Battery Pack Design Criteria and Selection 37 ,s ĞůůƐ ϭϬй D^ ϭϱй ϰϬй ,ŽƵƐŝŶŐ tŝƌŝŶŐ ϭϱй KƚŚĞƌ ϮϬй Figure 3 HEV battery cost breakdown. HEV, hybrid electrical vehicle. found in a fully electric vehicle that is designed to achieve long driving ranges. In a grid type application this may come through as a battery backup system designed to provide power for several hours. There is also a third type of application that is not frequently talked about. The typical PHEV uses what I refer to as a “balanced” battery. In this type of application the battery needs to operate some of the time like a fully electric vehicle and at other times like a traditional hybrid.
In this case the battery needs to meet both performance requirements. A power battery will have a relatively small amount of energy, usually less than 1.5 kWh. An energy battery needs much higher energy and will range anywhere from 7.5 kWh up to 80 kWh or even greater for very large applications, with the average automotive EV battery being about 24 kWh. In Appendices A through E I have included the battery goals that have been set out by the US Advanced Battery Consortium. These targets are set out for 12 V stop/start type batteries, 48 V stop/start micro-hybrids, and higher voltage HEV, PHEV, and EV type battery systems.
In all applications batteries must be connected with a combination of cells in series, to achieve system voltages, and in parallel, to achieve capacity. The trick is figuring out first what voltages the application needs so that you can back calculate to the required amount of cells to achieve that voltage; of course that assumes that you already know what type of cell you are going to use or at least the general chemistry. Finding the energy requirements can be a bit more difficult. In transportation applications if the efficiency of the vehicle is known it is easy enough to back calculate the desired energy, but for other applications it isn’t quite as easy as that.
Another important thing to understand about a lithium-ion battery is that it will naturally lose capacity over its life due to high temperatures, an increase in internal impedance, and other factors that will determine how long the battery will actually operate. And while lithium-ion does not have a “memory effect” as some of the nickel-based chemistries do, the continued impedance growth will eventually reduce the capacity to a level where it is no longer fit for use in the applications.