Since electric trucks do not have an unlimited supply of energy onboard, everything possible is done to make them as energy-efficient as possible. This includes taking advantage of the resistive properties of electric generators to assist in decelerating the truck.

This is called regenerative braking.

In conventional trucks, or any vehicle for that matter, brakes are used for deceleration. We think of brakes in terms of applying friction between the brake lining and the brake drum, or between the disc and the rotor, to slow the vehicle.

That’s what happens mechanically, but in engineering terms, the brakes are designed to convert the kinetic energy in the vehicle’s motion into another form of energy: heat.

The motion of the vehicle turns the wheels. The driver applies the brake, which forces the two surfaces together, causing resistance to the motion. As the truck slows and more kinetic energy is absorbed by the brakes, they get hot, and that heat energy is dissipated into the atmosphere.

In fact, that energy — and there’s a lot of it behind an 80,000-lb. truck traveling at 100 km/h — is wasted. It’s unrecoverable.

With electric vehicles, including fuel cell-powered trucks, kinetic energy can be recovered by using the motor as a generator.

“With regeneration, all we’re doing is reversing the polarity of the electric motors and turning them into electric generators,” says John Moore, electric vehicle product manager for Volvo Truck North America. “One of the advantages of electric trucks is we can recoup some of the braking energy and put it back in the batteries instead of losing it all as heat through the brakes.”

Motors and generators are one and the same

Fundamentally, an electric motor and an electric generator are the same thing, but in practice they work in opposite ways.

When an electric current flows through an electric motor, it creates an electric field which causes the magnets in the motor to rotate in a given direction. If you reverse the polarity of the current flowing through the motor, the motor will rotate in the opposite direction.

Quick Fact: most electric trucks do not have a reverse gear in the transmission. To make the truck travel backward, the electronic controllers reverse the polarity of the current flow from the battery, and the motor turns in the opposite direction.

In the case of a generator, the same physical device as a motor with copper wire windings and magnets, if some external force is used to turn the shaft of the generator, it will produce electric current.

Think of a wind turbine or the generators in a hydro-electric dam. In this example, wind or water flow causes the generator to rotate, which generates electricity.

Engineers refer to these differences as putting positive or negative torque on the driveline. Positive torque propels the truck forward with energy flowing from the battery through the motor to the wheels. Negative torque on the driveline means the wheels are turning the motor which creates electricity that flows back into the batteries.

“It’s similar in principle to a diesel engine in a truck providing power to the wheels while an engine brake acts as a giant air compressor driven by the wheels and the kinetic energy of the truck,” says Christian Appel, global head of product and programs at Nikola Motors. (Cash-strapped Nikola has since filed for creditor protection as it looks to find a buyer for itself).

Making energy as you drive

The capacity to store energy in a battery is limited by the size of the battery. In practical terms, a battery can only weigh so much and take up so much frame space. Thus, the range of the truck is constrained by the battery capacity.

“With electric trucks, there’s not a supply of energy that’s equal to a diesel and readily replaceable,” says Moore. “You can easily exhaust it. It’s not like diesel. So, in addition to making the trucks as efficient as possible, we also have the ability to recoup some of the braking energy and put it back in the batteries. In essence, we can make electricity as we drive, through braking.”

How much energy can be recovered varies with the application, the weight of the truck, and the terrain. Regen braking benefits city driving the most, where there’s more stop-and-go driving. Like any other truck, the heavier it is, the more brake force is required to stop it. Thus, more energy can be recovered.

Regen braking is generally not as useful in highway driving because you don’t do as much braking. But that’s not universally true.

Before Nikola filed for bankruptcy, it did a lot of testing on Interstate 17 between Phoenix and Flagstaff, Ariz. The difference in elevation between the two cities is about 6,000 ft (1,825 meters). Traveling south fully loaded, descending more than a mile in altitude, the Tre battery-electric truck could regenerate over 150 kWh of energy.

“Regen braking on those severe downgrade routes, we could recover enough energy to fully charge two Tesla Model 3 passenger cars,” Appel says.  

For regen braking to be most effective, the battery needs room to accept the charge. Moore says Volvo’s VNR Electric trucks won’t take a regen charge until the battery is down to about 93%.

“By the time you get to about 70% state of charge in the batteries, we then have full potential of regen we can put back into the batteries,” he says.

Regen braking works the same way with fuel-cell-electric trucks. They use batteries too, but because the battery is small, sometimes only half the capacity of a full battery electric truck, or less, managing the regen opportunity charging can be trickier.

The fuel cell consumes hydrogen to charge the battery, so efficiency is still critical, so recharging with regen braking becomes very advantageous. 

Regen braking can also be managed with geofencing and predictive cruise control, where the truck knows its location and can map out optimum charging opportunities along the route.

The driver’s role in regen braking  

Moore says that when drivers first get onto an electric truck, they tend to drive it like they stole it. They really like the performance, he says. After the novelty wears off, they get more serious about efficiency and learn how to maximize the potential of regen braking.

Basically, when you step on the accelerator, it goes. When you take your foot off the pedal, it coasts and slows down. There’s also a multi-position switch or a stalk mounted on the steering column that functions the same way as the controls for an engine brake in diesel.

It controls the level of braking torque the motor produces. There’s a range of regen power from position 1 (the lightest) to position 4 or 6 — depending on the truck maker – (the strongest).

Most trucks also have an automatic mode where the truck will optimize the regen braking action to suit conditions and terrain.

Unlike a diesel engine brake, regen braking is not an on-or-off proposition. The position of the accelerator pedal modulates the amount of braking torque the motor will apply.

For example, if the stalk was in position 2, and your foot was pressing the accelerator pedal down by 50%, the electronic controls would sense that you are not trying to decelerate. If you move the pedal to 25%, the motors would no longer be producing positive torque (power), but you might actually get 50% of the available negative torque (stopping power) in stalk position 2. If you took your foot off the accelerator, you would get 100% of the negative torque available in position 2.

The amount of negative torque is not linear in relation to the pedal position; we’re using these numbers to illustrate the point.

Depending on the weight of the truck, the speed, the traffic conditions, etc., drivers soon learn to manage the decelerating effect of regen braking, controlling speed without using the service brakes. This is known as one-pedal driving.

Obviously, there are situations where the service brakes are needed to slow the truck. Experienced electric truck drivers seeking the maximum efficiency and range from their batteries learn how to coast with traffic and begin decelerating early for traffic lights so they can avoid using the service brakes and generate the most regenerative charge at every stop.

Route planning and driver training

When determining which delivery routes are most suitable for electric trucks, fleets take the regen braking potential into account in order to maximize range.

For example, if the terminal is at a higher elevation than most of the stops on the route, a truck could be dispatched to run downhill while it’s fully loaded, and return to the terminal, traveling uphill, when it’s empty.

That’s obviously over-simplified, but those sorts of considerations can extend the advertised range of the truck but as much as 25% or more in some cases. Every little bit helps.

As noted earlier, drivers love the power and torque of electric trucks. Once they get over the initial excitement, they become more interested in learning how to drive them as efficiently as possible. OEM will provide all the training they need, but the best training opportunities come through mentoring and gamification.

Teaching drivers the principles of one-pedal driving opens the door to energy savings that can expand the routes and save a lot of brake wear. Volvo’s Moore says brakes on electric trucks will often last up to 30% longer than brakes on a diesel truck in the same operating environment.

Expect a bit of a learning curve for the driver and be prepared to invest some training resources to get the most from the regen charging opportunities.