Battery test and measurement modules for automation, testing and laboratories
Whether telecommunications, tools, leisure or electromobility - many modern devices and vehicles today operate independently of the mains using rechargeable batteries. For these to function reliably, the individual battery cells must have electrical properties that are as homogeneous as possible and their mechanical connections must also be tested 100 %. As hundreds of individual cells are sometimes required for larger batteries, the measurements for cell evaluation must be carried out quickly, reliably and safely during the production process. Modern Industry 4.0-capable measuring devices for all relevant parameters relating to single-cell assessment create the conditions for this.
A battery is a combination of identical units, in the case of electric batteries of individual battery cells, in order to achieve a higher voltage or capacity. However, this means that a battery is only as good as the weakest battery cell installed. Battery manufacturers must therefore test all individual cells and all pre-configured individual modules for ongoing production for their exact properties and classify them into quality groups. The battery in an electric car, for example, usually has several hundred cells installed in several modules. Testing this large number in the shortest possible time places special demands on the measurement technology. Many years of experience in the use of electrical and mechanical measurement methods is a prerequisite for summarising the key properties for battery production in a family of measuring devices. The modern devices can then be used individually or in combination. They are suitable for Industry 4.0 and can be easily connected to standard fieldbuses, integrated into production systems or used for research and development.
Testing the electrical properties of individual cells
The individual electrical properties of the single cells are the most important aspect of cell testing. If a cell with a lower capacity is installed in a series connection, the entire cell assembly can only deliver the lower amount of current. It can also lead to total failure, as the cell with the lower capacity is full first, but the others continue to be charged, although the cell with the lowest capacity overheats. The deviating cell can also overheat during discharge. Another problem is deviation in internal resistance, which leads to uneven charging and heating of the cells. For this reason, cells installed in batteries must have parameters that are as identical as possible (Fig. 1a, b). As battery packs are made up of individual cells of different formats (round, pouch, prismatic), the measurement technology must be able to test all variants.
With often thousands of individual cells, a high throughput during testing is essential. The use of measuring devices in the automated production testing of battery cells for seamless 100% quality assurance in fractions of a second therefore requires extensive expertise. The all-in-one module 2511 (Fig. 2) from measurement technology specialist burster (see company box) is a measuring device specially developed for such tasks. In addition to an AC/DC internal resistance measurement with values from 0.1 µΩ ... 300 mΩ in 4 measuring ranges from 10 ... 300 mΩ, it offers a four-wire measuring method for maximum precision and allows measurement and evaluation results to be generated in the shortest possible time. The measurements can be carried out in the frequency range of 1 kHz, 100 Hz, 10 Hz, 1 Hz and at voltages of 0 ... ±5 VDC. The cascadable battery measurement module allows statements on electrolyte and electrode quality compared to the manufacturer's target specifications to be made in just a few milliseconds via single to multi-channel applications and temperature measurement. The compact design and PROFINET and EtherCAT fieldbus interfaces enable easy integration into production. Matching PC software for intuitive parameterisation/configuration facilitates the creation of individual measurement programs (Fig. 3).
Testing high-current connections
In battery packs, the individual cells must be connected. To do this, steep busbar connectors, cables or spring contacts are pressed, screwed, welded or pressed onto the cells. Although it is important to adhere very precisely to the specified contact resistances in order to minimise power loss and self-heating of the battery. Given the high production volumes, measuring devices with very fast evaluation logic are also required here. With up to 100 measurements per second, the RESISTOMAT® model 2311 from burster is tailored to the requirements of high-speed applications in automation (Fig. 4a,b). Lead and contact resistances at the test contacts are eliminated by the four-wire measurement method. Integrated cable break detection monitors the test leads. Despite a measuring range of 20 mΩ ... 200 kΩ with a resolution of up to 1 μΩ and an accuracy of ≤ 0.03 % f.s., the test device achieves high-speed measurements from 10 ms per measurement including evaluation. Classifications and selections are performed by a 2- and 4-fold comparator with switched outputs. All device settings can be parameterised individually in up to 32 different measurement programs, optionally also via the Ethernet, USB (standard) or fieldbus interfaces (optional). The integrated data logger can store up to 900 measured values per measuring programme.
Determine mechanical measured values
Mechanical parameters are also important in battery production. Measuring tightening torques and tightening forces on the production line ensures that contact surfaces are securely mounted and contact resistances are minimised. It is also important to measure the forces that occur when pressing high-current contacts onto printed circuit boards or inserting individual cells into holding tubes or boxes. Recording and validating the joining forces immediately reveals any joining errors. A force/displacement press-fit monitoring system with window and envelope curve technology allows, for example, the recording of min/max peak values and force average values within a parameterised tolerance range. The DIGIFORCE® 9307 X/Y process controller (Fig. 5) makes it easy to implement such test methods. With a connection via PROFINET, EtherCAT, EtherNet/IP and Ethernet UDP, the module supports flexible sequence control and measurement data acquisition. Up to 128 measurement programmes enable a high degree of product variance. If, for example, the force displacement is monitored for the insertion of individual cell plates/cell blocks into a cell housing, several units can always be assembled simultaneously to optimise the cycle time and each individual plate can be recorded and evaluated using cascaded DIGIFORCE® controllers.
Long-term monitoring of the expansion behaviour of battery cells
However, it is not only time-critical measurements on the production line that are important; measuring devices are also needed for laboratory measurements on the battery behaviour of individual cells and battery packs, for example in development or for spot checks. This also makes it easy to determine mechanical measured values for long-term monitoring, e.g. of expansion during charging and discharging cycles or to determine the forces occurring with monitoring of temperatures or to optimise pressure distribution and durability. Thanks to its small design and three mounting holes, the 8526 compression load cell from burster is suitable for such tasks in a wide range of applications (Fig. 6). Even angular errors in the force application of up to 3° deviation from the axis of measurement have very little influence on the measurement result. Its wide measuring range from 0 ... 100 N to 0 ... 1 MN allows the exact recording of forces and expansion with temperature during charging and discharging of the cells, not only for battery cells, for example. This in turn allows an exact evaluation of the long-term behaviour and the ageing process.
Information on the images:
Image 1a/b: If possible, cells installed in batteries must have identical parameters (Author: burster)
Image 2: The 2511 all-in-one module is predestined for use in the automated production testing of lithium-ion battery cells for seamless 100% quality assurance in milliseconds. (Author: burster)
Image 3: Coordinated PC software for intuitive parameterisation/configuration facilitates the creation of individual measurement programmes (Author: burster)
Image 4a/b: The RESISTOMAT® model 2311 is tailored to the requirements of high-speed applications in automation. (Author: burster)
Image 5: When recording and validating the joining forces with the DIGIFORCE® 9307 X/Y process controller, any joining errors are immediately recognised. (Author: burster)
Image 6: Mechanical measured values for long-term monitoring of expansion during charging and discharging cycles or determining the forces that occur (Author: burster)
Company box: About burster
Whether individual metrological components or system solutions, burster mainly supplies users in mechanical and plant engineering, automation, automotive engineering with suppliers, electrical engineering, electronics and the chemical industry. The portfolio includes measuring and testing devices as well as standard sensors for mechanical and electrical measured variables, such as force, compression, torque and displacement sensors and milli and megohmmeters or resistance decades. Individual, customised OEM solutions are also possible, e.g. for many other industries and future markets such as medical technology, biotechnology and e-mobility. Our many years of experience in the construction of measuring devices and sensors always guarantees optimal solutions. Sensors, amplifier and transmitter modules, precision measuring devices and measuring systems for sensor signal processing are supplied to customers all over the world from the development and production site in Germany.
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Battery test and measurement modules from a single source for 100% quality control in fractions of a second