The state of charge (SOC) is a percentage of how much a battery is charged at any moment, while the depth of discharge (DOD) indicates how much of the battery’s capacity is used in a cycle. For instance, if a 10 kWh battery discharges down to 3 kWh (or 70% of its total capacity), the battery SOC is 30%, and the DOD is 70%. In general, most lithium battery systems are not discharged below 20% SOC to ensure some capacity is left for emergency situations and, in some instances, to ensure the battery is operated within the manufacturer’s warranty specifications.
Battery State of Health (SOH)
State of health (SOH) is a percentage of how much battery capacity is remaining. Battery capacity typically decreases by 1-4% annually, influenced by various factors, such as temperature, charge and discharge rates, energy throughput, and depth of discharge. This natural degradation process is often referred to as capacity fade. For example, a 10 kWh battery with 85% SOH after 7 years of use will have 8.5 kWh of usable capacity. Monitoring and measuring SOH is essential in determining a battery’s aging process and assessing whether the battery is within the manufacturer’s specifications. Most batteries with managed BMS units (closed-loop communication) will report the battery SOH to the inverter or battery controller.
Battery End of Life (EOL)
Battery end-of-life (EOF) is when its capacity has declined to a certain percentage of the original rated capacity. The EOL capacity, defined by the battery manufacturer, generally ranges from 60% to 80% SOH after 10 years, depending on the warranty. The warranty document should state that the battery will retain a specific percentage of the original capacity after a certain number of years or after a specific amount of energy throughput (kWh). Once the battery SOH has reached the EOL, the battery is not considered unusable and should still function for several years, but at a reduced capacity. However, some manufacturers may request that the battery be returned after it has reached its EOL.
