ACCELERATING TOWARDS THE FUTURE: THE RISE OF ELECTRIC TRUCKS
I was recently asked: “Electric buses exist. But will heavy trucks also electrify?” The implicit question was whether battery powertrains can be effective in freight vehicles. Electric trucks are coming fast, but they require a different set of design considerations than passenger electric cars or even buses.
A heavy truck, also known as Class 8 truck, is a vehicle with a gross weight above 15 metric tons. They are used for hauling heavy freight. Semi-trucks are among the most common in this category. In the United States, annual sales reach approximately 250,000 vehicles divided among major names such as Daimler Trucks, Volvo Trucks and Paccar. Some of the brands one may see on major interstate highways are owned by these manufacturers. For instance, Freightliner is a division of Daimler Trucks, Mack is part of Volvo AB and Peterbilt is a Paccar brand.
The trend towards electrification of heavy trucks received much attention as Tesla publicized its Semi, though all major manufacturers have their own respective initiatives. There were approximately 2,000 electric heavy trucks sold in 2022 – it is a very small figure of the total market but highlights the opportunity ahead for electrification.
The use case of a heavy truck is vastly different than that of a passenger vehicle. Consequently, the requirements of the electric powertrain, including the battery system, are very different. Whereas a passenger vehicle may see use for only a few hours each day, a heavy commercial vehicle operates for 16 to 22 hours each day, 350 days each year, under some extreme conditions ranging from -30 ℃ to + 50 ℃ and intense road vibrations. Towing a load requires high torques on the motors resulting in high current loads on the battery. A battery in an electric truck may undergo 2 to 3 charge cycles per day, and may receive fast charging once or twice a day.
Such operating conditions place significant constraints on the battery design. Owing to its heavy weight, the efficiency of the powertrain is typically between 1 and 2 kWh per mile – about ½ to ⅓ that of a passenger car -- which dictates the size of the battery for a given driving distance. The Tesla Semi has a battery capacity of up to 875 kWh for a claimed driving distance of 500 miles (800 km). The battery in the Volvo VNR Electric series is a little smaller at 565 kWh, boasting a driving range of about 275 miles (450 km). Naturally, the actual driving distance depends on road conditions and overall weight of the loaded vehicle. Power is another key specification. Typically, three electric motors provide a total power output between 300 and 500 kW (approximately 450 to 650 hp) – at par with its combustion-engine counterpart.
By comparison, the battery size of an electric sedan is closer to 75 kWh, a small fraction of the battery in a heavy truck. Truck batteries are assembled as super packs, i.e., connecting 4 to 6 battery packs in parallel, with each pack providing about 100 kWh operating at 400 V (think of 6 Tesla Model S working in tandem). Simple calculations indicate that at peak power, the motors may be drawing over 1,000 A from the battery!
The packs are assembled from individual lithium-ion battery cells sourced from suppliers such as LG Energy Solution, Samsung SDI, or Panasonic. Tesla historically used cylindrical cells from Panasonic. Cell naming convention often lack creativity. Called 2170 or 4680, they refer to the diameter and height of the cylinder (e.g., 21 mm x 70 mm). A 2170 cell has a typical charge capacity of about 5 Ah, whereas the 4680 offers about 9 Ah. To convert from charge capacity (Ah) to energy (Wh), we multiply the charge capacity by the cell nominal voltage of 3.8 V. Hence, a 100-kWh battery will be made by electrically assembling some 5,200 individual 2170-cells, or 2,900 4680-cells into one large battery pack.
The cells are connected in a single packin a hybrid configuration of series and parallel electrical connections. The series connection adds up the voltages from 96 series elements to reach avoltage of nearly 400 V. Newer pack architectures stack 192 cells in series to reach a pack voltage of 800 V. Each series element consists of a number of cells(15 ~ 30) connected electrically in parallel.
Mechanically, these cells are mounted on cooling plates for temperature control within a heavy steel enclosure that provides additional mechanical and vibration protection. Each pack weighs 500-600 kg (1,100 – 1,300 lbs.). Yes, you guessed right – a truck battery with 6 packs will weigh nearly 8,000 lbs.!
With increased pack complexity, the battery management system and software (BMS) play a critical role and needs to scale correspondingly. Normally, each pack has its own dedicated BMS tasked with monitoring and diagnostic functions. But in a heavy truck, an additional BMS unit sits on top of the six packs in parallel to manage their smooth operation.
Finally, we come to charging. The vehicle’s up-time is a critical metric for trucks and as such, DC fast charging is important. Present charging times vary between 45 and 90 minutes. Volvo Trucks specifies 60 to 90 mins of charge time at 250 kW of charging power. Tesla’s Semi can charge faster using 500 kW charging stations. The advent of bigger charging stations delivering 750 to 1,000 kW of power promises to bring the charging time to under 30 minutes – sufficient to charge the truck during scheduled driver breaks.
Regenerative braking (or regen) is another key consideration for battery charging – regen is when the motor runs in reverse polarity to slow down the vehicle (e.g., driving downhill) thus becoming a generator of electricity and storing it back into the battery. A 30-ton truck descending the Grapevines slopes outside of Los Angeles (about 4,000 ft in elevation drop) may add a whopping 100 kWh of energy back into the battery. Consequently, the BMS may reserve 10 to 15% of the battery’s capacity as buffer for energy received during regenerative braking. This also illustrates how route planning begins to differ between electric and combustion-engine trucks.
Next time you see an electric Class-8 truck on the road, turn down your radio and appreciate how noiselessly it drives. Not only are they much quieter than diesel-powered trucks, but they also produce significantly fewer emissions. This means that as more electric trucks are introduced onto our roads, we can look forward to cleaner air and a healthier environment for everyone. Additionally, with the increasing concerns about climate change and the need to reduce carbon emissions, the transition to electric trucks represents a critical step towards a more sustainable future.