Advanced Charging Solutions for the Next Generation of Commercial Electric Vehicles

As the electrification of heavy-duty or commercial vehicles becomes more and more accepted, charging batteries that are larger than those of electric vehicles becomes increasingly important. Because time is money, especially in logistics, the preferred options are to increase the charging power or distribute the idle time for charging. These preferences lead to three different charging scenarios.

Three scenarios for dedicated power electronics solutions

As the electrification of heavy-duty or commercial vehicles becomes more and more accepted, charging batteries that are larger than those of electric vehicles becomes increasingly important. Because time is money, especially in logistics, the preferred options are to increase the charging power or distribute the idle time for charging. These preferences lead to three different charging scenarios.

Scenario 1: Parking lot charging and fleet operation

Modern battery technology and cutting-edge power semiconductor solutions enable the design of efficient infrastructure. The above image depicts a contemporary parking garage for charging bus fleets.

Depot charging is the best option for local fleet operations, especially buses and delivery vehicles, which run on relatively fixed routes and sit idle during the night hours.

This form of charging reduces the need for charging power sources and provides more energy management options. Including fixed batteries, decoupling bus charging times from times of excess energy is also an option.

Common electric buses today have battery capacities ranging from 250 to 500 kWh, which enables them to run a shift without recharging. A single depot charger is only needed to charge one vehicle overnight, and even if 80% of 500 kWh is charged in 6 hours, 70 kW is enough. Of course, this is multiplied by the number of vehicles that must be charged simultaneously throughout the depot.

A typical charger schematic includes an input stage that adapts to the DC link voltage, an output rectifier, and a galvanic isolation stage between the two, as shown in Figure 2.

Figure 2: Bidirectional charger schematic and recommended components

Typically, chargers are built in a modular fashion from subsystems that can be stacked to increase output power. Most standard designs feature 15-60 kW per subsystem, with component selection varying by power output requirements and cooling preferences. While units in the 10 to 15 kW range with forced air cooling widely employ discrete components, higher power units are liquid cooled and primarily constructed from multiple power modules.

Parallelizing units is another option to increase output power while also building in functional system redundancy. Doing so enables the system to run at a lower power in the event of a single module failure rather than losing the entire system.

Parking lot charging also opens the door to the use of secondary grid services. Stationary energy storage helps reduce the load on the grid and can even support the grid during times of high energy demand. Scheduled charging and load balancing also become an option. Charging times could be aligned with periods of excess energy, which could result in lower or even negative energy prices at night.

Fleets with fixed schedules do not need to be fully charged at the same time. Sharing of energy between vehicles is also possible, and those not scheduled to be put into use can contribute their stored energy. Overall, parking lots that are part of larger industrial areas can also become solar power stations.

Scenario 2: Opportunity Charging

Fleet vehicles operating along scheduled routes can extend their range by adding smaller amounts of energy more frequently. This is called opportunity charging, and it works best if it is done in a fully automated manner.

There are two recommended solutions for opportunity charging.

A mechanical system called a pantograph allows large electrical contacts to be moved over greater distances and come into contact with their counterparts safely. Pantographs are a proven, reliable technology used extensively in tram and railway applications. Pantographs are classified into two systems, top-down and bottom-up, based on where they are installed. The bottom-up approach is where the system is mounted on the vehicle and is in contact with the station. The top-down mechanical system is part of the station and is lowered to the vehicle. Figure 3 shows how a pantograph is set up for charging.

Figure 3: Top-down pantograph for opportunity charging

The construction of infrastructure is still limited to the roadside. Therefore, such facilities can be built to upgrade existing power stations where a suitable power supply is available locally. Since this is rarely the case, buffering charging stations with battery storage is a widely accepted solution, which can separate the high-power charging of vehicles from the recharging of stationary batteries.

Typically we use power levels of 125-250 kW.

Before starting the charging process, the charging voltage and current are aligned between the charging station and the vehicle's battery management system. Due to the high powers involved, charging via a pantograph is always a direct current charge with direct contact to the vehicle's battery.

For future installations, pantographs are the recommended solution, especially for autonomous vehicles, as there are no plugs or wires involved that need to be handled precisely. These systems can easily handle vehicles of varying heights and can be constructed to allow for misalignment between stations and vehicles.

This is also popular on mobile devices such as smartphones, and wireless power transfer (WPT) can be considered to be upgraded to meet the needs of large-scale energy transmission. SAE J2594 describes wireless power transmission for vehicle-scale systems in detail. Wireless charging systems essentially have two independent parts, both of which exchange energy through magnetic flux. In order to avoid sacrificing too much transmission efficiency, SAE J2594 sets a goal of at least 80% transmission efficiency for them. As shown in Figure 4, a series compensated resonant circuit with an operating frequency range of 80-140 kHz can be used to meet this requirement.

Figure 4: Series compensated resonant WPT setup

Many input rectifier topologies are worth considering, including static diode rectifiers or thyristor-based versions as cost-optimized solutions. The Vienna rectifier is a common solution because it has excellent EMI performance, reduces the amount of work required for filtering, and has an adjustable DC link voltage. Driving the transmit coil at a high switching frequency of 80 to 140 kHz, as required by the standard, can consider using low switching loss IGBTs or silicon carbide MOSFETs for the DC-DC conversion stage.

Inductive chargers must be installed where vehicles can reach them. In contrast to pantographs, they would severely impact infrastructure, especially public transport. Therefore, inductive charging is a suitable solution for semi-public areas. For example, airport luggage trolleys can benefit from wireless power transfer, as the power level, the energy involved and the terrain conditions all match the requirements of the application.

Scenario 3: Single long-distance operation

Traveling on random routes, as required by long-distance logistics, will require individual high-power charging, similar to today’s gas stations. This high-power charging will need to be part of the existing infrastructure to seamlessly integrate electric trucks into the mobility landscape.

Using DC voltages of up to 1500 V and maximum charging currents of up to 3000 A, charging at rates of over 2 MW is possible.

At 2 MW, a 500 kWh charge can be completed in about 15 minutes, giving a further 300 km of range, which covers well the legally required rest periods that drivers must observe. However, a city's low-voltage three-phase grid of less than 400 V will not be able to support this power level.

In this case, the prerequisite for local power supply must be considered as medium voltage power supply. Although buffering with stationary batteries is a potential option, the storage capacity will become relatively large.

We have to use medium voltage transformers, which offer a promising option for megawatt chargers. Rather than scaling up the structure used for charging buses, we want to stick with the widely accepted scheme in electrolysis. Figure 5 depicts the relevant high-power setup.

Figure 5: High-power charging topology using B12C, also known as B6C-2P

This approach has only one energy conversion stage, and moving the galvanic isolation stage from a smaller single converter to a medium-voltage transformer can increase the efficiency of the power conversion stage to more than 99%. At the same time, it minimizes the number of resources per kilowatt installed, and devices built from pressed components reduce space requirements.

When moving into the megawatt range, thyristor-based solutions combine excellent efficiency with the unprecedented lifetime and reliability of capsule-type devices.

Such infrastructure systems require a large number of operating cycles and place particular expectations on service time. Designers need to consider both factors in the early stages of design. Although the technology and topology may seem outdated, the higher efficiency, lower cost and smaller space requirements make it an obvious choice. This approach will be crucial when future autonomous commercial vehicles require higher power ratings to further reduce charging times, as drivers will not need to rest.