Stationary electrical storage system and control method

A stationary electrical storage system comprising:

• a stationary storage battery including a plurality of nickel metal hydride secondary batteries connected in series with each other and configured to supply electric power to a load;
• a controller configured to repeatedly execute charging control with the use of an external power supply and discharging control in order to supply electric power to the load, the controller being configured to stop discharging the stationary storage battery to the load when a state of charge (SOC) of any one of the nickel metal hydride secondary batteries has become a first threshold, the controller being configured to stop charging the stationary storage battery from the external power supply when an SOC of any one of the nickel metal hydride secondary batteries has become a second threshold, wherein
• the controller is configured to additionally charge the whole stationary storage battery in charging control with the use of the external power supply such that:
• the nickel metal hydride secondary battery other than the nickel metal hydride secondary battery having the lowest SOC is allowed to be charged to a fully charged level beyond the second threshold by not stopping charging when the SOC of the any one of the nickel metal hydride secondary batteries has become the second threshold;
• and
• the SOC of the nickel metal hydride secondary battery having the lowest SOC becomes higher than or equal to the second threshold;
• a voltage sensor configured to detect a voltage value of each of the nickel metal hydride secondary batteries;
• and
• a current sensor configured to detect a current value flowing through the nickel metal hydride secondary batteries, wherein
• the controller is configured to determine the first nickel metal hydride secondary battery that has reached a voltage value corresponding to the first threshold on the basis of a prescribed correspondence map between the SOC of each nickel metal hydride secondary battery and the voltage value, and to stop discharging,
• the controller is configured to determine the second nickel metal hydride secondary battery that has reached a voltage value corresponding to the second threshold on the basis of the correspondence map, and to stop charging, and
• the controller is configured to additionally charge the whole stationary storage battery when an SOC difference between the first nickel metal hydride secondary battery and the second nickel metal hydride secondary battery is larger than or equal to a predetermined value.