Engineers and technologists are improving the automotive industry with electric vehicles that are increasingly efficient. One of the key technologies contributing to their energy efficiency and range optimization is regenerative braking. In this blog post, we will look at regenerative braking, how it works, and its role in enhancing the performance of popular EVs like Tesla, Ford Mach E, Rivian, Polestar, and almost all electric cars. Additionally, all 5000 Amazon Rivian electric delivery vehicles on the road and all Tesla Semi have regenerative braking. 

Understanding Regenerative Braking

Regenerative braking is referred to by insiders as “one-pedal driving” because the vehicle slows down, or “brakes” when the accelerator pedal is released. In addition, this technology allows EVs to recover and store energy during the braking process. In conventional internal combustion engine cars, braking dissipates kinetic energy as heat through friction. In contrast, regenerative braking converts this energy into electrical energy. This process helps recharge the EV's battery and contributes to its efficiency.

How Regenerative Braking Works

When you slow an EV, the electric motor operates in reverse, effectively becoming a generator. The kinetic energy of the vehicle is converted into electrical energy. This electricity is then directed back into the battery for later use. The efficiency of regenerative braking varies among different EV models, with some offering more aggressive regenerative braking profiles than others. In a blog post on Tesla’s website, Greg Solberg provides an explanation of regenerative braking in the early Tesla Roadster.

Tesla Roadster Regenerative Braking

Solberg describes it as the process of converting the vehicle's kinetic energy into chemical energy stored in the battery. Regenerative braking also functions as a means of slowing down the vehicle. The term "regenerative" comes from the energy being recaptured and stored in the battery for future use. Solberg explains, “The kinetic energy possessed by a moving vehicle depends on factors like its mass and speed, as indicated by the equation E = ½mv². In simple terms, if a vehicle is twice as heavy, it possesses twice the kinetic energy. Likewise, if it's moving at twice the speed, it has four times the kinetic energy. When a vehicle decelerates, the kinetic energy it carries needs a destination for dissipation.” 

In a traditional gasoline car, the majority of the kinetic energy is transformed into heat by the brake pads when the driver applies the brakes.

To understand regenerative braking further, we will look at how it applies to the Tesla Semi, a vehicle with a much larger mass. 

Tesla Semi: A Marvel of Physics

The Tesla Semi can reach 500 miles of range. One obvious reason the range is great for the Semi is the advanced aerodynamics. Semi is built like a bullet, not a barn wall. 

^{Aerodynamic Semi}

Let's take a look at the powertrain for the Semi, which includes three advanced carbon-sleeved rotors, similar to the Tesla Model S Plaid motor. One rotor is for efficiency/power generation, and two are for acceleration. It is notable that all three motors are not used at the same time in all cases to drive the Semi. The Semi also has a massive 900kW/h Battery Pack. 

^{Triple Plaid Motor Powertrain}

When the Semi drives up an incline, it uses energy from the battery pack, but when it goes back down the mountain on the other side, the battery pack is charged through regenerative events. As with smaller vehicles, its estimated that regenerative braking adds 10-15% more range with city driving and a negligible amount with flat highway driving, under optimum conditions such as an extended trip downhill, regenerative braking can recharge a vehicle up to 50%.

^{Tesla Semi Regenerative Braking Events}

An interesting observation is that for a battery to capture energy, it is an advantage if the battery has a bigger reservoir to allow for ample room to store that energy. 

When a battery is recapturing energy during a regenerative braking event, it does not have the same luxury as you find when you Supercharge a small vehicle, hence the regen limit in the Semi is around 70 - 100 kW/h.

In contrast, I can Supercharge my Model 3 at the faster rate of 250kW/h if the battery is 1) preconditioned, and 2) at a low state of charge.

For comparison, my Tesla Model 3 Battery has about 78.5 kW of usable energy storage, while the Semi has around 900kW/h of usable storage, which is 11 times more than the Model 3 I drive. Model 3 employs regenerative braking when in motion without the accelerator pedal pressed, slowing down the car and returning excess power to the battery. This helps extend the vehicle's driving range. However, the effectiveness of regenerative braking may vary depending on the battery's condition, like being cold or fully charged. Tesla allows owners to maintain consistent deceleration regardless of the battery's state, by enabling the traditional braking system to engage automatically when regenerative braking is limited. I enabled this feature in my Model 3 through the settings menu in the Touch Controls under Pedals & Steering by selecting "Apply Brakes When Regenerative Braking is Limited."

Brands Embracing Regenerative Braking

EV manufacturers have incorporated regenerative braking into their vehicles. Tesla, known for its cutting-edge software, frequently perfoms OTA updates to improve regenerative braking in its Model S, Model 3, Model X, and Model Y. The FORD Mustang Mach-E, Rivian R1T, and Polestar 2 also feature advanced regenerative braking systems.

My Experience

Every day, I experience regenerative braking in my Tesla because of the hills near my home in Austin and the stop-and-go traffic in the downtown. I often see the regenerative braking indicator in the vehicle. Regenerative braking also means my brake pad life is extended. I have not had to repair or replace my brakes or pads in over 90,000 miles of driving. 

EVs Benefit from Regenerative Braking

Regenerative braking represents a significant advantage for electric battery-powered vehicles compared to their gas-powered counterparts. It not only reduces costs associated with brake pads and rotor maintenance but also actively replenishes the battery while slowing down. Enabling your car's maximum regenerative setting and practicing smooth braking whenever possible can make a real difference in maximizing your EV's range. While it typically adds around 10-15% more range in city driving and a minimal amount on highways, in ideal scenarios like extended downhill descents, regenerative braking can boost your vehicle's charge by up to 50%. Over time, this adds up to substantial savings and increased efficiency. 

Special thanks to The Limiting Factor for insight into the Tesla Semi regenerative braking.