{ "node": { "id": "term:thermal-mass", "type": "Term", "label": "Thermal mass", "properties": { "id": "thermal-mass", "name": "Thermal mass", "aka": [ "thermal inertia", "thermal time constant" ], "section": "topic-thermal", "definition": "A body’s total heat capacity seen as a buffer against temperature change. A 500 kg pack at about 1 kJ/(kg·K) stores roughly 500 kJ per kelvin, so 10 kW of uncooled heat warms it only about 1.2 K a minute. Together with the thermal resistance to the coolant it sets the pack’s thermal time constant.", "formula": "τ = m·cₚ·R_th", "related": [ "specific-heat-capacity", "thermal-resistance", "thermal-model", "phase-change-material", "thermal-management" ], "example": { "title": "Calculate a thermal time constant", "text": "With mass 0.5 kg, cₚ = 900 J/(kg·K) and R_th = 2 K/W, the lumped thermal time constant is 900 s, or 15 min. The calculation assumes a nearly uniform internal temperature." }, "url": "https://lemonergy.com/learn/glossary/thermal-mass/", "mentions": [] } }, "edges": [ { "id": "edge:110", "source": "term:thermal-mass", "target": "workflow:charge-thermal", "type": "usedIn", "properties": { "sourceFile": "wiki-blocks.json" } }, { "id": "edge:4579", "source": "term:thermal-mass", "target": "topic:topic-thermal", "type": "belongsTo", "properties": { "sourceFile": "terms.json" } }, { "id": "edge:4580", "source": "term:thermal-mass", "target": "term:specific-heat-capacity", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4581", "source": "term:thermal-mass", "target": "term:thermal-resistance", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4582", "source": "term:thermal-mass", "target": "term:thermal-model", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4583", "source": "term:thermal-mass", "target": "term:phase-change-material", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4584", "source": "term:thermal-mass", "target": "term:thermal-management", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:4585", "source": "term:thermal-mass", "target": "visual:thermal", "type": "illustratedBy", "properties": { "sourceFile": "visual-pages.json" } }, { "id": "edge:4586", "source": "term:thermal-mass", "target": "reference:bernardi-energy", "type": "cites", "properties": { "sourceFile": "wiki-references.json", "scope": "direct technical reference" } } ], "neighbors": [ { "id": "workflow:charge-thermal", "type": "Workflow", "label": "Inspect pack temperature", "properties": { "id": "charge-thermal", "title": "Inspect pack temperature", "kind": "protocol", "summary": "The separate Charge session can include a lumped thermal node.", "steps": [ "Provide thermal mass and conductance.", "Compare two ambient temperatures." ], "example": "100 kJ/K thermal mass and 100 W/K conductance give a 1000 s no-source temperature-difference time constant.", "observe": "Temperature limits can pause charge; heater and mass are separate.", "limits": "One node does not resolve gradients, coolant geometry or runaway. Studio’s native charger instead uses external temperature.", "url": "https://design.lemonergy.com/", "label": "Open Design → Charge session", "evidence": [ "js/charger.js" ] } }, { "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:specific-heat-capacity", "type": "Term", "label": "Specific heat capacity", "properties": { "id": "specific-heat-capacity", "name": "Specific heat capacity", "abbr": "cₚ", "aka": [ "heat capacity", "specific heat" ], "section": "topic-thermal", "definition": "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.", "formula": "Q = m·cₚ·ΔT", "unit": "J/(kg·K)", "related": [ "thermal-mass", "thermal-characterisation", "thermal-model", "heat-generation", "thermal-diffusivity" ], "example": { "title": "Estimate a temperature rise", "text": "A 0.5 kg body with cₚ = 900 J/(kg·K) requires 4500 J for a 10 K temperature rise if heat loss and phase change are neglected. The heat capacity of the whole body is 450 J/K." }, "url": "https://lemonergy.com/learn/glossary/specific-heat-capacity/", "mentions": [ "pouch-cell" ] } }, { "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-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:phase-change-material", "type": "Term", "label": "Phase change material", "properties": { "id": "phase-change-material", "name": "Phase change material", "abbr": "PCM", "aka": [ "paraffin wax", "latent heat storage" ], "section": "topic-thermal", "definition": "A material that melts at a chosen temperature, around 30–45 °C for batteries, absorbing a large latent heat — roughly 200 J/g for paraffin wax — with little temperature rise. Packed around cells, often in a graphite matrix to lift its poor conductivity, it caps peaks during short heavy loads, but it must be cooled to reset and adds mass.", "related": [ "passive-thermal-management", "thermal-mass", "thermal-management", "thermal-conductivity" ], "url": "https://lemonergy.com/learn/glossary/phase-change-material/", "mentions": [ "graphite" ] } }, { "id": "term:thermal-management", "type": "Term", "label": "Thermal management", "properties": { "id": "thermal-management", "name": "Thermal management", "abbr": "TMS", "aka": [ "battery thermal management system", "BTMS" ], "section": "topic-thermal", "definition": "Keeping cells in their comfortable range, roughly 15–35 °C, and within a few degrees of each other: liquid cooling plates, refrigerant plates or immersion for heat removal, and heaters or waste heat for warming before fast charging in the cold.", "related": [ "cooling-plate", "thermal-interface-material", "operating-temperature", "heat-generation", "preconditioning", "temperature-sensor" ], "url": "https://lemonergy.com/learn/glossary/thermal-management/", "mentions": [ "fast-charging" ] } }, { "id": "visual:thermal", "type": "Visual", "label": "Heat approaches an equilibrium", "properties": { "title": "Heat approaches an equilibrium", "summary": "Thermal resistance sets the steady temperature rise; thermal mass sets the response time.", "readout": "Steady rise: 10 K. Time constant: 8.333 min.", "formula": "ΔT = P R_th [1 − exp(−t / (C_th R_th))]", "note": "C_th = 500 J/K, fixed ambient and heat input, uniform temperature.", "software": { "status": "reference", "label": "Reference concept", "body": "Analysis → Charge session supports optional lumped thermal behavior. This independent explanation uses a 500 J/K thermal mass and constant heat input; it is not an application simulation result.", "href": "https://design.lemonergy.com/" }, "html": "
Temperature above ambient
Temperature above ambientTime (min) versus Temperature rise (K). Lumped response. Interpretation follows the graphs.001515303045456060Time (min)Temperature rise (K)
Lumped response
" } }, { "id": "reference:bernardi-energy", "type": "Reference", "label": "A General Energy Balance for Battery Systems", "properties": { "title": "A General Energy Balance for Battery Systems", "publisher": "Journal of The Electrochemical Society", "url": "https://doi.org/10.1149/1.2113792", "type": "Research article", "scope": "Battery energy balance and irreversible and reversible heat contributions. Reduced thermal models require explicit assumptions and current-sign conventions.", "year": 1985, "authors": [ "Dawn Bernardi", "Ellen Pawlikowski", "John Newman" ], "accessed": "2026-10-27" } } ] }