Pulse resistance based online temperature estimation for lithium-ion cells

Knowing the temperature distribution within a battery pack is vital, because of the impact on capacity loss, power degradation and safety. Temperature measurements are usually realized with temperature sensors attached to a limited number of cells throughout the battery pack, leaving the majority of cells in larger battery systems unattended. This work presents a novel sensorless method for determining the temperature of a cell by exploiting the relation of the cell's overpotential and temperature exemplary using a 18650 nickel-rich/silicon–graphite cell, although the method is basically applicable to any cell. Current changes in the battery load are utilized as pulse excitation for the calculation of a direct-current resistance R_DC,Δt determined after a certain time Δt. Reference pulses at 10/20/30/40°C are recorded to investigate the influence of state-of-charge and pulse rise/fall-time, as well as the pulse-current amplitude and direction on R_DC,Δt. The analysis of the reference pulses shows that a Δt in the 10ms to 100ms regime has the greatest sensitivity to temperature and the least dependence on other parameters. The method is finally validated using a 6s1p-module with an externally constant temperature gradient applied to the serial connection, showing an average estimation error smaller than 1K for each cell.