Analysis of Internal Temperature Variations of Lithium-Ion Batteries During Fast Charging

One of the major challenges that limits the fast charging of Lithium-ion batteries is Lithium (Li) plating at low temperatures. To reduce Li-plating an increased environmental temperature is commonly used. However, the uncertainties in the measurement of key battery internal states such as temperature, is a limiting factor to find the best fast charging profile that considers battery performance, degradation, and safety of the electric vehicles (EVs). We have used our state-of-the-art instrumented cells equipped with internal data acquisition and microcontroller, forming smart cells, that enable sensor data to be transmitted via a USB to a data logger. We demonstrate here that commercially available 21700 format cells were successfully instrumented and gave direct information on internal temperature for continuous fast charging rates from C/2 to 2.5C. The internal temperature was found to be considerably higher than that of the surface of the cell (between 10 and 14°C at 2.5C charge rate). A gradient of up to 2°C was found between the positive and negative end of each cell that became more prominent for higher charge rates. Li-plating was detected for all C-rates below 25°C even though, the internal temperature rose above 30°C when the cells were charged at 2.5C with an ambient temperature of 0°C. At a higher ambient temperature of 40°C, the cell’s internal temperature rose (to ~62°C) beyond the safe limits defined by the manufacturer’s datasheet whilst the external temperature recorded (~52°C) was within the manufacturer’s defined safe operating limits.