Authors: Antonio Garcia Martinez, Carlos Micó, Imad Elkourchi, and Jaime Martín
CALBET (CALorimetric Battery Energy Tool) is an in-house thermodynamic model developed at CMT – Clean Mobility & Thermofluids, Universitat Politècnica de València (UPV) to analyze the thermal runaway (TR) behavior of lithium-ion cells under accelerating rate calorimetry (ARC) conditions. The model has been designed to provide a fast, robust, and physically grounded estimate of the total energy released during TR events—critical information for battery designers working on safety, pack thermal management, and propagation mitigation.
Originally derived from diagnostic models for diesel combustion analysis (CALMEC), CALBET adapts first-law thermodynamics principles to the context of batteries. It considers both internal energy changes and chemical reactions occurring inside the ARC canister, offering a much more complete picture than traditional approaches based solely on surface temperature variations.
The methodology and validation of CALBET were published in SAE Technical Paper 2023-24-0162, based on experiments conducted with commercial 18650 and 21700 cylindrical cells.
Why CALBET?
Most energy estimates during TR are based on temperature traces of the battery body alone, often neglecting:
- The mass and enthalpy of gases vented during the event,
- Exothermic reactions between vented gases and air trapped in the sealed canister,
- The heat exchange dynamics between gases, cell, and canister surfaces.
This results in significant underestimation of the total heat released—by as much as 30% to 50%, as confirmed experimentally. CALBET addresses these limitations through a physically informed energy balance that combines:
- Sensible internal energy variation of the gas mixture (from pressure and temperature),
- Heat of combustion of vented gases (using post-test GC data),
- Convective heat transfer between gas/cell/canister,
- Enthalpy correction for vented mass based on venting flow and gas properties.
Inputs & Requirements
To apply CALBET, the following test data are required from ARC experiments using a sealed canister:
- Temperature vs time:
- Cell surface (T_cell)
- Canister internal gas (T_gas)
- Canister wall (T_canister)
- Pressure vs time (P_gas)
- Post-test gas chromatography (GC) results:
- Concentrations of H₂, CO, CO₂, CH₄, C₂H₄, etc.
- Geometrical configuration:
- Cell type and mass
- Canister volume and surface area
- Vent hole specification
CALBET assumes cylindrical cells and constant vent gas composition, but these constraints can be relaxed in future extensions.
What CALBET Delivers
- Total energy released during TR, broken down into:
- Internal reactions (electrochemical and decomposition)
- External combustion of gases in air
- Instantaneous power release curves
- Mass flow rate of vented gases
- Temperature and energy balance diagnostics
- Normalized energy release factor (η_E) to compare different cells
CMT – Clean Mobility & Thermofluids, Universitat Politècnica de València.
a research and postgraduate educational centre fully involved in the R+D of Applied Thermo-Fluid Science and incorporating more than 100 people. For more than 40 years we have conducted applied studies for optimizing industrial systems behaviour and assisting in its development as well as basic research for better understanding the relevant physical processes involved. From the deep scientific knowledge to the real-life problems of the automotive, aerospace, railway, marine… industry, we use to combine experimental tests conducted in our state-of-the-art facilities, and theoretical studies assisted by High Power CFD, providing relevant technical and scientific results. Our interdisciplinary approach covers different research areas and aims at Excellence and Innovation.