NOVELTY - The apparatus (1) has a connecting device connected in parallel with a battery. A direct current/direct current (DC/DC) converting circuit is connected in parallel with capacitors by selecting on or off condition via a switch to perform a parallel connection. A converting circuit is connected in series with each capacitor via the switch to perform a serial connection. Another connecting device is utilized for switching each capacitor that is charged from the parallel connection to the serial connection by the battery and carries out a discharge output to an exterior part of the apparatus.    USE - DC-power-supply apparatus for use in an electric power storage system (claimed) for a hybrid vehicle and an electric vehicle.    ADVANTAGE - The apparatus ensures that the battery with variation in performance can be utilized in an effective manner. The apparatus ensures that variation of electric power generation amount in an electric power storage system can provide photovoltaic power generation or wind power generation in a large power generation equipment so as to relieve variation of electric power demand, thus increasing utilization factor of power generation equipment. The apparatus ensures that discharge capability of each battery can be utilized in an effective manner. The apparatus comprises a semiconductor switching element that is provided with high-speed response and high durability. The apparatus improves conversion efficiency. The apparatus ensures that an electric power storage system can be manufactured in an inexpensive manner.    DETAILED DESCRIPTION - The battery is selected from a group consisting of a lithium ion battery, a nickel-hydrogen battery, a lead acid battery and a secondary battery. An INDEPENDENT CLAIM is also included for an electric power storage system comprising a converter.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a DC-power-supply apparatus.'(Drawing includes non-English language text)'    DC-power-supply apparatus (1)    Alternating current/DC converter (101)