Measurements for a long battery life
A visit to the battery lab
What’s expected of a battery installed in a train with a service life of 30 years? The question of service life was probably the most important one facing the team developing the battery-powered train. After all, the better they understand the battery’s performance, the more they can get out of it. The battery lab performed the measurements.
“Imagine a triangle with the corners representing service life, performance, and weight. You want to optimize the triangle, but pulling on one corner makes things worse at the opposite end.” That’s how Andreas Meyer, team leader for Lithium-Ion Battery Technology at Engineering, describes the efforts to develop the battery system for the new battery-powered trains from Siemens Mobility. “The more we know about the battery’s service life and its electrical and thermal performance, the more we can keep this triangle in balance. The most important element right now is service life.” The proof will be provided by the batteries in the 20 Mireo Plus B trains that will enter service in Baden-Wuerttemberg starting in December 2023. Siemens Mobility already has a joint passenger service project with Austrian Federal Railways (ÖBB) in operation, in the form of the Desiro ML ÖBB Cityjet eco. In September 2019, this project became the first of its kind to be registered anywhere in Europe.
Emission-free on non-electrified branch lines
Battery-powered trains are a key to the rail component of the transformation of the transport sector. In many countries, significant parts of the rail network are not electrified, or in other words there are no overhead lines. In Germany, for example, that applies to 40 percent of the tracks, mainly regional branch lines. These use diesel railcars, although the rail services hope to move toward drive systems with no air or noise pollution, and especially no CO2 emissions. Battery-powered trains are particularly suitable in situations where the train can connect to an overhead line on parts of the track. The trains can also recharge at the station. And every time the brakes are applied, the kinetic energy that’s recovered is fed into the electricity storage system.
Huge development underpinning the battery system
The train’s battery module consists of many individual high-performance battery cells, which Siemens Mobility designed in close collaboration with the battery manufacturers. Meyer also notes the close collaboration with the battery lab at Siemens Technology: “Thanks to the measurements they supplied, we were able to substantially reduce the amount of battery power needed on each line. Since the battery is the most costly traction component in the train, that has a direct impact on the overall price of the system.”
The knowledge accumulated over decades by the lab assists all Siemens Business Units that use batteries in their products or plan to do so. For the train, it investigated the cells that were being considered. A single cell is about the same size as two smartphones alongside and on top of each other in a solid metal housing. In temperature and climate chambers, the cells were exposed to typical load profiles in ambient temperatures ranging from very hot to freezing cold, in other words, a sequence of charging and discharging cycles. Sometimes the train draws a lot of electricity from the battery, and sometimes a lot of braking energy flows back into the system; sometimes it has to charge quickly using powerful currents, and then it will roll along again with minimal load on the battery. Depending on the question being investigated, the measurements were taken at high resolution or using highly simplified profiles. A current of up to 400 amperes was applied to the cells at a voltage of approx. 3V. The stronger the currents, the warmer the cells become, and their performance falls, explains materials scientist Frank Steinbacher. The lab measures just how strongly the cell heats up, and where exactly the heating takes place. Temperature probes measure how hot the contacts become, and infrared cameras record the surface temperature distribution.
Train maintenance benefits from the battery model
In ageing tests, the cells were exposed to simplified, typical load profiles over extended periods of between 9 and 18 months. The trend represented by the measurement curves can be extrapolated over the age of the battery, which is currently the most important parameter for Siemens Mobility. Measurements were also taken on the battery module in order to optimize its design, comments Barbara Schricker, a physical engineer with a focus on electrochemistry: “There are a lot of places where you can go wrong, both in the design and geometry of the cooling and in the electrical connection system.”
Based on the measurements, the battery researchers create a model of the battery, a kind of digital twin, which is integrated into the simulation model of the train. This enables the train to run through the planned routes at a virtual level to enable values to be derived for range and service life, among other factors. The digital twin of the battery will also be valuable when the train is operating for real. Siemens is responsible for maintenance and guarantees train availability for 30 years. By comparing the real condition data for the battery against the predictions based on the model, they can work out its service life with certainty, since the measurements will indicate when the battery will be due for replacement as much as 1-2 years in advance.
Skills for all that use batteries
The battery lab at Technology works with all Siemens businesses that use batteries. The framework of core technologies, energy storage in this case, serves as a company-wide platform for collaboration and sharing information. Examples of this kind of collaboration include ferries with electric drives, constructed by Siemens Energy in Norway, decentralized solutions using energy storage devices and converters such as those implemented in the Galapagos Islands by Smart Infrastructure, and mobile x-ray systems from Siemens Healthineers. With Smart Infrastructure, the battery lab is working together with data analysts from Technology to develop a solution for determining the condition and service life of battery storage devices. These insights are then used to optimize storage system operation. “This is where our understanding of the influencing factors affecting battery performance plays a part. Our co-workers in data analysis identify patterns in the huge volumes of data gathered in actual use,” explains Manfred Baldauf, Principal Key Expert for Battery Storage Systems at Technology.
The batteries of the future are still an important topic, too: Currently, the battery lab is working with colleagues from France on software for the simulation and design of future batteries as part of the EU project Modalis2. “This also involves simulating the next generation of lithium-ion batteries using simulation software,” explains Steinbacher. In this project, Technology is responsible for battery characterization. The project also involves material and battery manufacturers, automakers, and the Software Business Unit of Siemens Digital Industries.
Christine Rüth, October 2020
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