  There is a need for techniques for efficient and accurate measurement of the
impedance of rechargeable batteries at extra-low frequencies (ELFs, typically
below 10uHz), as these reflect real usage and cycling patterns, and their
importance in fractional battery circuit modeling is becoming increasingly
apparent. Major impediments include the time required to perform such
measurements, and `drift' in impedance values when measurements are taken from
the same battery at different times. Moreover, commercial impedance analyzers
are generally unable to measure at frequencies of the order of microhertz. We
describe here our use of programmable two-quadrant power supplies to deliver
multiple small signal measurement tones in the presence of large signal
`working' currents, and our use of these data to generate impedance
measurements with good precision and in reasonable time.
