Heat flux

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Cooling plate

A flat aluminium channel assembly under or between the cells through which coolant, usually a water–glycol mix, is pumped. Heat crosses from cell to plate through a thermal interface material; the plate also sets the pack’s temperature uniformity.

Heat transfer coefficient

The heat carried away per unit area per kelvin of difference between a surface and the fluid passing it. Typical values are 5–25 W/(m²·K) for still air, 25–250 for forced air and hundreds to thousands for pumped liquid, which is why fast-charging packs are liquid-cooled. Thermal models use it as their cooling boundary.

Thermal conductivity

How readily a material conducts heat: the heat flux per unit temperature gradient. Cells are strongly anisotropic — around 25–45 W/(m·K) in-plane, along the metal foils, but below 1 W/(m·K) through-plane, across separators and coatings — so the side a cell is cooled from decides how hot its core runs.

Thermal gradient

A change of temperature with position — within a cell from core to surface, or across a pack from coolant inlet to outlet. Packs are commonly designed to keep cells within about 5 °C of each other, because warmer cells age faster and, in parallel, carry more current, so a gradient grows into an imbalance.

Thermal resistance

The temperature difference needed to drive one watt of heat along a path, in kelvin per watt. A 1 mm layer of gap filler at 3 W/(m·K) over 100 cm² has about 0.03 K/W, so 30 W through it costs 1 K. Resistances in series add like electrical ones, which is how cell-to-coolant paths are budgeted.

References & further reading

Technical references

1. University Physics Volume 2, §1.6: Mechanisms of Heat Transfer

   OpenStax, Rice University

   Technical reference · Conduction, convection, radiation and thermal resistance.

Further reading on this topic

These sources provide broader context; they are not presented as citations for every statement in this entry.

2. A General Energy Balance for Battery Systems

   Dawn Bernardi, Ellen Pawlikowski, John Newman · Journal of The Electrochemical Society · 1985

   Topic background · Battery energy balance and irreversible and reversible heat contributions. Reduced thermal models require explicit assumptions and current-sign conventions.