  Large-scale grid-connected lithium-ion batteries are increasingly being
deployed to support renewable energy roll-out on the power grid. These battery
systems consist of thousands of individual cells and various ancillary systems
for monitoring and control. Although many studies have focused on the behaviour
of single lithium-ion cells, the impact of system design choices and ancillary
system controls on long-term degradation and efficiency of system, containing
thousands of cells, has rarely been considered in detail. Here, we simulate a 1
MWh grid battery system consisting of 18900 individual cells, each represented
by a separate electrochemical model, as well as the thermal management system
and power electronic converters. Simulations of the impact of cell-to-cell
variability, thermal effects, and degradation effects were run for up to 10000
cycles and 10 years. It is shown that electrical contact resistances and
cell-to-cell variations in initial capacity and resistance have a smaller
effect on performance than previously thought. Instead, the variation in
degradation rate of individual cells dominates the system behaviour over the
lifetime. The importance of careful thermal management system control is
demonstrated, with proportional control improving overall efficiency by 5 %-pts
over on-off methods, also increasing the total usable energy of the battery by
5 %-pts after 10 years.
