Over 1000 V in the future? Integration of HV battery and drivetrain in the electric car
400 V, 800 V, 915 V: Voltage levels in electric vehicles seem to be unwaveringly rising. Some suspect that all our HV batteries will use voltage levels beyond 1000 V in the future. However, is a higher voltage preferable in all cases? Or are there disadvantages to switching from the current EV voltage standard of 400 V?
At first glance, higher voltage presents convincing arguments. More power can be provided at a higher voltage – thus, higher charging speeds are possible. Concurrently, it’s possible to reduce the weight of the car as well, due to the smaller dimensions of the car’s cable-sections – which has a positive effect on acceleration and range.
SHORTER EV CHARGING TIMES WITH LIMITED CURRENTS
One reason why the trend towards higher voltage levels is continuing, especially in large, powerful electric cars, is the limitation of charging currents. Due to legal limitations, charging times for very large HV batteries can only be shortened if the voltage is increased. Charging times are calculated by multiplying the charging voltage and charging current (P=U•I) – as such, the power can be increased with the same charging current if the voltage is increased.
This way, charging speed increases while charging times for HV batteries are significantly reduced. Compared to an average charging process, total charging time can be shortened to less than 50%. This makes a decisive contribution to shortening the overall journey time on longer journeys that require a charging stop. Ideally, EVs can now even match (or at least approach) travelling times of conventional combustion engines.
However, the optimization potential is not unlimited. Increasing charging speed via higher voltage levels is capped by the physical limitations of how much electricity HV battery cells can store – regardless of the battery’s voltage level. Once this limitation is reached, further increasing the voltage will only have minor effects on loading speed. In this respect, the development towards ever higher voltage levels will reach a ceiling, due to the cell’s limitations on the one hand and the charging infrastructure on the other.
A HIGHER VOLTAGE LEVEL CAN REDUCE WEIGHT AND FACILIATE THE BATTERY'S FIT INTO THE EV
At higher voltage levels, lower current intensity is needed to provide the same electric capacities. That also means that conductors with smaller cross-sections can be integrated, which also reduces power loss considerably. Let’s look at an example: A charging capacity of 200 kW at the industry-standard voltage level of 400 V results in a power loss of 85 W per meter of cable if connecting cables of 50 mm2 are used. If this power loss is considered acceptable, at 800 V, a vehicle cable cross-section of just 12.5 mm2 is enough to reach the same performance; at 1000 V, cross-sections of just 8 mm2 are sufficient.
Not only can this reduce the vehicle’s weight considerably and increase vehicle dynamics, performance, and range in the process; the cables themselves are more flexible and can be installed into the vehicle more easily. Meanwhile, the insulation requirements from 400 V to 1000 V don’t change. Double insulation is already used for 400 V for high-voltage cabling in the vehicle, which is also sufficient for higher voltages.
Conductor cross-sections for different voltage levels with the same power loss: significantly less conductor cross-section is required at higher voltages
This change holds significant weight reduction potential. At 400 V, the average weight of a 150 kW copper cable sits at 1.27 kg/m (0.853 lb./ft); At 800 V, the weight almost halves to 0.673 kg/m (0.452 lb./ft). And the positive effects of lower current requirements for identical performance are not limited to charging and supply lines either. The smaller cable cross-section also allows the windings in the motors to be designed more compactly, which reduces the size of the motors. According to a German OEM, the overall weight of the vehicle has been reduced by doubling the voltage. Naturally, material costs are also reduced to the same extent.
HIGHER VOLTAGE LEVELS LEAD TO HIGHER COSTS
Although material costs are reduced, the higher voltage level raises costs elsewhere. First, 400 V architecture is the dominant voltage level in the industry right now – which means that components optimized for 400 V are produced in larger quantities and by more suppliers than the respective 800 V components – and are therefore cheaper. Also, the existing charging infrastructure right now is dimensioned with 400 V in mind, while 800 V charging stations are few and far between. So, until 800 V charging infrastructure is established on the road, some systems must be installed in duplicate in HV battery-powered vehicles. For example, an additional DC/DC converter may be needed to convert the 400 V from the charging station to the 800 V in the vehicle.
Meanwhile, 800 V also place higher demands on certain devices. For example, the inverter within the powertrain must be able to switch higher voltages, and the conversion to low voltages during start-up (900 V to 30-40 V) is also more complex – a process necessary to check the function of all electrical devices before starting.
LONGER DISCHARGE TIME OF THE ELECTRIC CAR AT HIGHER VOLTAGE LEVELS
Due to the higher voltage levels, a lot more energy is stored in the EV’s intermediate systems, such as its cable network. The amount of energy increases as the square of the voltage. For example, in a system in which an amount of energy of 120 J remains at 400 V, 480 J remain at 800 V and even 750 J at 1000 V. This poses a challenge for the legally required rapid discharging ability of the vehicle system.
The legal requirements state that in the case of an emergency shutdown of the vehicle (e.g., after a crash), all parts of the vehicle must free of any electric potential (higher than 60 V, at least) after 5 seconds. In the case of the “conventional” 400 V, a single electric discharge unit suffices to remove any residual potential (e.g., an electric drive unit).
At higher voltage levels however, residual potential is higher – and more EDUs are needed to completely discharge all components within 5 seconds. Current trends indicate a shift towards self-discharging components, a technically feasible, albeit quite expensive solution to this problem. Additionally, costs are also increased by the more complex battery management system (BMS) which is needed at higher voltage levels due to more battery cells connected serially.
TECHNICAL CHALLENGES CAUSED BY HIGHER VOLTAGE LEVELS
Inverters, which are needed for an EV’s motor control, also face higher efforts at higher voltage levels. The electronic circuits (usually silicon IGBTs are used) within the inverter may reach their limits when voltage peaks occur; Switching to a different material such as silicon carbide (SiC) is required to ensure the required high switching power.
The switching losses depend linearly on the voltage, current, switching duration, and switching frequency. With constant power, higher voltage and lower current, the power loss can be mitigated by using fast-switching SiC-IGBTs. Another (expensive) solution could be simply switching the inverter architecture. Whether the higher efficiency are worth the higher costs should be coordinated with the OEM in both cases.
One problem that remains is electromagnetic interference (EMI) within the inverter occurs more frequently at higher voltage levels. EMIs may cause malfunctioning of the EV’s electronic assemblies, but they may also damage the insulations of the motor windings, which shortens the engine’s lifetime. They can be prevented from spreading along the cables and lines by installing improved filter units. Meanwhile, a shielded HV on-board power supply protects surrounding assemblies form EMI.
HIGHER VOLTAGE LEVELS DO NOT SOLVE ALL CHALLENGES WITH EVs
In conclusion: Switching to a voltage level higher than 400 V is by no means a perfect one-size-fits-all solution to increase efficiency, performance, and fast charging capacities of EVs. HV batteries definitely lead to large improvements in all regards, but only do so alongside a notable increase in costs. Whether switching 800 V or higher provides a competitive advantage or not must be gauged for each vehicle project individually.
Overall, higher voltages synergize best with high-performance BEVs sporting very large batteries. Here, the much higher charging power potential is much more relevant – and end customers can be expected to pay more for the improved technology. EVs of smaller sizes on the other hand benefit more from 400 V, as the lower voltage level is not only cheaper, but also provides several additional advantages.
Advantages and disadvantages of 400 V and 800 V powertrain systems: HV disadvantages can be fixed with technical improvements, although higher costs should be expected.
AN EXPERIENCED HV BATTERY DEVELOPMENT PARTNER WILL BENEFIT THE EV PROJECT
Designing and developing new EVs may often come with expensive mistakes. To avoid those from the get-go, cooperating with a competent and experienced development partner such as Magna is always advantageous – for both established OEMs as well as for new start-ups entering the EV market. Years of experience with electric mobility and a diverse range of EV projects and customers have fostered Magna’s broad knowledge in dealing with the respective wishes and requirements of its clients in a timely, flexible, and project-specific manner. For example, Magna can offer flexible solutions and system variants to OEM customers and manage the integration of many diverse components from suppliers all around the world.
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