An inexpensive a.c. impedance measurement system to elucidate the solid-phase electrochemical reactions inside the active materials of batteries during charge anti discharge has been developed by using a cheap galvanostat and a personal computer. In order to develop a simple, in situ, a.c. impedance measurement system for evaluating cells, the impedance of manganese dioxide in alkaline solution under constant-current discharge has been measured. To obtain the impedance, a discrete sinusoidal current is superimposed on the cell under the constant-current polarization, and the resulting transfer function is calculated by a single, sine wave, correlation method. Software developed in this study makes it possible to measure the impedance without using a frequency-response analyzer. A large sinusoidal current of magnitude up to that of the discharge current can be superimposed while keeping the sinusoidal potential response in the linear range, and this enables reliable impedance measurements to be conducted. The impedance and frequency characteristics thus obtained reflect well the discharge behaviour of manganese dioxide in alkaline solution.