A battery device for an electronic apparatus includes a rechargeable secondary battery, a fuel cell, and a run time computing unit receiving data representative of a voltage versus residual-capacity characteristic of the secondary battery and data representative of a residual capacity of the fuel cell and determining therefrom a residual capacity and a run time of the secondary battery and a run time of the fuel cell and estimates of which of the respective power capacities of the secondary battery and the fuel cell unit will reach the respective, predetermined residual capacities thereof earlier. If it is estimated that the fuel cell will, the computing unit determines a run time of the battery device and supplies data representative of the thus determined run time of the battery device to a power management unit of the electronic apparatus.