Thermal conductivity

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Biot number

The ratio of the resistance to heat conduction inside a body to the resistance to convection at its surface. Below about 0.1 the body heats almost uniformly and a lumped thermal model is adequate; large, strongly cooled cells with poor through-plane conductivity exceed it and need a model that resolves the internal gradient.

Thermal characterisation

Measuring the thermal properties a model needs: specific heat capacity in a calorimeter, thermal conductivity through and along the layers, the entropy coefficient from OCV against temperature, and heat generation under load. Without them a thermal model is a guess.

Thermal diffusivity

How quickly temperature evens out inside a material: thermal conductivity divided by density times specific heat. Heat spreads a distance L in roughly L²/α, so crossing a 10 mm cell at about 0.2 mm²/s through-plane takes several minutes, while aluminium, near 100 mm²/s, equalises almost at once.

Thermal interface material

A soft, thermally conductive paste or pad that fills the gap between cells and the cooling plate so heat does not have to cross air. It is dispensed by the kilogram in a pack and is a surprising share of its cost.

Thermal model

A model of cell temperature: at its simplest one thermal mass with heat generation inside and convection outside, at its most detailed a 3D simulation of the pack. Temperature feeds back into every rate, so electrical and thermal models are usually solved together.

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.