modular battery system with at least two battery modules , wherein each battery module comprises a cooling member , through which a coolant at least partially flows , and a battery cell , wherein the battery cell is disposed at the cooling member such that a heat - conducting contact is set up between the battery cell and the cooling member and a coolant supply is provided by way of a coolant line for distributing the coolant at the cooling member or for draining the coolant from the cooling member , wherein the coolant line comprises coolant line modules that can be connected together and that form the coolant line at least partially , and wherein a diverting unit is provided in the coolant line for adjusting the flow length of the coolant in the coolant supply to divert the flow direction of the coolant in the coolant line , and the coolant line comprises two flow channels at least in a longitudinal section , the coolant being fed substantially in opposite directions in said flow channels .