Basic Terminology 29 Current—Current is the measurement of the flow of electrical charge, which may be carried by electrons moving through a wire or circuit board and by ions moving through an electrolyte between the anode and cathode and is measured in Amperes. Cycle—The term cycle refers to the process of discharging and then charging a battery. A complete discharge and then charge is known as one cycle. A cycle may run at various levels of power and/or voltage or even using a constant rate of charge and discharge, depending on the application requirements.
A cycle may be full, completely discharging and then charging the cell, or partial, only discharging to a set level and then recharging back up to starting level. Depth of Discharge (DOD)—The DOD is a measurement of how much of the cell or pack energy will be used for that application. You will typically use lithium-ion battery only somewhere between 20% and 90% of the total amount of energy in order to prevent overcharge at the top and manage low end voltage. So if we think of the battery as a 10-gallon gas tank PHEV, it may only use 60%, or 6 gallons, of its energy—so the DOD would be 80%.
For a HEV or micro-HEV, the battery may only use 30–50% of the available energy or 3–5 gallons in this example. And an EV may use 80–90% of the total energy available, about 8–9 gallons in this example. During system development one design decision that is commonly made, especially for hybrid applications, is to reduce the Depth of Discharge (DOD) during operation in order to achieve greater cycle life and improve safety in the energy storage system. Electrodes—The term electrodes refers to the combined pair of anode and cathode when it is assembled in a battery cell.
Electrolyte—The electrolyte is liquid, gel, or other material that is used as the medium to transfer the lithium-ions back and forth between the anode and cathode. Electric Miles per Gallon (eMPG)—eMPG refers the “electric miles per gallon” equiva- lent rating that is used by the US Environmental Protection Agency (EPA) to offer an equivalent comparison for consumers to traditional mpg used on ICE vehicles. End of Life (EOL)—The EOL of a battery is reached when the battery’s maximum power and energy have been reduced to about 80% of their BOL measurements.
The general rule of 80% is based on either power or energy that has dropped to a point where accel- eration (power) or range (energy) is no longer “satisfactory” to the consumer. However, depending on the application, a lower EOL could be used. Energy—The term energy, which is measured in kilowatt hours (kWh), refers to the amount of energy that a battery will store, think of it as being analogous to the size of the gas tank. Energy Density—Energy density is the measurement of how much energy a cell or pack contains in relation to either its mass or its volume. Energy density is measured either in watt hour per kilogram (Wh/kg) or as Watt hour per Liter (Wh/L).
When it is used to compare to mass in the Wh/kg, it is referred to as Gravimetric Energy Density. When it is referred to in relation to volume in the Wh/L form, it is referred to as Volumetric Energy Density.