A testing device contains measuring circuitry adapted to apply time-varying electrical excitation to a cell or battery, to sense a time-varying electrical response, and to thus determine components of complex immittance at n+m discrete frequencies, where n is an integer equal to or greater than two and m is an integer equal to or greater than one. Computation circuitry utilizes measured complex immittance components at the n discrete frequencies to evaluate the elements of a 2n-element equivalent circuit model. It then calculates the complex immittance of this model at the m discrete frequencies and mathematically compares components of the calculated immittances with components of the measured immittances at the m discrete frequencies. The results of this comparison establish the degree of cell deterioration without regard to the battery's manufacturer, group size, or its electrical ratings.