Sensorless Individual Cell Temperature Measurement by Means of Impedance Spectroscopy using Standard Battery Management Systems of Electric Vehicles 2020-01-0863
Li-Ion ion technology is state of the art for actual hybrid and electrical vehicles. It is well known that Li-Ion performance and safety characteristics strongly depend on temperature. Thus, reliable temperature measurement and control concepts for lithium ion cells are mandatory for applications in electrical cars. Temperature sensors for all individual cells increase the battery complexity and cost of a battery management system. Therefore, temperature is measured on module level in current battery packs, providing a poor observation of the individual cell temperature. Sensorless impedance-based temperature measurement concepts have been published and are validated in laboratory studies. However, dedicated test equipment is usually required, which is not available for automotive series application. This work describes a practical approach to enable impedance-based sensorless internal temperature measurement for all individual cells using state-of-the art battery management system components. Excitation is provided by introducing fast switching of different operation modes of a standard commercial active balancing system based on flyback DC to DC converters. For data acquisition, also an established commercial battery monitoring circuit unit is used. To overcome bandwidth limitations, a sub-sampling scheme is presented, which allows to determine the impedance at higher frequencies than the sampling rate. Impedance calculation is performed by means of an efficient digital signal processing concepts with low demand on memory and processing power. Thus, the method can be integrated into existing battery management systems with low implementation effort. The concept is demonstrated on a 4-cell submodule of 26 Ah automotive Li-ion cells. Also low-cost battery management systems without active balancing capability can be enhanced by the presented impedance evaluation algorithm if an appropriate excitation is applied - for example, by using the arbitrary driving current during nonstationary driving phases.