Batteries for e-cars and mobile devices are easy to use, but frequent charging and discharging and the associated aging processes impair the performance of battery cells and shorten their service life. In the SPARTACUS research project, Fraunhofer ISC researchers are using a range of different sensors to monitor the internal status of battery cells. The data enables the battery management system to optimize charging and discharging processes and the load on individual cells in the battery module. This can significantly speed up charging – and even extend the life of the batteries.
In the high-tech age, batteries are regarded as uncomplicated helpers. They let the alarm clock ring in the morning, supply smartphones with energy and help the mobile Bluetooth speaker to make music. They also power e-cars and e-scooters and thus play a crucial role in the mobility revolution. Once the battery is empty, it is recharged. But the simple usage concept contrasts with the amazingly complex inner workings of the batteries. This offers a lot of potential for improving performance and extending lifespan.
Researchers at Fraunhofer ISC in Würzburg want to make better use of this potential. Together with project partners, Fraunhofer experts are working on a technology in which sensors register the status of the battery cell as part of a Europe-wide research initiative in the SPARTACUS project. With the help of this data, the integrated battery management system can optimally control the charging and discharging currents. “A major advantage here is that the charging time is reduced by up to 20 percent without sacrificing battery performance and service life,” explains Gerhard Domann, project coordinator at Fraunhofer ISC, Würzburg.
Ultrasonic sensors generate a sonogram
The concept of SPARTACUS (Spatially resolved acoustic, mechanical and ultrasonic sensing for smart batteries) exploits the phenomenon that a number of complex electrochemical and physical processes take place in the battery cell. The acoustic, mechanical and thermal sensors used at Fraunhofer ISC monitor and measure the processes. Ultrasonic sensors, for example, emit sound pulses that pass through various layers of the battery cells and are detected again by sensors. If, for example, a cell expands during charging or a defect occurs at an electrode, this will affect the propagation time of the sound signal. In this way, a kind of sonogram of the battery cell is created. Equally important: the pressure-sensitive mechanical sensors. They register how certain components change their volume during charging or discharging. Thermal sensors, in turn, measure the temperature changes.
“All sensor data is transmitted to the battery management system, where it is evaluated. It is thus able to control the electrical currents when discharging and charging the battery in such a way that, on the one hand, the maximum possible power is available but, on the other hand, this is done so gently that the relevant functional layers (anodes, cathodes, etc.) of the battery are not excessively stressed. This is how we extend the service life of the battery cell,” says Domann. The seemingly simple energy storage device becomes an intelligent battery that monitors its cells and actively controls the electrical currents during operation and charging.
The resulting extension of service life is worthwhile not only for batteries in e-cars, in which several 100, sometimes even more than 1000 individual cells are installed, but also for stationary systems.
Analysis of the aging processes
But there are more effects observed by the sensors. The mechanical stress of use and the natural aging processes leave traces in the sensitive inner life of the cells. In the case of the battery cell electrodes, this may lead to cracks and detachment of the graphite layer, or tree-like metal structures may form on the outside of the electrodes, known as dendrites. All these phenomena weaken the performance and in extreme cases can create a short circuit in the cell, which can lead to a battery fire. “The multifunctional sensor array helps us better understand the complex electrochemical processes in the battery and adjust battery management accordingly,” Domann says. Fraunhofer ISC has many years of experience in electrochemistry, as well as in the development of sensors and battery technology. This expertise flowed into the development of the multifunctional sensor arrays.
The second life of the battery
The sensor technology developed in SPARTACUS can be used for the entire life cycle of a battery product. It already helps to optimize quality during design and development. In production, ultrasonic sensors ensure that the finished product comes off the line in optimum condition. “When the battery’s performance inevitably degrades after a few years and less than 80 percent of the original power is available, the sensors from the SPARTACUS project could also be used to qualify the battery for a second life, or secondary use for a less demanding application,” Domann says.
In the future, the technology will not only be applicable to lithium-ion batteries, but can also be applied to solid-state batteries or lithium-sulfur batteries, for example. Fraunhofer ISC and its project partners have already tested the functionality and practicality of the technology. In the next step, the research team will work on the concrete implementation and corresponding prototypes.
SPARTACUS is not an isolated project. It is part of the large-scale EU research initiative BATTERY 2030+, which aims to significantly improve the sustainability and performance of batteries in Europe for various application scenarios.