ENERGY STORAGE SYSTEM AND RECHARGEABLE BATTERY CONTROL METHOD

A power storage system includes a plurality of secondary battery packs and an upper controller. When causing an additional discharge secondary battery pack that is not being discharged to start being discharged by connecting the additional discharge secondary battery pack in parallel with a discharging secondary battery pack that is being discharged, in a state in which some of the secondary battery packs are being discharged, the upper controller predicts discharge start timing at which inter-terminal voltages of the additional discharge secondary battery pack and the discharging secondary battery pack correspond to each other. Subsequently, the upper controller causes the additional discharge secondary battery pack to start being discharged at the discharge start timing.