High Precision On-Line Impedance Measurement for a Li-Ion battery

This paper presents a highly precise battery impedance measurement algorithm. The proposed algorithm consists of fast Fourier transformations, peak, and phase difference estimations. Additional offset clipping and amplifying circuits expand the range of the signal measurement. An opamp based offset clipping circuit increases the ac ripple to DC voltage ratio without phase delay. Amplifying circuit gives more resolution to the ripple voltage. These proposed instrumentation ensure the precise measurement of ripple voltage and current. The proposed algorithm determines the peak magnitude of ripple voltage with tolerance of 0.12mV, and the phase difference tolerance is 0.2%. To validate the proposed approach, a 40Ah 13.6V Li-Ion battery charger was built using a Opal-RT controller, a synchronous buck converter, and offset clipping and amplifying circuits.