With increasing levels of electrification of on-road, off-road and stationary applications, use of larger lithium-ion battery packs has become essential. These battery packs require large capital investments and pose a significant risk of self-annihilation without rigorous safety evaluation and management. One of the most extreme safety issues is thermal runaway. At the elevated temperatures that occur after a short-circuit, exothermic decomposition of the cell materials begins. Eventually, when the self-heating rate of the cell is greater than the rate at which heat can be dissipated to the surroundings, the cell temperature rises exponentially. Testing these battery packs to validate design changes which meet safety standards can be cost prohibitive.
The objective of 03-R8968 was to develop a computer-aided engineering (CAE) model capable of predicting thermal runaway and fire propagation behavior at the cell, module and pack-level. A comprehensive testing and simulation workflow was established in this project to calibrate and validate the numerical modeling approach with the test data for each of the individual sub-models: electrochemical, internal short circuit and thermal abuse. The development of the model was split into two main phases: conventional battery models for nominal operating conditions, and exothermic chemistry models that trigger and perpetuate thermal runaway to simulate the resulting elevated temperatures. Commercial Panasonic NCA cells with 4.8 Ah capacity (found in the Tesla Model 3) were used for this project.
The coupled 3D-CFD model was successful by meeting the following metrics of the project:
Accurate cell voltage and temperature prediction at different discharge rates
Prediction of the onset of thermal runaway, peak cell temperature and post peak temperature behavior within 15% of the thermal stability test data for a single NCA cell
Prediction of the onset of thermal runaway, peak cell temperature and post peak temperature behavior within 15% of the nail penetration test data for a single NCA cell
Prediction of the onset of thermal runaway in all cells, peak cell temperature and cell-to-cell thermal propagation reasonably well with nail penetration test data for a nine-cell and 72-cell module