An electricity storage system comprising: a plurality of chargeable and dischargeable batteries (3-1 to 3-30); a plurality of charging and discharging devices (2-1 to 2-30) connected on a DC side to the respective batteries and on all AC sides in parallel to a power system, and configured to charge and discharge the connected batteries; and a battery controller connected to an Energy Management System EMS via signal lines configured to receive a charging and discharging power value of the entire system, and to distribute the charging and discharging power value of the entire system as an instruction value to each of the charging and discharging devices at a constant cycle or at an arbitrary timing, wherein the battery controller comprises: a selecting unit configured to select the battery which the characteristic parameters relating to the internal status thereof are to be obtained; and a SOC data obtaining unit configured to obtain data on the SOC of each battery and data on temperature of each battery; an arithmetic determining unit configured to calculate and determine whether or not the SOC and a maximum chargeable and dischargeable power are within respective predetermined ranges; an execution determining unit configured to determine whether or not to execute a test charging and discharging to the selected battery based on the above determination; a distribution rate determining unit configured to give, to the charging and discharging device connected to the selected battery, a charging and discharging instruction to perform the test charging and discharging to the selected battery, and to distribute, to all the batteries except the selected battery, a value obtained by subtracting a charging and discharging power value for the test charging and discharging from a charging and discharging power value for the entire system; and the battery controller is configured to obtain the characteristic parameters of the selected battery based on the test charging and discharging. a plurality of chargeable and dischargeable batteries (3-1 to 3-30); a plurality of charging and discharging devices (2-1 to 2-30) connected on a DC side to the respective batteries and on all AC sides in parallel to a power system, and configured to charge and discharge the connected batteries; and a battery controller connected to an Energy Management System EMS via signal lines configured to receive a charging and discharging power value of the entire system, and to distribute the charging and discharging power value of the entire system as an instruction value to each of the charging and discharging devices at a constant cycle or at an arbitrary timing, wherein the battery controller comprises: a selecting unit configured to select the battery which the characteristic parameters relating to the internal status thereof are to be obtained; and a SOC data obtaining unit configured to obtain data on the SOC of each battery and data on temperature of each battery; an arithmetic determining unit configured to calculate and determine whether or not the SOC and a maximum chargeable and dischargeable power are within respective predetermined ranges; an execution determining unit configured to determine whether or not to execute a test charging and discharging to the selected battery based on the above determination; a distribution rate determining unit configured to give, to the charging and discharging device connected to the selected battery, a charging and discharging instruction to perform the test charging and discharging to the selected battery, and to distribute, to all the batteries except the selected battery, a value obtained by subtracting a charging and discharging power value for the test charging and discharging from a charging and discharging power value for the entire system; and the battery controller is configured to obtain the characteristic parameters of the selected battery based on the test charging and discharging. a selecting unit configured to select the battery which the characteristic parameters relating to the internal status thereof are to be obtained; and a SOC data obtaining unit configured to obtain data on the SOC of each battery and data on temperature of each battery; an arithmetic determining unit configured to calculate and determine whether or not the SOC and a maximum chargeable and dischargeable power are within respective predetermined ranges; an execution determining unit configured to determine whether or not to execute a test charging and discharging to the selected battery based on the above determination; a distribution rate determining unit configured to give, to the charging and discharging device connected to the selected battery, a charging and discharging instruction to perform the test charging and discharging to the selected battery, and to distribute, to all the batteries except the selected battery, a value obtained by subtracting a charging and discharging power value for the test charging and discharging from a charging and discharging power value for the entire system; and the battery controller is configured to obtain the characteristic parameters of the selected battery based on the test charging and discharging.