Turning cells into sensors: how e-car batteries get smarter and safer

A new management system promises to make electric car batteries safer, more sustainable, and longer-lasting. By turning cells into sensors to detect hidden faults and temperature shifts, this "electronic brain" helps prevent failures and boost performance. "We integrated it into a car and proved it works," its developers say. "Now the challenge for the industry is to make it affordable."

If you've attended "The Battery Show Europe" in Stuttgart, Germany, over the past few days, you may have come across a small demo car with a battery installed under the seat and 14 cells in series. At first glance, nothing extraordinary for Europe's largest battery technology fair, this year themed "Driving sustainability, resilience and innovation in Europe's battery industry." Yet, if you looked a bit closer, you might have noticed a display showing a series of figures. The true innovation of this prototype lies precisely here: the data shown doesn't just include the battery's charge level, but also the state of health and temperature of each individual cell, thus promising to enhance safety, lifespan, and sustainability. It's a "battery management system" (BMS), the electronic brain that controls the battery, which its developers have named zBMS. Its novelty lies in the "z" that precedes the traditional acronym. "For the industry and the scientific community, Z stands for the impedance," explains Andreas Hutter, Leader of the Battery Systems group at the Swiss technology transfer center CSEM. To illustrate what that means, Hutter uses the image of an amount of water being pushed through a pipe: "The water flowing through the pipe represents an electric current, and the pressure pushing the water represents the voltage. Impedance in an electrical circuit is like the resistance to the flow of water in the pipe. If the diameter becomes smaller, the resistance to current flow increases. Similarly, in electrical terms, increased impedance reduces the amount of current that can flow." In lithium-ion batteries, the current is essentially generated by electrons flowing between an anode, a negative electrode, and a cathode, a positive one, with ions moving back and forth to charge and discharge the battery. "It's kind of a race, but full of hurdles, and any time they bump into an obstacle, some of them risk getting trapped or lost," notes Hutter. And since every time this happens the battery's capacity and performance are reduced, the first step toward improvement is to identify those obstacles. "Compared to today's practices, we measure this impedance at different frequencies, through a non-invasive technique called 'impedance spectroscopy' that allows us to make these obstacles visible, without opening the battery," he adds.

Md Sazzad Hosen, Part-time Professor and Senior Battery Researcher at the Free University of Brussels (VUB), also coordinates NEMO, the European consortium behind the development of the zBMS. "Thanks to electrochemical impedance spectroscopy (EIS), you can have more precise information on the battery's performance and lifetime and predict if it's overheating, or posing safety concerns," he says. To make it simple, electrochemical impedance spectroscopy is essentially a technique that allows impedance to be identified without the use of sensors. "The first advantage is that you don't need additional hardware," says Jan Philipp Schmidt, Professor for Systems Engineering of Electrical Energy Storage Systems at the University of Bayreuth. "Adding a sensor inside a cell is already quite complex. Then, when it comes to temperature, for instance, sensors only measure it at a single point, whereas with impedance spectroscopy you can determine the temperature of the entire battery."

One of the main benefits of applying this technique, according to the zBMS developers, is that it ensures good performance even in the case of some internal failures. As batteries consist of hundreds of cells arranged in series and interconnected, if some of them are damaged or age too quickly, the entire series is affected. "If in a battery module there are 15 cells, and two of them get damaged, the individual status check via impedance measurement and the cell management system will allow us to bypass them and rely on the remaining 13," Hosen explains. "And this will ensure safe and reliable operation." This is what experts call "balancing." This process enables two things, clarifies Hutter: "First, we prolong the lifetime of the entire system by keeping all cells at the same health status and, second, we can use the hardware to spot and skip failing cells."

Schmidt emphasises that further research is now crucial, but other barriers still hinder the widespread adoption of impedance-based battery management systems. "The technology is advancing very fast, and that's good news," he says. "But while in traditional systems you have a sensor that you just design once and use across different applications, with impedance spectroscopy, the cell itself becomes your sensor. This means that, yes, you can gather much richer and more detailed data, but also that every time a new generation of higher-performing cells is introduced, all this information needs to be validated from scratch."

A fundamental step, however, has already been taken, he adds. While impedance spectroscopy itself is quite an old process, dating back to the late 19th or early 20th century, the idea of using it to gather diagnostic data for EV batteries is relatively recent. "Around 2011, 2012 it started being used to replace sensors and to measure cells' temperature, and this gave researchers the idea to implement this functionality beyond the lab," notes Schmidt. This is precisely one of the main contributions the NEMO project hopes to bring to future generations of battery management systems. "We miniaturised the system, we integrated it in a car and we proved that it works in an operational environment. It could not only improve safety and extend battery life by 20%, but also make the validation for second-life purposes much easier," states Hutter. Yet, integrating it into car batteries today would raise the final product's cost from around €10 to €13, and an additional cost of nearly 30% is currently a dealbreaker, he adds, "So, now the challenge ahead for the industry is to make it affordable."

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