A vehicle traction battery assembly is provided. The vehicle traction battery assembly may include an array of battery cells, a thermal plate in thermal communication with the array and defining a coolant path, and an electromagnet. The electromagnet may be positioned proximate to the path and configured to selectively output a magnetic field to influence movement of magnetic particles within coolant flowing through the path to control the flowing. The assembly may also include at least one sensor located proximate to the array and configured to output a signal indicative of a temperature of at least one of the battery cells. A controller may be configured to, in response to the signal, direct the electromagnet to adjust the magnetic field.