Hot spot

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Arrhenius law

The rule that a rate constant is proportional to exp(−Ea/RT), so a thermally activated reaction speeds up steeply with temperature. Near room temperature many degradation reactions roughly double their rate for every +10 °C.

Joule heating

Heat produced by current flowing through a resistance, equal to the current squared times the resistance. Doubling the current quadruples it: a 1 mΩ cell gives 10 W at 100 A but 40 W at 200 A. It dominates heat generation at high C-rates and also warms busbars, welds and connectors.

Temperature sensor

Usually a negative-temperature-coefficient thermistor glued to a busbar or cell surface, with a few per module. Because sensors read the surface, the hottest point inside a cell can run several degrees higher than the BMS sees.

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 runaway

Self-accelerating heating in a cell: heat speeds up reactions that release more heat, until the cell vents, catches fire or ruptures. An internal short, overcharge or external heating can trigger it.

Thermography

Imaging surface temperature with an infrared camera, which converts the radiation a surface emits into a temperature map. It reveals hot welds, busbar joints and uneven cell heating at a glance, but bare metal reflects more than it emits, so shiny surfaces are usually coated matt black before measuring.

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.