Key Parameters of Energy Storage Batteries Explained
Batteries are central to electrochemical energy storage systems. With declining costs, improved energy density, enhanced safety, and extended lifespans, energy storage is now scaling rapidly. This article details critical battery parameters for professionals.
1. Battery Capacity
Battery capacity is one of the key performance indicators measuring a battery’s capability. Battery capacity is divided into rated capacity and actual capacity. The amount of electricity discharged by a battery under specific conditions (discharge rate, temperature, cut-off voltage, etc.) is called the rated capacity (or nominal capacity). Common units for capacity are mAh and Ah (where 1 Ah = 1000 mAh). Taking a 48V, 50Ah battery as an example, its capacity is calculated as 48V × 50Ah = 2400Wh, equivalent to 2.4 kWh.
2. Battery Discharge C-Rate
C is used to represent the rate capability of battery charging and discharging.
Charge/Discharge Rate = Charge/Discharge Current / Rated Capacity
For example: For a battery with a rated capacity of 100Ah, discharging at 50A corresponds to a discharge rate of 0.5C. 1C, 2C, 0.5C represent the discharge rate, indicating the speed of discharge. Using the full capacity in 1 hour is called 1C discharge; taking 2 hours to discharge fully is termed 0.5C discharge (since 1/2 = 0.5C). Battery capacity is often tested using different discharge currents. For a 24Ah battery, a 1C discharge current is 24A, and a 0.5C discharge current is 12A. The larger the discharge current, the shorter the discharge time.
When discussing the scale of an energy storage system, it is often expressed as System Maximum Power / System Capacity (kW/kWh). For instance, an energy storage station rated at 500kW/1MWh means its maximum charge/discharge power is 500kW, and its system capacity is 1MWh. If discharged at its rated power of 500kW, the station’s capacity would be depleted in 2 hours, corresponding to a discharge rate of 0.5C.
3. SOC (State of Charge)
State of Charge, abbreviated as SOC, refers to the ratio of the remaining capacity of a battery after a period of use or prolonged storage to its capacity when fully charged. It is usually expressed as a percentage. Simply put, it indicates the remaining charge level of the battery.
4. DOD (Depth of Discharge)
Depth of Discharge (DOD) measures the percentage of discharged capacity relative to the battery’s rated capacity. For the same battery, the set DOD depth is inversely proportional to its cycle life. Deeper discharge depths lead to shorter battery cycle life. Therefore, it is important to balance the required operating time of the battery with the need to extend its cycle life.
If the change in SOC during the process from fully discharged to fully charged is recorded as 0% to 100%, it is best in practical applications to operate each battery within the 10% to 90% SOC range. Operating below 10% may cause over-discharge, leading to irreversible chemical reactions that can damage the battery’s lifespan.
5. SOH (State of Health)
State of Health (SOH) represents the current battery’s energy storage capability relative to a new battery. It is the ratio of the current battery’s fully charged energy to that of a new battery. Currently, the definition of SOH mainly focuses on capacity, stored charge, internal resistance, cycle count, and peak power, with energy and capacity being the most widely used metrics.
Generally, when a battery’s capacity (SOH) drops to around 70% to 80%, it can be considered to have reached its End of Life (EOL). SOH describes the current health status of the battery, while EOL indicates the battery has reached its end-of-life point and needs replacement. By monitoring the SOH value, the time when the battery reaches EOL can be predicted, allowing for appropriate maintenance and management.