{ "node": { "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" ] } }, "edges": [ { "id": "edge:4587", "source": "term:heat-flux", "target": "topic:topic-thermal", "type": "belongsTo", "properties": { "sourceFile": "terms.json" } }, { "id": "edge:4588", "source": "term:heat-flux", "target": "term:heat-transfer-coefficient", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4589", "source": "term:heat-flux", "target": "term:thermal-conductivity", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4590", "source": "term:heat-flux", "target": "term:thermal-resistance", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4591", "source": "term:heat-flux", "target": "term:thermal-gradient", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4592", "source": "term:heat-flux", "target": "term:cooling-plate", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:mentions/heat-flux/cylindrical-cell", "source": "term:heat-flux", "target": "term:cylindrical-cell", "type": "mentions", "properties": { "sourceFile": "terms.json", "method": "build_glossary.mention_index + mentions", "field": "definition", "meaning": "Text mention; not a causal claim" } }, { "id": "edge:4594", "source": "term:heat-flux", "target": "visual:fourier", "type": "illustratedBy", "properties": { "sourceFile": "visual-pages.json" } }, { "id": "edge:4595", "source": "term:heat-flux", "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:heat-transfer-coefficient", "type": "Term", "label": "Heat transfer coefficient", "properties": { "id": "heat-transfer-coefficient", "name": "Heat transfer coefficient", "abbr": "h", "aka": [ "convection coefficient", "film coefficient" ], "section": "topic-thermal", "definition": "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.", "formula": "Q = h·A·ΔT", "unit": "W/(m²·K)", "related": [ "heat-transfer-modes", "heat-flux", "biot-number", "thermal-model", "cooling-plate", "coolant" ], "example": { "title": "Estimate convective cooling", "text": "For an illustrative h = 20 W/(m²·K), area 0.1 m² and surface-to-fluid temperature difference 10 K, hAΔT gives 20 W of cooling. The coefficient depends on the actual flow and geometry." }, "url": "https://lemonergy.com/learn/glossary/heat-transfer-coefficient/", "mentions": [] } }, { "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" ] } }, { "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-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": "term:cooling-plate", "type": "Term", "label": "Cooling plate", "properties": { "id": "cooling-plate", "name": "Cooling plate", "aka": [ "cold plate" ], "section": "topic-thermal", "definition": "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.", "related": [ "thermal-management", "thermal-interface-material", "immersion-cooling" ], "url": "https://lemonergy.com/learn/glossary/cooling-plate/", "mentions": [ "thermal-gradient", "coolant" ] } }, { "id": "term:cylindrical-cell", "type": "Term", "label": "Cylindrical cell", "properties": { "id": "cylindrical-cell", "name": "Cylindrical cell", "aka": [ "18650", "21700", "4680" ], "section": "topic-chemistries", "definition": "A cell made by winding the electrodes into a jelly roll and sealing it in a round metal can. Sizes are named by their dimensions: a 21700 cell is 21 mm across and 70 mm long.", "related": [ "jelly-roll", "can", "tabless-design", "prismatic-cell", "pouch-cell", "centre-pin" ], "lesson": "fundamentals/cell-production/#formats", "example": { "title": "Shape versus ratings", "text": "A cylindrical shape does not supply the model’s capacity, resistance or voltage limits; those belong to the actual cell." }, "url": "https://lemonergy.com/learn/glossary/cylindrical-cell/", "mentions": [ "winding" ] } }, { "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" } } ] }