battery-thermal-management

name	battery-thermal-management
description	Battery thermal management — heat generation (Bernardi model), cooling strategies (air/liquid/phase change), thermal runaway, cold plate design, pack temperature uniformity, SOC/SOH.
metadata	{"priority":7,"promptSignals":{"phrases":["battery thermal management","battery cooling","thermal runaway","battery pack temperature","cold plate battery","BMS thermal","EV battery cooling"],"minScore":3}}

Battery Thermal Management — Complete Skill

Heat Generation in Battery Cells

Bernardi Heat Generation Model

Q_gen = I × (U_oc - V_terminal) + I × T × (∂U_oc/∂T)

I = current [A]; U_oc = open-circuit voltage; V = terminal voltage T × (∂U_oc/∂T) = reversible heat (entropy term; positive for charging, negative for discharging)

Simplified (Joule heating dominant at high C-rate): Q_gen ≈ I² × R_internal [W/cell] R_internal: NMC 18650 ≈ 20–50 mΩ; LFP prismatic ≈ 0.5–2 mΩ (varies with SOC, T)

At typical conditions (1C discharge, 50% SOC, 25°C): Q_gen ≈ 0.2–1.5 W/cell depending on chemistry and format

Cell Chemistries and Thermal Properties

Chemistry	Nominal V	Energy density [Wh/kg]	T_optimal	T_max	Notes
NMC	3.6V	150–260	15–35°C	55°C	EV common; high energy
LFP	3.2V	90–160	20–40°C	60°C	Long life; safer; lower V
NCA	3.65V	200–260	15–35°C	50°C	Tesla; highest energy
LTO	2.3V	50–80	-30–40°C	55°C	Long cycle; fast charge
LiCoO₂	3.7V	150–200	15–35°C	50°C	Consumer electronics

Specific heat capacity c_p: NMC: 1000–1100 J/kg·K; LFP: 1000 J/kg·K Thermal conductivity: Radial (in-plane): 20–40 W/m·K (electrolyte + electrode layers) Axial (through-thickness): 1–3 W/m·K (much lower; determines heat path)

Temperature Uniformity Requirement

ΔT within pack ≤ 5°C (target); ≤ 10°C (acceptable) Non-uniform T → cell aging imbalance → capacity fade → accelerated degradation

Module design: tab cooling (effective for prismatic); side cooling; bottom cooling

Cooling Strategies

Air Cooling (Forced Convection)

Simplest; lowest cost; lowest thermal performance h_air ≈ 10–50 W/m²K; limits heat removal to ~500 W/kWh

Turbulent channel: Nu = 0.023 Re^0.8 Pr^0.4 → h = Nu × k_air / D_h Air at 25°C: k = 0.026 W/mK; ν = 15.7×10⁻⁶ m²/s; Pr = 0.71

Limitations: poor at high C-rate (>2C); large temperature gradient along flow path; noise

Liquid Cooling (Cold Plate)

Indirect (cold plate under pack): h_liquid ≈ 1000–5000 W/m²K (turbulent water flow) Can handle 2–5 kW/module

Cold plate design: Channels: rectangular, parallel; ΔP = 12μLQ/(nwh³) [laminar flow; n = channels, w = width, h = height] Target: T_outlet - T_inlet ≤ 5°C; T_cell,max - T_coolant,in ≤ 10°C

Thermal resistance from cell to coolant: R_thermal = t_cell/(k_cell × A) + R_contact + t_coldplate/(k_Al × A) + 1/(h_channel × A_channel)

Coolant: 50% ethylene glycol/water; k = 0.4 W/mK; operating -40 to +65°C

Immersion Cooling (Direct)

Dielectric fluid (mineral oil, 3M Novec) directly contacts cells h ≈ 200–1000 W/m²K (natural), 500–5000 W/m²K (forced) Handles very high power density (> 5C); temperature uniformity excellent Complexity: fluid containment, dielectric selection, leak risk

Phase Change Material (PCM)

PCM stores latent heat during peak load → releases slowly Paraffin wax (n-eicosane): T_melt = 36–42°C; h_fg = 240 kJ/kg k_PCM ≈ 0.2 W/mK → enhance with metal foam or graphite matrix (k → 5–20 W/mK)

Thermal Runaway

Mechanism:

1. Heat source (overcharge, short, external heat, dendrite)
2. Temperature > 70°C → SEI decomposition
3. Temperature > 90–130°C → exothermic reactions (NMC anode + electrolyte)
4. Temperature > 150°C → separator melts; internal short
5. Temperature > 200°C → cathode decomposition; O₂ release + electrolyte combustion
6. Runaway at > 300°C; uncontrolled vent + fire + potentially explosion

Key parameter: Self-Heating Rate dT/dt|onset = Q_gen / (m × c_p) - Q_cooling / (m × c_p) Thermal runaway if dT/dt positive and accelerating

Prevention: BMS (Battery Management System): overvoltage, overtemperature, overcurrent cutoff Cell-level fuses, PTC (Positive Temperature Coefficient) resistors Firewall between cells; venting to external; fire suppression spray

State of Charge (SOC) and State of Health (SOH)

SOC = Q_remaining / Q_nominal × 100% Methods: Coulomb counting (integrating current), OCV lookup, Kalman filter fusion

SOH = Q_current_capacity / Q_initial_capacity × 100% Degradation mechanisms: lithium plating (low-T), SEI growth, particle cracking, gas evolution End of life: SOH < 80% (EV); SOH < 70% (stationary storage second life)

Thermal Model

Lumped model (per cell): m c_p dT/dt = Q_gen - Q_conv - Q_cond Q_conv = h A (T_cell - T_coolant) Q_gen = I² R_int

Coupled electro-thermal model: R_int = R_int(T, SOC) → coupled equations; solve iteratively in MATLAB/Python/COMSOL

Output

Provide: Q_gen per cell [W] at operating C-rate, ΔT expected across pack [°C], cooling strategy recommendation, cold plate flow rate and ΔP [kPa], T_cell,max vs. T_limit, thermal runaway onset temperature, BMS protection triggers.