a.c. impedance measurements are made on spiral-wound, size 4r5A, NirMH batteries, which are prepared by using direct-seal technology. Both the positive and the negative electrodes are fabricated by filling in a porous nickel substrate with the active materials. The main purpose of the investigation is to determine the causes. of the early cycling deterioration of this foam-type battery. The results demonstrate that the cycle-life curve can be divided into three parts. After little change in capacity and voltage, deterioration of the NirMH battery occurs as the voltage performance decreases. This is followed by a sharp decrease in both the discharge capacity and the voltage performance. The decrease in voltage performance is due to drying of the separator, which increases the total ohmic resistance of the battery, while the decrease of discharge capacity is due to an inactive surface that increases the charge-transfer resistance of the battery.