Sensorless Cell Temperature Estimation over the Lifetime of Lithium-Ion Batteries

Abstract

The temperature distribution within a battery pack has a major impact on capacity loss, power degradation and safety issues. Therefore, monitoring the temperature of the cells in the battery pack is an essential task of a battery management system. Temperature is usually measured with external temperature sensors attached to a few crucial cells throughout the battery pack, leaving the majority of cells in larger battery packs unattended. In our recent work, we developed a method to determine cell temperatures using the temperature dependent voltage response to a current change in the load of the battery [1]. Thereby the temperature of each cell in a battery can be monitored without the need for additional temperature sensors as long as current and voltage values of the cell are available. The method relies on a DC resistance (RDC) reference to relate the voltage response to a temperature value. Since the resistance of a cell changes with the aging of the cell, the reference, and ultimately the temperature estimation method, loses its applicability with proceeding cell degradation. For this reason, we investigated the estimation method’s behavior during aging and possible ways to adjust the RDC reference. For the aging experiment, we used the same setting as in our initial work. A 6s1p module was cycled for 200 cycles, with every 5th discharge cycle being a randomized current profile, which is used for the evaluation of the temperature estimation method. The state of health (SOH) and a RDC offset are used to scale and shift the RDC reference. SOH and RDC offset were gained from checkup cycles, which were performed every 20th cycle. To maintain the online applicability of the method, only this values (SOH and RDC offset), which can be monitored by a battery management system (BMS), are considered for the adjustment of the RDC reference. We assume the SOH is a given value from the BMS and the RDC offset can be calculated from a defined or randomized pulse. The RDC offset can be used for the adjustment under the following two terms: (1) The system is in a thermal equilibrium, e.g. an electric vehicle that is parked for several hours. (2) There is at least one temperature sensor in the system. This sensor is needed to relate the RDC offset to a certain temperature. This simple adjustment strategy increased the estimation accuracy and allows the method from [1] to be used for the entire lifetime of the battery module with an average temperature estimation error of 2 K. To our knowledge, this is the first approach for resistance/impedance based temperature estimation of lithium-ion cells considering aging.[1] S. Ludwig, I. Zilberman, M.F. Horsche, T. Wohlers, A. Jossen - Pulse resistance based online temperature estimation for lithium-ion cells; Journal of Power Sources; 2021; DOI: https://doi.org/10.1016/j.jpowsour.2021.229523. Figure 1