A diagnosis method for SOH of batteries includes the steps of: providing data of discharge voltages and currents of a battery unit; setting an acceptable value of a predetermined period of time and an acceptable range of variation rate of discharge current for the battery unit; calculating variation rates of discharge voltages and currents of the battery unit within the predetermined period of time by using the data of discharge voltages and currents; recording the data of discharge voltages and currents of the batteries and variation rates thereof; comparing the data of discharge voltages of the batteries and the variation rates thereof if the variation rates of discharge current are fallen within the acceptable range of current variation with selected reference data; and converting results of compared data of discharge voltages of the battery unit and the variation rates thereof into a SOH index.