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News – What`s happening in energy research

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In the DriveBattery project, researchers are developing a new control and interconnection concept for electric vehicles. This is intended to ensure that the battery systems are more efficient and reliable. The picture shows the eGolf, the first fully electric car from Volkswagen.
© Volkswagen AG
Battery systems in electric vehicles
27.05.2015

The photo shows the first concept version of a battery master switch from Infineon.
© Infineon Technologies AG

Increased range, reduced costs

One million electric vehicles by 2020 – that is the declared goal of the German Federal Government. However, existing battery systems are not suitable enough for everyday use. In the joint DriveBattery2015 project, developers are therefore designing a new control and interconnection concept. Their aims: to increase the range, reduce costs and make electric cars more reliable. The first phase of the DriveBattery project has now come to an end – time for an interim balance.

Electromobility has considerable strategic importance for the German Federal Government, since electrically driven vehicles are expected to make a significant contribution to climate protection. One problem area, however, are the battery systems used in electric vehicles, which in terms of their short range, power density and reliability still have scope for improvement. In addition, the current batteries are still very expensive. In order to make progress in this area, the developers in the joint DriveBattery2015 project are focussing on the battery system. They are developing an intelligent control concept, cost-effective battery modules based on microelectronics and are working on integrating the battery system so that it is crash-proof. Previous battery systems have generally consisted of a rigid connection of homogeneous battery cells. An optimal design can typically only be achieved by using battery cells specially tailored to the application.

Greater safety thanks to semiconductors

The battery master switch in an electric car must safely disconnect the battery from the loads in the event of an accident. Previously this was a purely mechanical switch. This has the disadvantage that the emergency stop process can take more than ten milliseconds. This is sufficient time for a short circuit to occur that could damage the battery permanently. In the project, the researchers are developing a suitable semiconductor switch for this purpose, which is 1,000 times faster and can therefore switch off the battery before any damage can occur. The new semiconductor-based battery master switch also reduces the volume and weight, and because its on-state resistance remains the same, it does not age.

More reliable and greater range

Until now, the battery systems for electric vehicles have consisted of large cells installed in series. “That is sound and is not in itself wrong, but does have some drawbacks,” says Dr Wilhelm Maurer, coordinator of the DriveBattery project. For example, if a single cell fails then the entire battery system fails too and the electric car is no longer roadworthy.
The Technische Universität München is therefore investigating smaller, differently interconnected cells and, based on the data acquired, has developed a mathematical model. This is also the source of the developers’ simulation-based proposal to interconnect the cells both in series and in parallel. This therefore enables the battery system to be adapted to different load scenarios. If a cell fails, the driver will still manage to get his or her electric car safely home.

The range can be increased by using, among other things, so-called cell balancing. This is because even with perfectly manufactured cells, small differences in the production increase during the lifetime of the battery. This has a noticeable effect on the performance of the battery: for example, a difference of two per cent between the cells is sufficient to cause the entire battery to only reach 92 to 95 per cent of its nominal capacity. “With corresponding electrical measures, also known as active balancing, we have managed to compensate for the difference between the cells so that the battery maintains its full capacity,” says Maurer.

Entire value chain makes the joint project successful

“The expertise of the project partners unifies the entire value chain and goes even beyond this. That was also the reason why we wanted to do research within a network,” adds Maurer.

The first phase of the joint DriveBattery2015 project was launched as part of the National Platform for Electromobility and was funded by the German Federal Ministry for Economic Affairs and Energy with around 5.8 million euros. The second phase begins in summer of this year. The researchers then want to further improve the battery management and the master switch and build a demonstrator. With this they want to provide a practical demonstration that the system is more efficient, safer and cheaper.

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Addresses

Project coordination, sensors and battery master switch
Infineon Technologies AG

Model development and simulation
Deutsche ACCUmotive GmbH & Co. KG

Integration of the sensors and demonstrator
Sensor-Technik Wiedemann GmbH

Requirements and specifications
TWT GmbH

Interconnection, models, charge balancing systems
TU München, KLIMA

Modular concept, HiL testing & fault simulation
RWTH Aachen

System modelling and analysis
TU Dortmund

Associated partner
Daimler AG

Associated partner
Adam Opel AG

Associated partner
Volkswagen AG