Capacity expansion system and method for battery pack in energy storage system

The embodiment of the invention provides a capacity expansion system and method for a battery pack in an energy storage system. The system comprises at least two battery packs connected in parallel, a resistance adjusting module and a control module, and the resistance adjusting module comprises a variable resistor connected with each battery pack in series; the control module is used for determining the number M of existing battery packs in the at least two battery packs connected in parallel; based on the initial internal resistance value and the current internal resistance value of the at least one existing battery pack, the initial internal resistance value and the current internal resistance value of the to-be-merged battery pack in the at least two parallel battery packs, and the resistance values of the variable resistors respectively connected in series with the at least one existing battery pack, determining the resistance value of the variable resistor connected in series with the to-be-merged batte