  Hybrid energy storage systems (HESS) have carved a niche in the industry.
HESS improve the system efficiency, reduce the overall cost and increase the
lifespan of the system. The proton exchange membrane (PEM) fuel cell is
hybridized with Li-ion batteries (LIB) for vehicular applications, robotic
applications etc. In applications which have geometrical space constraints, the
temperature of the energy storage elements is influenced by convective heat
transfer. In this paper the thermal analysis of the geometry of PEM-LIB hybrid
system is carried out using COMSOL Multiphysics Software package for different
discharge rates (C rates) of the LIB and different voltages of the PEM cell.
The additional rise in temperature of the LIB pack when placed in close
proximity with PEM cell was in the range of 0.03-0.6$^0$C at 4C. The cell
temperature of the LIB pack increased with increase in C rate and decrease in
PEM cell voltage.
