30 Chapter 3 Energy Storage System—The term ESS is used in various forms, but it generally means the complete battery pack system. The ESS is the combination of cells that are mechanically and electrically connected, along with the appropriate thermal, electronics, and mechanical structure to house the entire unit. In essence, it is everything that is “in the battery box.” High Voltage (HV)—any system with a voltage over 60 V is considered “high voltage” and must include appropriate protections (HVIL, safety disconnect, orange cabling, etc.) in order to prevent harm to the workers or anyone who may come in contact with the system.
Impedance—Impedance is basically a measurement of how much the materials within the cell slow down, or impede, the flow of current when a voltage is applied. It is measured in Ohms and is represented by the symbol Ω . AC impedance is generally used in relation to measuring at the cell level. Impedance basically builds on the concept of DC resistance, which is a measure of magnitude (where there is no difference in phase shift between voltage and current), by adding the concept of phase to the measurement. Jelly Roll—The jelly roll is the combined assembly of anode, separator, and cathode that are either stacked or rolled together and inserted into the can or pouch ( Figure 5 ).
Figure 5 Schematic of a cylindrical lithium-ion battery. LIB—An acronym that is occasionally used in place of the terminology lithium-ion battery or li-ion. Miles per Gallon—This is the standard measurement of fuel efficiency that is applied to almost all ICE vehicles in the United States. In other regions, kilometers per gram CO 2 (kg/CO 2 ) is used for the same purpose.