E-Mobility Engineering 014 l InoBat Auto dossier l In Conversation: Brandon Fisher l Battery monitoring focus l Supercapacitor applications insight l Green-G ecarry digest l Lithium-sulphur batteries insight l Cell-to-pack batteries focus

temperature and even the pressure inside the cells. New materials for lithium-ion cells, particularly silicon anodes, can provide much higher energy densities, up to 800 Wh/litre, to provide longer range in the same size battery pack or smaller, lighter packs. But these materials can swell during charging, creating more pressure in the cell. During development, the exact temperature, voltage, current and pressure profile can be captured to be used in the BMS that controls the pack during operation. That then has an impact on the overall vehicle design. For instance, the battery pack can be designed with a series of modules that can be switched off if one of the cells is identified as acting outside the safe limits. This isolates any risks, but still allows the vehicle to operate in a ‘limp home’ mode to get to a safe place to stop or even to a garage. More accurate monitoring of the battery pack can also improve the vehicle’s performance. Today’s lithium- ion battery packs are charged up to 80% of the total capacity, and discharged to 20%, to prevent damage from over-charging. More accurate modelling allows high limits for safe charging and lower limits for safe discharging without damaging the cells. With more accurate battery monitoring during operation, this can add hundreds of kilometres to a vehicle’s range or hours more for industrial vehicles between charges. This distributed battery pack system supports packs with high cell counts by connecting multiple high-accuracy The battery monitoring system is a mix of sensors, voltage measuring chips, comms chips and the BMS itself. Battery packs can extend up to 800 V and beyond to support the demanding loads of an EV’s motor. This translates into more than 200 lithium-ion cells, each operating at 3.6 V and stacked together in series inside the vehicle. Small variations in the construction of each cell can lead to different performances, with higher or lower current drain than expected. All this needs to be monitored to within a few millivolts, floating at 800 V. If a cell drains too fast the internal structure can heat up and be damaged. Modern lithium-ion batteries with a liquid electrolyte have an optimum operating temperature between 15 and 35 C, but are capable of working outside that range. Next-generation solid-state batteries work at higher temperatures, currently around 70 C. While the impact of this damage might not show up immediately, it can lead to higher discharge rates, higher temperatures and the cell swelling up. In the worst case, it can lead to the cell splitting, letting in oxygen and the cell catching fire. Once that happens with one cell, it can quickly spread to the rest of the battery module, the pack and even the entire vehicle. That means high-voltage packs increasingly need more sophisticated technologies with a safety architecture to report cell diagnostics in a safe, timely and reliable manner. This is then linked to the design of the pack with safety in mind. Measuring the current of each cell is a key technique to identify problems before they become apparent with higher temperatures. However, it needs a combination of accurate current measurements over the lifetime of the battery pack and a detailed model of the pack as a comparison. This model is built during development, and can take into account the exact physical construction of the battery pack, the Stability of measurement is key for monitoring, and this system supports packs with high cell counts (Courtesy of Analog Devices) Summer 2022 | E-Mobility Engineering 33 Focus | Battery monitoring