Charge efficiency of Mm (misch metal)-based hydrogen storage alloy (MmNi(3.6)Mn(0.4)Al(0.3)Co(0.7)) electrode was evaluated in the temperature range of 25 to -40 degreesC. The charge efficiency obviously decreased less than 0 degreesC. From the rate of hydrogen absorption in solid-gas phase, exchange current density and charge-transfer resistance measured at various temperatures, it was inferred that the rate-determining step in the charge process of the negative electrode at relatively low temperatures was the charge-transfer reaction on the electrode surface. The addition of Mo to the alloy was effective for improving the electrocatalytic activity, leading to the improvement of the charge efficiency at relatively low temperatures. (C) 2001 Elsevier Science Ltd. All rights reserved.