NOVELTY - The low-cobalt hydrogen occlusion alloy has a calcium-copper type crystal structure. The corrosion amount calculated by a specific test is 600 mumol or less. The alloy is used for a battery by which charging-and-discharging is carried out in the central region of charging-and-discharging depth.    USE - As cathode active material for battery (claimed), such as nickel-hydrogen battery, used as power supply for small and portable electric equipment, power tools, electric vehicles, storage apparatuses of natural power sources, fuel cells, and heat pumps.    ADVANTAGE - The low-cobalt hydrogen occlusion alloy improves the initial activity, durability, and cycle characteristics of battery.    DETAILED DESCRIPTION - The low-cobalt hydrogen occlusion alloy has a calcium-copper (CaCu5) type crystal structure given by the formula: MmNiaMnbAlcCod, where Mm is misch metal, a is 3.7-4.7, b is 0.3-0.65, c is 0.2-0.5, d is greater than 0 and at most0.15, and a+b+c+d is 4.9-5.5. The corrosion amount calculated by the following test is 600 mumol or less. A nickel-hydrogen battery cell is assembled using the hydrogen-occlusion-alloy fine particles, grain-refined to an average particle diameter of 45 mum, for the cathode, nickel hydroxide for the anode, 1.3-31 wt.% potassium hydroxide aqueous solution (50 cc) for the electrolyte liquid, and a non-woven fabric for the separator. The cell is dismantled after charging for 6 hours and performing 20 cycles of discharge to 0.7 V by 0.2 C, and charging for 0.25 hours and performing 480 cycles of discharge of 1 Cx0.25 hours in the region of the hydrogen storage capacity 0.4-0.6. The hydrogen occlusion alloy used for the cathode is immersed in sulfuric-acid aqueous solution (25 cc) of pH 2 at 25degreesC for 10 minutes, and then in another sulfuric-acid aqueous solution (25 cc) of pH 2 at 25degreesC for 10 minutes. The operation is repeated using 24 sulfuric-acid aqueous solutions. The dissolution amount of each metallic element in the solutions is determined. The accumulated total elution-amount curve with respect to immersion elapsed time is obtained for every metallic element from the relationship between the dissolution amount and immersion elapsed time, and the accumulated total elution amount corresponding to the point of inflexion of the accumulated total elution-amount curve is obtained for every metallic element. The sum total of the accumulated total elution amount of all the metallic elements gives the corrosion amount. An INDEPENDENT CLAIM is included for battery.