State-of-Charge Monitoring and Battery Diagnosis of NiCd Cells Using Impedance Spectroscopy

With respect to aeronautical applications, the state-of-charge (SOC) and state-of-health (SOH) of rechargeable nickel-cadmium batteries was investigated with the help of the frequency-dependent reactance Im Z(omega) and the pseudo-capacitance C(omega) in the frequency range between 1 kHz and 0.1 Hz. The method of SOC monitoring using impedance spectroscopy is evaluated with the example of 1.5-year long-term measurements of commercial devices. A linear correlation between voltage and capacitance is observed as long as overcharge and deep discharge are avoided. Pseudo-charge Q(omega) = C(omega).U at 1 Hz with respect to the rated capacity is proposed as a reliable SOH indicator for rapid measurements. The benefit of different evaluation methods and diagram types for impedance data is outlined.