A novel online measurement technique for AC impedance of batteries and other electrochemical systems

Traditional impedance measurement techniques rely heavily on taking voltage measurements to obtain the internal impedance of batteries. Because of the very low impedance, voltage output signals will be very low as well. These voltage sources require high gain amplification and, with the combination of low volt signals and low input impedance to the amplifier, there is a high susceptibility to noise and EMI. Also, sensitivity of the measurement is directly related to the impedance of the battery. As battery cells get larger and larger, the internal impedance gets smaller and smaller. The voltage measurements in turn get smaller as well as reducing the sensitivity of the measuring circuit. Increasing the input current is one way of improving the output signal but this would require moving from amps to tens of amps of current to drive the circuit. Frequently occurring online measurements at such high current not only puts a higher power requirement on the circuit but injects more energy into the battery. This potentially contributes to heating and disrupting normal cell reactions and in turn distorts the measurement accuracy. The technique presented in this paper uses only current measurements and very low excitation current (patent pending). A description of the technique is given and compared to other common configurations. Experimental data was collected using the novel circuit design along with the 4-wire method. This data was collected from two different battery sizes and chemistries (lead-acid, nickel-cadmium). Different tests were conducted: in the presents of EMI, offline, and online under various loading conditions.