Basic Terminology 33 ĞƉƚŚŽĨŝƐĐŚĂƌŐĞ dŽƚĂůĂƉĂĐŝƚLJ ^ƚĂƚĞŽĨŚĂƌŐĞ Figure 8 DOD, SOC, and total capacity of a lithium-ion cell. State of Health (SOH)—SOH is an interesting measure because it does not represent any “standard” measurement; different battery companies, controls companies, and applica- tion users may have different definitions. In general, SOH refers to the current state of health of the battery as compared to its beginning of life measurement. In other words, SOH is intended to tell you how long the battery will take to reach its end of life (EOL).
In essence, SOH is a measure of internal resistance, capacity, voltage, self-discharge, the battery’s ability to accept charge, and the total number of charge–discharge cycles that the battery has completed at that point in time. The state of health calculation is an algorithm that is programmed into the battery management system main controller. State of Life (SOL)—The SOL is often used synonymously with the state of health. However, it may also be a separate algorithm that is intended to determine how much life is remaining in the battery based on some of the same measurements as are used in the state of health calculation.
SOx—The term SOx is used to represent all of the “State of” calculations at the same time, including state of health, state of charge, state of life, etc. Voltage (V)—Voltage is the potential of the charge in a battery, but for clarity it can be thought of as being analogous to the pressure in a hose. Now we have a base understanding of the terminology and lingo that are used in battery design. Chapter 4 will begin discussing some of the basic calculations, questions, and methodology for doing a high-level battery concept development.