What are the key indicators used to define the State of Health (SOH) of a battery?

State of Health (SOH) represents the overall condition of a battery relative to its original, new state. It's a crucial metric for Battery Management Systems (BMS) because it indicates the battery's remaining performance capabilities and expected lifespan. Several key indicators are used to define SOH, each reflecting different aspects of battery degradation. The most commonly used indicator is capacity fade. Capacity fade refers to the reduction in the maximum amount of charge a battery can store compared to its initial capacity when new. This is usually expressed as a percentage of the original capacity. For example, if a new battery could store 1000 mAh (milliampere-hours), and after a period of use, it can only store 800 mAh, its capacity fade is 20%, and its SOH based on capacity fade would be 80%. Another key indicator is internal resistance increase. As a battery ages, its internal resistance increases, which limits its ability to deliver power. This increase is due to factors like electrolyte decomposition, electrode corrosion, and the formation of solid electrolyte interphase (SEI) layers. The internal resistance is typically measured using electrochemical impedance spectroscopy (EIS) or by analyzing the voltage drop during a pulse discharge. The SOH can be defined based on the percentage increase in internal resistance relative to its initial value. For example, if the initial internal resistance was 50 mΩ (milliohms), and it increases to 75 mΩ, the increase is 50%, which would negatively affect the SOH. Cycle life is another important indicator, representing the number of charge/discharge cycles a battery can undergo before its capacity fades below a certain threshold (usually 80% of its original capacity). SOH can be estimated based on the number of cycles the battery has completed relative to its expected cycle life. Voltage response changes also indicate SOH. As a battery ages, its voltage profile during charging and discharging changes. For instance, the voltage may sag more quickly during discharge or the charging voltage may reach the upper limit sooner. These changes in voltage response can be analyzed to estimate the SOH. Self-discharge rate is another factor. An increase in the self-discharge rate, meaning how quickly a battery loses charge when not in use, can indicate degradation and a decline in SOH. This is often due to internal short circuits or increased parasitic reactions. While capacity fade and internal resistance increase are the most widely used indicators, a comprehensive assessment of SOH often involves considering multiple indicators to provide a more accurate representation of the battery's overall condition. The specific weight given to each indicator can vary depending on the battery chemistry, application, and operating conditions. A BMS uses these SOH indicators to optimize charging and discharging strategies, predict remaining useful life, and ensure safe operation.