Rapid recharge

Professor Dr Kai André Böhm discusses the workings of an innovative research project that has achieved 400km range in just 15 minutes.

As the availability of Battery Electric Vehicles (BEV) with ranges exceeding 500km continues to increase, the acceptance of e-mobility is still restricted by lengthy charging durations. The research project “Direct Superfast Charging for the Electric Vehicle” (abbreviated as “D-See”), funded by the German Federal Ministry of Economics and Climate Protection, aimed to decrease the charging time for 400km of real-world driving performance to just 15 minutes.

Over the span of four and a half years, the participating partners – Bochum University of Applied Sciences, hofer powertrain, innolectric, Keysight Technologies, Sensor-Technik Wiedemann (STW), and Voltavision – have thoroughly analysed and optimised the entire energy flow chain. This encompasses everything from the power grid to the charging electronics, the charging cable, and finally, the vehicle battery. The outcome of this research is a fast-charging station with a 450kW charging capacity and a prototype vehicle suitable for reproduction. Together, these advancements enable the energy required to drive 400km to be charged into the vehicle battery in just 15 minutes.

State-of-the-art and project objectives

To achieve a range of 400km in real-world driving, a mid-size passenger car requires approximately 88kWh of energy. One of the fastest charging cars globally, if not the fastest, is Porsche’s Taycan 4S. It has the capacity to charge at just under 170kW on average between 0% and 80% state of charge (SoC). However, a maximum charge of 150kW of this power is actually stored in the battery. Consequently, the Taycan would require at least 35 minutes to charge 88kWh – if its battery were large enough.

To charge 88kWh in 15 minutes, a net charge rate of 352kW is necessary, signifying an acceleration of 2.3 times compared to the state of charge. However, this approach comes with a drawback: most losses in the cables, connectors, and the battery are proportional to the square of the current, leading to power losses that are over five times higher.

Additionally, every component used had to meet the high project requirements and thus was subjected to rigorous tests in terms of efficiency, cost, comfort, and service life. Existing norms and standards also had to be taken into consideration, including compliance with DIN SPEC 70121. This specification outlines the fundamentals of DC fast charging.

Research and development

To enable charging powers of up to 450kW, Keysight Technologies designed new advanced and modular power electronics for the fast-charging process in Silicon Carbide (SiC) technology. With the development of the DC Charging Controller, a fully developed communication unit, the establishment of standard-compliant charging communication during the DC charging process has been achieved by Innolectric.

The greatest challenge in this project was found in the high voltage battery. Several approaches have been analysed in deep detail, like optimisation of the charging strategy (eg. so-called boost-charging) and external cooling for heat exchange with an external cooling system integrated in charging station. But these approaches result in extremely high power losses of up to 40kW and in greatly reduced battery lifetime.

The optimal solution for the battery has been found in optimisation of the power to energy ratio, thus enabling an efficient compromise between heat generation, service life, and range. In the same cell format and chemistry a li-ion cell can be optimised for highest energy density or for higher power by variation of parameters like particle sizes, porosity, conductive additives and anode and cathode thickness.

The various cell formats that made it to the shortlist were tested by the high-voltage test service provider Voltavision. The appropriate battery was developed by hofer powertrain and the University of Bochum. The final battery prototype uses cells with a P/E ratio of 3.5 and an energy density of 210Wh/kg. The new high power prototype battery with Li-ion technology has a voltage of 645-903V and can be charged at a constant rate of 460A.

The results

In this project, the entire energy flow chain from grid to the vehicle was optimised, built and validated using a prototype vehicle and charging system. In the final charging test a constant charge rate of 460A was applied to the vehicle, resulting in 392kW charging power. After exactly 15 minutes, 98.1kWh had been charged by this. Due to the optimally chosen P/E ratio, the battery temperature increased by 26°C only, so that no extensive cooling was necessary.

To proof the amount of energy stored, the vehicle was discharged at 70kW, which is far more than average driving requires. Even so, 90.3kWh could be discharged. By this, 103% of the project target was reached. The overall efficiency of charging plus discharging process is at a very high 92%. These 90.3kWh result in either 410km real-world driving or 560+km WLTP range is it is communicated by vehicle manufacturers, charged in only 15 minutes.

Charging 400km real-world range in only 15 minutes is possible today. By the results of this project, such high charging rates will come at definite costs like reduced efficiency, reduced battery lifetime, convenience losses and increased production costs. As a solution, we suggest increasing the P/E ratio moderately, so that a minor reduction in EV range enables for fast recharging without the mentioned disadvantages.

Prof. Dr. Kai André Böhm is at hofer powertrain