Thermal Management 117 the adjacent mounting frame on the left. The heat transfer from the cell to the cooling plate uses conduction both in the direct conduction of heat from the cell to the cooling plate and to the liquid cooling channels. These channels then use convection to remove the heat through a liquid medium away from the cells. At the top of the cell, a simple copper bus bar is shown being connected to the tab of the cell. This demonstrates the heat transfer through direct conduction from the cell to the bus bar.
Finally, the frame piece to the left of the cell may be used for mechanical structure of the module; however, even if it is not directly in contact with the cell it will experience radiative heating from the cell. Lithium-ion chemistries tend to operate best between about 10 and 35 °C; this is referred to as the optimal temperature range. This is where you want the batteries to be at most of the time. However, most all lithium-ion chemistries will still operate down to about −20 °C and up to about 45 °C; this is known as the operational range. In this temperature range, no reduction in battery life would be expected to be experienced during normal operation.
Between −20 and −40 °C the electrolytes may begin to freeze and the cold temperatures increase the impedance within the cell thereby resisting the flow of ions and reducing capacity and performance, and above 60 °C many lithium-ion cell chemistries begin to get more unstable; this is known as the survival temperature range ( Figure 2 ). This brings us to another concept that we should consider, at least briefly, in our thermal system design—thermal mass. Thermal mass is essentially the amount of heat that each object within the pack that has mass (all of them!) can absorb.
Much in the same way that a battery can only store so much energy, all of the components in the battery pack can absorb a certain amount of heat, which is directly relative to their mass and the specific thermal properties of that particular material. For example, a copper bus bar will be able to absorb more heat than a plastic cover. Above its maximum ability to absorb heat, the material will begin to breakdown, plastics will melt and/or burn, and metals will begin to melt. Thermal mass will not generally drive major design considerations, other than in some of the compo- nents that are directly in contact with the areas of heat generation.
However, in a passively Figure 2 Lithium-ion cell temperature ranges.