State of health and charge measurements in lithium-ion batteries using mechanical stress

Despite the fundamental importance of state of health (SOH) and state of charge (SOC) measurement to lithium-ion battery systems, the determination of these parameters is challenging and remains an area of active research. Here we propose a novel method of SOH/SOC determination using mechanical measurements. We present the results of long term aging studies in which we observe stack stress to be linearly related to cell SOH for cells aged with different cycling parameters. The observed increases in stack stress are attributed to irreversible volumetric expansion of the electrodes. We discuss the use of stress measurements for SOC determination, which offers the advantage of being more sensitive to SOC than voltage as well as the ability to measure SOC in the presence of self discharge. Finally we present a simple model to explain the linear nature of the observed stress-SOH relationship. The inherent simplicity of the mechanical measurements and their relationships to SOH and SOC presented in this paper offer potential utility for the improvement of existing battery management systems.