{ "node": { "id": "term:thermal-conductivity", "type": "Term", "label": "Thermal conductivity", "properties": { "id": "thermal-conductivity", "name": "Thermal conductivity", "abbr": "k", "aka": [ "λ", "through-plane conductivity", "in-plane conductivity" ], "section": "topic-thermal", "definition": "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.", "formula": "q″ = −k·dT/dx", "unit": "W/(m·K)", "related": [ "thermal-resistance", "thermal-diffusivity", "thermal-characterisation", "thermal-interface-material", "biot-number", "thermal-model" ], "example": { "title": "Calculate a slab heat flow", "text": "An illustrative 2 mm layer with k = 1 W/(m·K), area 0.01 m² and a 10 K temperature difference transfers 50 W in steady one-dimensional conduction. Contact resistances and edge losses are omitted." }, "url": "https://lemonergy.com/learn/glossary/thermal-conductivity/", "mentions": [ "heat-flux", "thermal-gradient" ] } }, "edges": [ { "id": "edge:4544", "source": "term:thermal-conductivity", "target": "topic:topic-thermal", "type": "belongsTo", "properties": { "sourceFile": "terms.json" } }, { "id": "edge:4545", "source": "term:thermal-conductivity", "target": "term:thermal-resistance", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4546", "source": "term:thermal-conductivity", "target": "term:thermal-diffusivity", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4547", "source": "term:thermal-conductivity", "target": "term:thermal-characterisation", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4548", "source": "term:thermal-conductivity", "target": "term:thermal-interface-material", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4549", "source": "term:thermal-conductivity", "target": "term:biot-number", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4550", "source": "term:thermal-conductivity", "target": "term:thermal-model", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:mentions/thermal-conductivity/heat-flux", "source": "term:thermal-conductivity", "target": "term:heat-flux", "type": "mentions", "properties": { "sourceFile": "terms.json", "method": "build_glossary.mention_index + mentions", "field": "definition", "meaning": "Text mention; not a causal claim" } }, { "id": "edge:mentions/thermal-conductivity/thermal-gradient", "source": "term:thermal-conductivity", "target": "term:thermal-gradient", "type": "mentions", "properties": { "sourceFile": "terms.json", "method": "build_glossary.mention_index + mentions", "field": "definition", "meaning": "Text mention; not a causal claim" } }, { "id": "edge:4553", "source": "term:thermal-conductivity", "target": "visual:fourier", "type": "illustratedBy", "properties": { "sourceFile": "visual-pages.json" } }, { "id": "edge:4554", "source": "term:thermal-conductivity", "target": "reference:openstax-heat", "type": "cites", "properties": { "sourceFile": "wiki-references.json", "scope": "direct technical reference" } } ], "neighbors": [ { "id": "topic:topic-thermal", "type": "Topic", "label": "Thermal management and heat transfer", "properties": { "id": "topic-thermal", "name": "Thermal management and heat transfer", "lede": "How heat is made, moved and removed in cells and packs, and the hardware that keeps a battery at the right temperature.", "context": "Select spatial detail for the question.", "example": "Equal total heat in different layouts can produce different gradients. Average temperature can miss a hot spot.", "lesson_url": "https://lemonergy.com/learn/fundamentals/battery-aging/", "lesson_label": "Open temperature and aging" } }, { "id": "term:thermal-resistance", "type": "Term", "label": "Thermal resistance", "properties": { "id": "thermal-resistance", "name": "Thermal resistance", "section": "topic-thermal", "definition": "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.", "formula": "R_th = ΔT / Q = L / (k·A)", "unit": "K/W", "related": [ "thermal-conductivity", "thermal-interface-material", "cooling-plate", "heat-flux", "thermal-model" ], "example": { "title": "Translate heat into a temperature difference", "text": "A 2 K/W thermal path carrying a steady 5 W heat flow has a 10 K temperature difference across it. The temperature at the cold boundary is needed to obtain an absolute hot-side temperature." }, "url": "https://lemonergy.com/learn/glossary/thermal-resistance/", "mentions": [ "watt" ] } }, { "id": "term:thermal-diffusivity", "type": "Term", "label": "Thermal diffusivity", "properties": { "id": "thermal-diffusivity", "name": "Thermal diffusivity", "abbr": "α", "section": "topic-thermal", "definition": "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.", "formula": "α = k / (ρ·cₚ)", "unit": "m²/s", "related": [ "thermal-conductivity", "specific-heat-capacity", "thermal-gradient", "biot-number", "thermal-model" ], "example": { "title": "Compare heat-spreading rates", "text": "For an illustrative material with k = 1 W/(m·K), density 2000 kg/m³ and cₚ = 1000 J/(kg·K), α = 5×10⁻⁷ m²/s. A higher α means a temperature disturbance spreads faster for the same geometry." }, "url": "https://lemonergy.com/learn/glossary/thermal-diffusivity/", "mentions": [] } }, { "id": "term:thermal-characterisation", "type": "Term", "label": "Thermal characterisation", "properties": { "id": "thermal-characterisation", "name": "Thermal characterisation", "aka": [ "heat capacity test", "entropy test" ], "section": "topic-testing", "definition": "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.", "related": [ "heat-generation", "entropy-coefficient", "thermal-model", "accelerating-rate-calorimetry" ], "url": "https://lemonergy.com/learn/glossary/thermal-characterisation/", "mentions": [ "open-circuit-voltage", "thermal-conductivity", "specific-heat-capacity" ] } }, { "id": "term:thermal-interface-material", "type": "Term", "label": "Thermal interface material", "properties": { "id": "thermal-interface-material", "name": "Thermal interface material", "abbr": "TIM", "aka": [ "gap filler" ], "section": "topic-thermal", "definition": "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.", "related": [ "cooling-plate", "thermal-management" ], "url": "https://lemonergy.com/learn/glossary/thermal-interface-material/", "mentions": [] } }, { "id": "term:biot-number", "type": "Term", "label": "Biot number", "properties": { "id": "biot-number", "name": "Biot number", "abbr": "Bi", "section": "topic-thermal", "definition": "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.", "formula": "Bi = h·L / k", "related": [ "heat-transfer-coefficient", "thermal-conductivity", "thermal-model", "thermal-gradient" ], "example": { "title": "Check a lumped-temperature assumption", "text": "For h = 10 W/(m²·K), characteristic length L = 0.005 m and k = 1 W/(m·K), Bi = 0.05. Small Bi supports a lumped-temperature approximation; the characteristic length must match the geometry." }, "url": "https://lemonergy.com/learn/glossary/biot-number/", "mentions": [ "heat-transfer-modes" ] } }, { "id": "term:thermal-model", "type": "Term", "label": "Thermal model", "properties": { "id": "thermal-model", "name": "Thermal model", "aka": [ "lumped thermal model" ], "section": "topic-thermal", "definition": "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.", "related": [ "heat-generation", "thermal-management", "arrhenius", "newman-model", "thermal-characterisation" ], "url": "https://lemonergy.com/learn/glossary/thermal-model/", "mentions": [ "thermal-mass", "heat-transfer-modes" ] } }, { "id": "term:heat-flux", "type": "Term", "label": "Heat flux", "properties": { "id": "heat-flux", "name": "Heat flux", "abbr": "q″", "section": "topic-thermal", "definition": "The rate of heat flow through a surface per unit area. Dividing a cell’s heat by its cooled area gives the flux the cooling must carry: a 21700 cell shedding 5 W through its roughly 46 cm² side sees about 1 kW/m². Cooling only one face concentrates the flux and steepens the gradient inside.", "formula": "q″ = Q / A", "unit": "W/m²", "related": [ "heat-transfer-coefficient", "thermal-conductivity", "thermal-resistance", "thermal-gradient", "cooling-plate" ], "example": { "title": "Compare equal heat across different areas", "text": "A 10 W heat flow spread uniformly across 0.02 m² has an average flux of 500 W/m². Concentrating the same heat into half the area doubles the average flux." }, "url": "https://lemonergy.com/learn/glossary/heat-flux/", "mentions": [ "cylindrical-cell" ] } }, { "id": "term:thermal-gradient", "type": "Term", "label": "Thermal gradient", "properties": { "id": "thermal-gradient", "name": "Thermal gradient", "aka": [ "temperature gradient", "temperature spread", "temperature uniformity" ], "section": "topic-thermal", "definition": "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.", "related": [ "hot-spot", "cell-to-cell-variation", "cooling-plate", "thermal-management", "biot-number" ], "url": "https://lemonergy.com/learn/glossary/thermal-gradient/", "mentions": [] } }, { "id": "visual:fourier", "type": "Visual", "label": "Heat flows down a temperature gradient", "properties": { "title": "Heat flows down a temperature gradient", "summary": "Thermal conductivity scales heat flux for the same spatial temperature gradient.", "readout": "A +100 K/m gradient gives -200 W/m². Through 0.01 m², that is -2 W in the positive coordinate direction.", "formula": "q″ = −k dT/dx; heat rate = q″ × area", "note": "One-dimensional, constant isotropic conductivity. Layered cells generally need direction-specific values and boundary conditions.", "software": { "status": "reference", "label": "Reference concept", "body": "Fourier’s law is a reference conduction relationship. A lumped temperature model does not resolve internal temperature gradients or directional conductivity.", "href": "../chapter-1/" }, "html": "
Fourier conduction
Fourier conductionTemperature gradient (K/m) versus Heat flux (W/m²). Signed heat flux. Interpretation follows the graphs.-2,000-1,000-1,000-500001,0005002,0001,000Temperature gradient (K/m)Heat flux (W/m²)
Signed heat flux
" } }, { "id": "reference:openstax-heat", "type": "Reference", "label": "University Physics Volume 2, §1.6: Mechanisms of Heat Transfer", "properties": { "title": "University Physics Volume 2, §1.6: Mechanisms of Heat Transfer", "publisher": "OpenStax, Rice University", "url": "https://openstax.org/books/university-physics-volume-2/pages/1-6-mechanisms-of-heat-transfer", "type": "Textbook section", "scope": "Conduction, convection, radiation and thermal resistance.", "accessed": "2026-10-27" } } ] }