Full Battery Pack Modelling: An Electrical Sub-Model Using an EECM for HEV Applications

With a transition towards electric vehicles for the
transport sector, there will be greater reliance put
upon battery packs; therefore, battery pack
modelling becomes crucial during the design of the vehicle.
Accurate battery pack modelling allows for: the simulation
of the pack and vehicle, more informed decisions made
during the design process, reduced testing costs, and implementation
of superior control systems. To create the battery
cell model using MATLAB/Simulink, an electrical equivalent
circuit model was selected due to its balance between
accuracy and complexity. The model can predict the state of
charge and terminal voltage from a current input. A battery
string model was then developed that considered the cell-tocell
variability due to manufacturing defects. Finally, a full
battery pack model was created, capable of modelling the
different currents that each string experiences due to the
varied internal resistance. The model was then validated with
real-life data from the “Hill Route” section of the First Group
Millbrook Fuel Economy Test Version 5.0 drive cycle of a
mild hybrid electric bus. Results showed a strong correlation
with the measured data and both the state of charge and
terminal voltage simulations of the model. For the string
model, results showed that there was a slight variance in the
state of charge between cells in a string with varied capacities.
However, terminal voltages between cells did not vary significantly
with variances in internal resistance. Future work
includes the creation of a thermal sub-model and an ageing
sub-model, which considers whether the location of a cell
within a pack has a correlation with its degradation. These
sub-models will then be integrated and used as a full battery
pack model.