  Low ambient temperatures significantly reduce Lithium ion batteries' (LIBs')
charge/discharge power and energy capacity, and cause rapid degradation through
lithium plating. These limitations can be addressed by preheating the LIB with
an external heat source or by exploiting the internal heat generation through
the LIB's internal impedance. Fast external heating generates large temperature
gradients across the LIB due to the low thermal conductivity of the cell, while
internal impedance heating (usually through AC or pulse charge/discharging)
tends to be relatively slow, although it can achieve more uniform temperature
distribution. This paper investigates the potential of combining externally
sourced resistive film heating with bidirectional pulse heating to achieve fast
preheating without causing steep temperature gradients. The LIB is modeled with
the Doyle Fuller Newman (DFN) electrochemical model and 1D thermal model, and
reinforcement learning (RL) is used to optimize the pulse current amplitude and
film voltage concurrently. The results indicate that the optimal policy for
maximizing the rate of temperature rise while limiting temperature gradients
has the film heating dominate the initial phases and create the ideal
conditions for pulse heating to take over. In addition, the pulse component
shares the heating load and reduces the energy rating of the auxiliary power
source.
