SoC Li-ion battery monitoring with impedance spectroscopy

The state of charge (SoC) of Li-ion coin batteries is investigated as a function of impedance variation. Batteries are submitted to highly stressed conditions to simulate the operation of active loads and impedance is measured in real time both for charge and discharge. In particular, modulus and phase dependence as a function of SoC is studied with impedance spectroscopy in the range 40 Hz to 20 kHz. Phase is found to be the most sensitive parameter. It decreases with battery discharge and increases with battery charge. Impedance is simply modelled with a 3 parameters circuit, two resistors and a constant phase element (CPE). Selected frequencies are then proposed to estimate the SoC of the battery.