Modeling of Li-Ion Battery Packs as Basis for Design of Battery Thermal Management Systems

Li-ion batteries are one of the most popular battery types on the market, due to their
prime properties such as high capacity, low self-discharge rate, zero-maintenance, high
energy density and long lifetime. However, safety still remains a major drawback, due
to overheating and thermal runaway. The shortcomings of safety were reflected in the
recent accidents, where fires and explosions were reported in cell phones, electric cars,
laptops, e-hovers and even airplanes. The goal of this thesis is to generate knowledge,
understanding and methods to ensure safety in Li-ion cells and packs.
For achieving the goal, two main mathematical models and a side model were developed
as follows:
1. A lumped venting model for analyzing the thermal runaway behavior in a cylindrical
Li-ion cell, which includes the venting of gases and solids. The venting model
was developed by deriving the energy and the mass conservation equations, and
the results were compared against experimental data from the open literature;
2. A simplified thermal runaway model for investigating the propagation of thermal
runaway in a battery pack designed by NASA for astronaut spacesuits. A simplified
model was initially built for single battery cells with an internal short circuit
device (ISCD) implanted inside, used for triggering thermal runaway at low temperatures.
The simplified lumped model was then coupled with a 2D thermal
FEM for investigating the pack design. The simplification consists of implementing
an efficiency factor term in the energy equation which accounts for the energy
leaving with the gas and solids;
3. An electrochemical model for investigating the discharging process of prismatic
large-cell battery packs designed by the project partner company Banke A/S. The
cells were charged at different ambient temperatures, and the capability of phase
change materials to protect the cells was investigated. Moreover, the influence
of including the fluid flow equations was analyzed. A 1D electrochemical model
was developed by using the mass, charge and energy conservation equations for
a single cell and coupled with a 2D thermal model.
By comparing the pressure, the temperature, the vapor quality and mass loss, the
results from the venting model showed good agreement with analytical calculation,
within a deviation of 3.5%. When compared against the experimental data, it was
found that the model predicted the amount of energy measured experimentally for
different states of charge, within a deviation of 1.75%. The venting model is the only
model available in the open literature for analyzing in detail all the stages of thermal
runaway in a cylindrical cell.
The results from the simplified thermal runaway model showed that by fitting an efficiency
factor for modeling the battery cells with an internal short circuit device, the
maximum temperature predicted by the model matches the maximum temperature
measured experimentally. By using the simplified model and coupling it with a 2D
model of the battery pack, the model showed good agreement with the experimental
data, with a slight difference (from 5 to 10 C) in the peak temperatures. The simplified
model is the only model available in the open literature for modeling the internal short
circuit device developed by NASA and its partners.
The third electrochemical modelwas initially validated experimentally, and good agreement
with experimental data was found. The results showed that by having phase
change materials around the battery cells in a pack, and discharging the cells at extreme
ambient temperatures, no significant improvements are achieved. It was found
that at low ambient temperature, having air is better than phase change materials. . .