In a power supply 
system using various kinds of storage batteries such as a 
large capacity storage battery or a small capacity storage battery, an 
electric power path is complicated, which may increase failures such as a 
short circuit trouble. There is a problem in that, when a 
short circuit trouble between storage batteries occurs, damage to a smaller capacity storage battery is serious. Accordingly, in a power supply 
system according to the present invention, the relationship among voltages, current capacities, and 
electric power capacities of various kinds of storage batteries such as a 
large capacity storage battery or a small capacity storage battery is defined so that damage to the storage batteries may be minimized even when malfunction occurs in, for example, switching a switch, and energy transfers from a higher 
voltage storage device.