Abstract: A heat transfer system (100) for transferring heat from at least one electrical cell (124, 125, 126, 127). In particular, but not exclusively, the invention may be embodied as a heat transfer system (100) for transferring heat from at least one electrical cell (124, 125, 126, 127), the heat transfer system (100) comprising a generally planar and substantially flexible cooling bladder (107) comprising an internal fluid channel (117), wherein the bladder (107) is configured to be thermally coupled with an electrical connector (102), such as the terminal (128), of an electrical cell (124, 125, 126, 127) such that heat may be transferred from the electrical connector (102), or terminal (128), to said fluid channel (117).