeds, and this is why every hybrid or battery-powered vehicle also has conventional hydraulic brakes. The good news is that because the regenerative system is also slowing the vehicle down, the brakes don’t have to work as hard. Drivers usually find their hybrids or EVs require new brakes far less often than regular vehicles do.

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Driving behaviour determines energy captured

Hybrids generally get better fuel economy in city traffic than on the highway, and it isn’t always because of speed. They need regenerative braking to charge their batteries, but if you maintain a steady speed and don’t slow down on the highway, the battery is feeding the electric motor (which will either augment the gas engine, or run the vehicle by itself, depending on driving conditions) without getting anything back from the regenerative system. When it gets too low, the battery stops working with the gas engine, and instead siphons some of its power to recharge.

How much energy a regenerative braking system captures depends on several factors — the driver being one of the most important ones. It’s estimated that a system’s ability to capture energy can range from about 16 to 70 per cent, and that’s all in how the vehicle is being driven.

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The highest rates of return are achieved when drivers slow down well in advance, while those who storm up to a stop and slam on the brakes at the last moment see the least amount of efficiency, especially since they’re depending heavily on the vehicle’s conventional brakes. It really is necessary to adjust your driving in an electrified vehicle if you’re going to get the most out of it.

And that’s only part of the bigger picture when designing EVs and hybrids to be as efficient as possible. A larger, heavier vehicle will have more momentum, with more kinetic energy to capture — but then it will take more energy to get it moving again from a stop than a smaller vehicle would need. A regenerative system adds weight and complexity, which increases the cost. And sometimes energy has to be wasted if the battery’s full, because it can’t be overcharged. It’s all part of what keeps automotive engineers awake at night.