State of charge determination of LiFePO4 batteries using an external applied magnetic field

In recent years, lithium ion (Li-ion) batteries have become ubiquitous energy sources for various battery-powered applications. Due to the good availability, the moderate raw material costs and the thermal stability, lithium iron phosphate (LiFePO
₄
) has become a common cathode active material for secondary cells. Batteries of this cell chemistry are further characterized by a high power density and are therefore often used in automotive applications. Unfortunately these cells show nonlinear and very flat open circuit voltage (OCV) characteristics, therefore the state of charge (SOC) can not be estimated accurately by means of model based estimation algorithms. This paper investigates an alternative way to determine the cell's SOC, whereas instead of the OCV in combination with Coulomb counting, the variation of the inductance of an external sense coil is used to quantify the SOC.