Thermal diffusivity

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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.

Specific heat capacity

The heat needed to raise one kilogram of a material by one kelvin; multiplied by mass it gives a body’s heat capacity in J/K. Lithium-ion cells sit near 1,000 J/(kg·K), so a 0.5 kg pouch cell absorbs about 500 J per degree — the figure a thermal model uses to turn heat into temperature rise.

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 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.

References & further reading

Further reading on this topic

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

1. 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.

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

   OpenStax, Rice University

   Topic background · Conduction, convection, radiation and thermal resistance.