NOVELTY - The method involves acquiring an open circuit voltage (OCV) of one of multiple serially-connected battery cells i.e. solar cells. A state of charge (SOC) of the cell is determined based on the OCV of the cell. A differential value in the OCV of the battery cell per change of the SOC of the battery cell is determined in accordance with the SOC of the battery cell. Cell balancing of the cells is activated (S14) when the differential value is greater than a reference value. A voltage difference between a maximum voltage and a minimum voltage among the OCVs of the battery cells is determined.    USE - Method for balancing serially-connected battery cells i.e. solar cells, in a tray battery e.g. lithium manganese oxide battery and lithium iron phosphate battery, of a battery system for storing energy from an energy generation system i.e. solar energy generation system, or a power grid by a cell balancing device in an energy storage system (all claimed) to supply power to a load e.g. home or factory. Can also be used for rechargeable secondary battery e.g. nickel-cadmium battery, lead storage battery, nickel metal hydride battery, lithium ion battery, lithium polymer battery, and for wind power generation system and tidal power generation system.    ADVANTAGE - The method enables efficiently performed cell balancing according to multiple types and characteristics of the battery cells by using a cell balancing device of the energy storage system. The method enables stopping an operation of a power converting unit to minimize or reduce power consumption by a converter if power is not produced by the energy generation system.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a cell balancing device    (2) an energy storage system comprising a power conversion system.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart illustrating a cell balancing method.    Step for repeatedly charging and discharging battery cells (S10)    Battery cell voltage measuring step (S11)    Step for determining whether voltage difference between maximum voltage and minimum voltage is smaller than reference voltage (S12)    Step for determining whether differential value is greater than reference inclination (S13)    Cell balancing activating step (S14)