A detection device and method for accurately detecting the battery capacity remaining in the device independent of the operating mode of the device. When the battery-powered device is operating in a low load mode, the battery discharge voltage is A/D converted and the remaining battery capacity is determined based on the resulting digital signal. When the battery-powered device is operating in a high load mode, the battery discharge voltage is compared with specific threshold voltages, and an interrupt process is run based on the comparison result to calculate the remaining battery capacity.