Battery coolant circuit control

A thermal-management system for a traction battery of a hybrid vehicle, the thermal-management system comprising:

• a refrigerant subsystem including a compressor, a condenser, a battery chiller, and an evaporator;
• a coolant subsystem including
• a chiller loop arranged to convey coolant through the chiller to transfer heat from the coolant subsystem to the refrigerant subsystem,
• a radiator loop arranged to convey coolant through a radiator to transfer heat from the coolant subsystem to outside air, the radiator loop having a bypass valve configured to route coolant around the radiator via a bypass line when the bypass valve is actuated to a bypass position and configured to route coolant to the radiator when the bypass valve is actuated to a radiator position,
• a proportioning valve having an inlet connected to an outlet side of the traction battery via an outlet conduit to receive coolant from the traction battery, a first outlet connected to the chiller loop to route coolant to the chiller, and a second outlet connected to the radiator loop to route coolant to the radiator, wherein the proportioning valve is actuatable to proportion coolant between the radiator and chiller between 0 and 100 percent, inclusive, and
• a temperature sensor disposed on the outlet conduit and configured to output a signal indicative of a coolant temperature;
• and
• a controller configured to, in response to the proportioning valve directing at least a portion of the coolant to the second outlet and the outside air temperature exceeding the coolant temperature, actuate the bypass valve to the bypass position.