Energy storage system

An energy storage system is described that is connectable to at least one of a power generating system, a grid and a load, the energy storage system including a battery system, wherein the battery system comprises a plurality of battery cells, a cell balancing unit for performing cell balancing of the plurality of battery cells, the cell balancing unit comprising a plurality of cell balancing resistors, each arranged to be selectively connected across a respective one of the battery cells and a cell balancing controller arranged to vary a time for which the cell balancing resistors are connected across the battery cells in dependence on the temperature of at least one of the cell balancing resistors.