A cooling system for a vehicle battery is described. The system has a cooling loop in heat transfer communication with the battery, for removing heat therefrom with a coolant, a first cooling device selectively coupleable to the cooling loop, using a refrigerant fluid to transfer heat with the coolant, and a second cooling device selectively coupleable to the cooling loop, using ambient air to transfer heat with the coolant. Various temperature sensors for sensing an ambient temperature and a battery temperature are provided, and connected to a controller that implements a coolant flow bypass function by commanding operation of a valve to bypass the selected one of the cooling devices in response to the battery and the ambient temperatures.