A user machine provided with a system of a software battery is connected to a host machine for providing a battery through a communications line, and a specific battery in a user machine is charged. The user machine includes a program for communications with the host machine and a software battery management system. The host machine includes a program having a function of listing available batteries, and transmitting a specified battery. With such a configuration, a user can select any battery from among the batteries in the host machine through communications, and charge for his or her own machine.