As the world accelerates towards a sustainable future, the electrification of commercial vehicles is becoming increasingly imperative. This technical presentation explores the rising demand for megawatt charging solutions in the electric commercial vehicle industry, technical hurdles, cost-efficiency, and performance advantages.
We delve into the costs to enable megawatt charging onboard commercial vehicles. These technical enablers must be understood when evaluating the vehicle’s desired performance and charging speed. Our presentation will review these key technological requirements, including high-capacity batteries, high fault current protection systems, and advanced thermal management systems. Each of these can be a limitation on available charging speed of the vehicle but addressing them comes at additional cost.Using an analysis of available time in the usage of commercial vehicles in a variety of business models, we will identify required charging speeds to electrify that business model. The flexibility of when and where to charge a vehicle becomes the primary benefit of faster charging. We present 3 potential business models that will require different charging solutions:
Local Delivery – Low energy use with high available downtime can be electrified with relatively slow 150kW charging.
Medium Haul – Medium energy use with medium available downtime can be electrified with currently available charging speeds of 350kW.
Long Haul – High energy use with low available downtime will require 1MW charging to be widely available before electrification.
While all BEV applications could benefit from the flexibility of faster charging, and in fact, potentially most applications could be electrified with widely available 1MW charging. It is important to understand that not all will require megawatt charging. Many business models can go fully electric and manage vehicle cost by using slower charging speeds.
Through innovative research and development, Eaton is at
the forefront of developing scalable and efficient onboard charging solutions and can help OEMs and fleets understand the trade-offs between cost and benefit specific to each vehicle’s application.
Cost of Faster Charging
Central to this discourse is the concept of charging speed, a pivotal factor influencing the practical deployment and utilization of electrified vehicles. Charging speed dictates the operational dynamics, dictating the feasibility of various business models transitioning to electrification. While some applications may seamlessly integrate electric vehicles (EVs) into their existing business frameworks, others may necessitate the advent of novel technologies or creation of new business models to utilize the strengths of electrification.
The assertion that expedited charging equates to superior performance holds merit; however, a comprehensive analysis of the constraints faced by vehicle manufacturers and fleet operators is imperative. This encompasses evaluating the technical and economic implications of vehicle electrification, ensuring that the transition is not only technologically viable but also sustainable in the long-term market landscape.
Battery Cells
Battery cells, akin to the human preference for climate-controlled environments, exhibit optimal performance within specific temperature parameters. Ideally, these cells operate efficiently within a thermal window of 15 to 35 degrees Celsius. It is noteworthy that the preferred operational temperature range may vary contingent upon whether the cells are in
the process of charging or discharging.Figure 1shows the relationship between temperature of a battery cell and it’s relative performance. The range for performance is greater during battery discharge than it is for charging.
This analogy underscores the necessity for sophisticated thermal management systems in electric vehicles (EVs). Such systems are designed to maintain the battery cells within their ‘comfort zone,’ thereby ensuring consistent performance. Furthermore, integrating intelligent features like route planning enables the vehicle to anticipate upcoming charging sessions, allowing for the pre-conditioning of the battery. This proactive approach facilitates optimal charging efficiency.
The second factor that can effect a cell’s ability to take in energy is its current state of charge. Drawing a parallel to the process of individuals entering a movie theater and locating a seat, charging a battery pack can be similarly conceptualized. When the ‘theater’—or battery pack—is unoccupied, the process is straightforward. However, as occupancy increases, finding available ‘seats’—or charging points within the cells—becomes increasingly challenging. Figure 2 shows this relationship graphically. As the state of charge approaches full capacity, the rate of charging can degrade to the point at which charging is un-economical. The driver will be better served driving to a second charging location using energy and plugging in at a lower state of charge than to sit and wait at the reduced speed.
This state of charge relationship can also effect a vehicle’s ability to use the drive motor as a secondary retarder. Therefore, a vehicle must maintain a sizable buffer in the battery pack to ensure sufficient charge rate and storage to perform regenera-tive braking. A sophisticated route planning tool should be able to predict energy consumption as well as predicted energy recovery to optimize efficiency.
Battery Size and C-rate
A critical aspect of battery chemistry is the relationship between a pack’s peak theoretical charging speed and its capacity.
The pursuit of faster cell chemistries is achievable; however, it often incurs higher costs and a compromise on longevity. The commercial electric vehicle (EV) market has shown a predilection for Lithium Iron Phosphate (LFP) chemistry, which strikes
a balance between cost-effectiveness and durability.
Recent advancements in LFP chemistry have elevated the pack’s C rate from approximately 2 to a range of 3 to 4. However, even with a C rate of 3, attaining a charging power of 3.75MW—the upper limit of the Mega Charging Standard (MCS)—a vehicle would necessitate an onboard battery capacity nearing 1000kWh. Conversely, a charging power of 1MW could be achieved with a 400kWh battery. While substantially larger than the batteries found in light vehicles, this size remains within a more manageable spectrum.
The highlighted example shows that a 3C battery connected to a 1 Megawatt capable charger will be limited in charging speed if the battery is below 350 kWh. Above that size, the charger’s max power can be achieved.
It is important to note that a larger battery not only extends the vehicle’s range and accommodates greater energy recovery through regenerative braking but also impacts the vehicle’s cargo capacity. This trade-off highlights the intricate balance between enhancing EV performance and maintaining practical utility.
Potential Thermal Bottlenecks
In the context of charging, thermal constraints are a significant factor to consider. Each element within the battery and charging infrastructure is susceptible to becoming a constriction point. The losses within the electrical system adhere to the I2r rule, indicating that resistance losses are proportional to the square of the current. When currents range from 800 to 3000 Amps, even marginal increments in resistance, particularly around contactors, can lead to substantial heat generation.
Figure 4 Shows the thermal analysis of Eaton’s Breaktor® circuit protection technology. It provides insight into these dynamics, showing an ability to carry 350A continuously, but much higher currents for limited durations. Incorporating active cooling mechanisms can enhance the duration for which components can operate within safe thermal parameters. It is crucial to recognize the individual limitations of each component and to manage the thermal thresholds at a system-wide level to ensure safe and efficient operation. The best way to manage this onboard a vehicle is using temperature sensing and transfer functions to monitor the internal temperature of components. Using an active monitoring system will allow a vehicle to demand maximum power for as long as possible while staying within thermal limitations.
Power Distribution
High-voltage system design encompasses not only the opera-tional parameters but also the protection strategies required to ensure safe operation and minimizing the impacts of a fault event. In scenarios where multiple battery packs are employed, each pack may necessitate the management of only a segment of the charging current and exposure to a proportional segment of potential fault conditions. For this reason, each pack’s Battery Disconnect Unit (BDU) may resemble the requirements of a light vehicle application.
As shown in Figure 5, Breaktor 900 emerges as an optimal protection solution for individual battery packs. Individually capable of handling 500kW charging for a limited duration and safeguarding against faults up to 20kA, it meets the require-ments of the individual pack after the MCS power is divided evenly. However, its capabilities do not extend to the high currents encountered at the MCS port.
At the MCS port, the system must be fortified against full system faults from either the charger, or multiple battery packs at once. This could be fault current of up to 70kA. The protection mechanism here is required to be current-limiting to shield the vehicle systems effectively. This is where Eaton’s truck series EV fuses become particularly relevant. These fuses can be arranged in parallel to forge a scalable protection strategy that spans the entire MCS current spectrum. Nonetheless, these fuses must be meticulously coordinated with a suitably sized pair of contactors to facilitate normal switching operations, ensuring a robust and reliable high-voltage system design.
Charging Session
Synthesizing the elements, let’s visualize an actual EV charging session. Calculating charging time lies in the area under the curve. Most charging systems are current limited meaning there is a maximum current output for the charger. Multiplying this by the system voltage will yield maximum peak charging power. Enhancing the current limit on either the charger or the vehicle —through thermal mitigation strategies or the augmentation of contactors for example—can elevate the peak charging speed, thereby diminishing the duration required for a full charge.
In the scenarios involving 1MW and 500kW charging sessions, the system sustains the peak charging rate only transiently before encountering thermal bottlenecks or intrinsic pack limitations that necessitate a reduction in charging speed. Conversely, a low power charging application, operating beneath these thresholds, can maintain peak speed for the majority of the charging duration. All three charging sessions deliver the same 600kWh, but it takes substantially different amounts
of time to do so.
These charging profiles are pivotal for Original Equipment Manufacturers (OEMs) and fleet operators to comprehend. They must align the vehicle’s charging capabilities with its designated operational role. This alignment is crucial in ensuring that the vehicle’s performance meets the expectations set by its intended use case. Next, we will delve into how these considerations translate into practical applications for OEMs and fleet operators.
Required Charging Speed
In order to understand how fast is fast enough, we must first examine a selection of commercial vehicle applications. Using a combination of real world data from the NACFE Run on Less initiative and a theoretical long haul trucking application we will look at the amount of energy used in a day vs the available time a vehicle has to charge.
Local Delivery Application
The first application is a local delivery vehicle from for NACFE data provided by Penske. In this application, the vehicle performs a short local delivery route covering approximately 200 kilometers each day in a single shift. This means the vehicle only uses about 250kWh and has over 15 hours of down time at the depot to add back charge. Theoretically, this could be managed by commercially available level II AC charging, but the vehicles would have to be plugged in the entire time they are not moving. In practice, Penske uses 150kW DC fast charging that is able to re-fill the truck in 2 hours at the end of its shift before the truck can be moved to a regular parking spot at the depot, this concentrates the infrastructure required to handle multiple vehicles and keeps them operationally flexible. Table 2 and Figure 7 represent this application.
Regional Delivery Application
The second example to examine is also from the Run on Less Electric Depot. Here Pepsi is running a fleet of Tesla semi-trucks on a regional delivery route. The vehicle travels up to 965 kilometers in a 24hr period over two driving shifts. This leaves a limited window at the end of each shift of 3 hours to add back charge. Using a similar method of looking at all available time, this could theoretically be managed by 250kW charging, but again that would require maximum charging for every moment the vehicle is not moving.
In practice, Pepsi uses 600kW charging to re-fill the trucks in
2 two hour sessions giving increased flexibility and potential up time. Table 3 and Figure 8 show this application.
Long Haul Application
The last scenario to review is a theoretical example of a long haul sleeper cab. This is often considered one of the hardest applications to electrify because it currently relies on a dispersed network of re-fueling stations rather than a controlled depot that is owned by the fleet or operator.
Legally long haul drivers in most countries are required to take rest at certain intervals. In an ideal state, the 10 hour rest period as well as all regular stops during the day would be available to charge the vehicle, an operator would be able to cover 965 kilometers per day and only require 150kW charging. But with the distributed nature of where this charging would need to occur, consistently finding a plug every time the truck is not moving will likely be impossible.
For this reason, we must assume the truck is only able to find a plug a limited number of times during the day and does not plug in during it’s overnight rest period reducing the available window to charge down to around 3 hours per day. This more closely models the business model used today with rest stops and diesel refueling stations. Using this available time, adding back the required range theoretically requires an average of 666kW charging. Understanding everything we have learned about charging limitations of peak charging and flexibility, a 1MW or higher charger will be required. Table 4 and Figure 9 represent this theoretical scenario.
This is not to say megawatt charging solves all issues associated with electrifying long haul trucking, charger availability and grid connection will be challenging. But this analysis shows that if 1MW charging is widely available, electrifying even the most difficult business case could become a reality.
How Fast is Fast Enough?
Finally, let’s pull the whole picture together. Each application will exhibit a different amount of energy used during the day vs the available time to charge. Figure 10 represents this graphically with shaded regions to represent the three applications examined. By graphing the curves of different peak charging rates, we can identify a threshold below which an application could feasibly be supported by a given charger.
Incorporating the three examples we examined into this graph reveals how each could be accommodated by a distinct charging network. It also indicates that a majority of applications could be adequately served by the widely available 1MW charging infrastructure.
While the MCS standard does permit charging capacities
of up to 3.75MW, it’s important to note that the costs associated with enabling such high-power charging tend to increase exponentially. Therefore, the financial justification for enabling such high-power charging will likely be limited to niche applications. This approach ensures a balance between operational efficiency and cost-effectiveness.
Conclusion
Each commercial vehicle application will be unique and require a unique assessment of its energy requirements and business case for electrification. Understanding the limitations and cost of increasing charging performance allows fleets and OEMs
to analyze cost vs the flexibility benefits afforded by faster charging. There is no one-size-fits-all answer. Many applications can be adequately served by the charging technology of today and the advent of widely available Megawatt charging should open the window of electrification to most of today’s commercial vehicle applications.
About Eaton: Eaton has long been a leading supplier in the commercial vehicle space. As our product portfolio grows for electrification, Eaton is the ideal partner for OEM customers to understand that cost benefit equation. Our expertise in power distribution and protection can help OEMs design cost effective High Voltage architectures that meet their customer’s needs.
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