E-Mobility Engineering 017 l ECE Doosan electric excavators dossier l In Conversation: Matt Faulks l Battery testing focus l Battery Show North America 2022 report l Ariel Hipercar digest l Cathode materials insight l Thermal management focus

of 66 Ah, was dismantled to extract a single cell. Each cell is composed of 35 electrode pairs, in addition to aluminium and copper current collectors, the dimensions of which are 22 x 26 cm. The battery was placed 6 mm below a 4 x 4 array of low-cost single-axis magnetometers. Each magnetometer in this fluxgate array (FGA) was configured to record a component of the magnetic flux density in a plane above the cell. To shield the sensor array from external magnetic noise, it was placed inside two layers of 1 mm-thick, high-permeability mu-metal that channels magnetic flux through its walls and acts to reduce the ambient magnetic flux density in the volume occupied by the FGA and battery. A motorised three-axis track moved the cell in a 2D plane beneath the array with a positioning reproducibility accuracy of around 10 µm. The magnetic field was recorded in three states. First, a battery cycler, acting as a constant current source/ sink, was used to charge or discharge the cell. A background magnetic field of 10 µT was then recorded with the cell disconnected from the battery cycler and positioned away from the FGA. This measurement was used to track the offset drift of the FGA, which varies with temperature. Third, the passive field, believed to be due to nickel plating on the copper current collector, was measured. This was stable to charging and discharging to a value of 100 nT in the 10 µT field. The choice of charging rate is a trade-off between capturing a narrow SoC ‘window’ versus the time taken to translate the FGA array over a region of interest, so each measurement took 12 minutes to scan an area of around 150 cm2 with a resolution of 5 mm. The results were tested against simulations using finite element model. The tests showed magnetic fields of over 100 µT, agreeing with the finite element simulations. Fully understanding the bubble formation process could allow better lithium-air batteries that create fewer bubbles. This would lead to more compact and stable batteries that hold on to their charge for longer. The video data from the bubble- forming process led the team to propose that as the battery discharges, a sphere of lithium superoxide jets out from the battery’s electrode and becomes coated with lithium oxide. The sphere’s interior then goes through a chemical reaction that forms lithium peroxide and oxygen. Oxygen gas is also released and inflates the bubble. When the battery charges, the lithium peroxide decomposes, and leaves the former bubble to look like a deflated balloon. Magnetic sensing Another non-invasive testing approach is to use arrays of sensors to map the magnetic field above EV pouch cell. The current density distribution reconstructed from this map, using finite element analysis simulations, confirm the results to within a percentage point. Current density is critical in causing SoC inhomogeneities, predicting heat generation, SEI formation, inhomogeneous extraction of lithium ions and lithium plating. A deeper understanding of these effects will help to maximise battery lifetime. The ability to spatially resolve regions of high electrical conductivities will allow the monitoring of defect formation, such as dendrite growth, in real time. It will also help with the investigation of conduction phenomena and characterisation of new battery chemistries with more accuracy than open-circuit voltage measurements. One particular area of interest is inhomogeneous current density distribution within a material. This distribution is important in understanding lithium-ion transport mechanisms, including long-term ageing. Direct measurements of current density also complement the modelling and simulation of battery behaviour during constant-current discharge processes. While thermal imaging can provide some of this data, the measurements of magnetic fields are instantaneous. The highly sensitive magnetometers used to measure the fields perform a non-invasive, in-operation and contact- free measurement of local current densities inside the cell through the magnetic fields that exist outside the cell. The technique is suitable for lithium- ion pouch cells and other types of batteries and cell formats with the appropriate adjustment of the computational current reconstruction protocols. To demonstrate the technique, a pouch battery, with a rated capacity The current flow in a lithium-ion battery gives rise to a magnetic field which is measured by an magnetometer array (Courtesy of Mark Bason, University of Sussex) January/February 2023 | E-Mobility Engineering 39 Focus | Battery testing