Power supply apparatus with chargeable battery and charge/discharge method

A power supply apparatus which drives a load and in which a plurality of chargeable batteries are connected in parallel between the node of an external power source and the load, and the common ground of the external power source and the load, comprising:

• charge state monitoring means for judging whether the external power source is in a charging state in which the external power source drives the load and the external power source outputs a sufficient voltage for charging the batteries, or in a non-charging state in which the external power source does not drive the load and a discharging current should be supplied from the chargeable batteries to the load;
• switch means connected in series to each of the plurality of batteries;
• and
• on/off controlling means for controlling the on/off operation of the switches according to the output of the charge state monitoring means and the charging/discharging state of each of the plurality of batteries.