In a battery pack (1), control means (3) reads identification information or operating state detection information out of identification information memory means (4) or battery operating state detection means (10) and inputs it to a modulation circuit (5). The modulation circuit (5) modulates the information into an information signal which can be superimposed on a direct-current power transmission line and sends it to a charging device (20) or a loading device (30) through a positive charge and discharge terminal (11). The charging device (20) or the loading device (30) demodulates the sent information signal to take out the identification information or the operating state detection information and allows power source control means (25, 35) to control the output of charging energy from a charging power source (23) or the output of discharging energy to a load (33). Thus, in addition to the transmission of direct-current power, information can be transmitted only by the connection of two positive and negative terminals.