The electrochemical properties of Ni/MH batteries with a series of Co-substituted superlattice alloys as the negative electrodes in sealed cells were investigated and compared to those of a conventional AB5 alloy. A small amount of Co (<5 at%) in the superlattice alloys is found to improve the discharge capacity, increase the discharge voltage, improve the low-temperature performance, and extend the cycle life. However, it is also responsible for the deteriorations in charge retention, 45 °C voltage stand, and high-temperature performance. Alloy C3 with 4.7 at% Co shows well-balanced properties − superior performance at low temperature and in cell capacity, charge retention, cycle stability, but is inferior at high-temperature when compared to the AB5 alloy. The failure mechanisms of the cells were investigated with scanning electron microscopy, inductively coupled plasma and X-ray diffraction analysis. The failure mechanisms of alloy C3 were identified as a combination of electrolyte dry-out and positive electrode swelling.