​Understanding Key Performance Parameters of Energy Storage Batteries

Gaining insight into the key performance parameters of energy storage batteries is crucial for understanding how they are used and how they perform within a storage system. Below is an explanation of several main parameters:
    1.    Cycle Life
This refers to the number of times the battery can be fully charged and discharged. The length of the cycle life is directly related to the battery’s durability and usage cost. For instance, in scenarios requiring long-term stable energy storage, batteries with a long cycle life are needed. Under proper usage conditions, lithium iron phosphate (LFP) batteries can achieve a high number of cycles. However, some batteries (such as ternary lithium batteries) have faster capacity degradation and shorter lifespans, affecting their suitability for long-term energy storage projects.
    2.    Capacity
Typically expressed in ampere-hours (Ah). The energy (Wh) can be calculated as Power (W) × Hours (h) = Voltage (V) × Ampere-hours (Ah). For example, a 48V100Ah battery indicates a capacity of 4.8 kWh. The capacity determines how much energy can be stored in a single charge. When selecting a battery, one should consider specific storage needs. For home energy storage systems, factors such as household electricity consumption and the desired duration of stored power should be taken into account to determine the appropriate battery capacity.
    3.    Charge/Discharge Efficiency
This refers to the energy conversion efficiency during the charging and discharging process. The charge/discharge rate (C-rate) equals the charge or discharge current divided by the rated capacity. For example, if a 100Ah battery is discharged at 15A, the discharge rate is 0.15C. Charging and discharging efficiency affects energy loss during these processes. A high-efficiency battery uses energy more effectively during charging and discharging, reducing waste and significantly contributing to the overall economics and performance of an energy storage system.
    4.    Depth of Discharge (DOD)
This is the percentage of the battery’s rated capacity that is actually discharged. For the same battery, a deeper DOD typically results in a shorter cycle life. Improving one aspect of performance can often compromise another. For example, at 80% DOD, lithium batteries may achieve 6,000–12,000 cycles. Therefore, in actual use, controlling the depth of discharge properly is necessary to prolong battery life.
    5.    State of Charge (SOC)
This represents the percentage of remaining battery capacity relative to its rated capacity. An SOC of 0% means the battery is completely discharged, while an SOC of 100% means it is fully charged. As an important parameter in a Battery Management System (BMS), SOC helps reflect remaining battery capacity and operating status in real time. This allows users to understand the current power level and plan charging and discharging more effectively.
    6.    State of Health (SOH)
This encompasses factors such as capacity, power, and internal resistance. It is defined as the ratio of the battery’s capacity—when discharged from full charge at a certain rate down to its cutoff voltage—to its nominal capacity. In simpler terms, it is the ratio of the battery’s current performance parameters to its rated parameters after some period of use. A brand-new battery is 100% SOH, while a fully degraded battery is 0%. According to IEEE standards, if, after some time in service, the fully charged capacity is less than 80% of the rated capacity, the battery should be replaced. Monitoring SOH helps detect performance decline early, allowing timely action.

​Battery Safety and Environmental Considerations
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Safety and environmental concerns cannot be overlooked when using batteries. Below are some relevant points and corresponding measures:
    1.    Safety Risks and Preventive Measures: Overcharge and Over-Discharge
Lithium batteries used improperly—such as being overcharged or exposed to high temperatures or impacts—can undergo internal thermochemical reactions, resulting in thermal runaway. If thermal runaway propagates within a battery module, it can cause a system-level fire. Additionally, toxic and flammable gases may be released, making firefighting difficult. To prevent such risks, choose batteries that comply with relevant safety standards (e.g., IEC62619). At the same time, the Battery Management System (BMS) plays a key role and should be certified under IEC61508 to ensure the battery does not operate beyond its limits. Some storage systems also adopt multi-stage charging (three-stage charging), including constant current, constant voltage, and float charging, to improve safety and avoid overcharging.
    2.    Battery Module Safety Integration Risks
Battery modules and racks should meet the requirements of UL1973 and IEC62619. Selecting batteries certified by UL9540A means they have been tested to simulate thermal runaway and to check whether a fire would spread. Batteries should be installed in sturdy battery cabinets that keep each unit separate, helping to prevent a fire from spreading to other cabinets. The cabinet housing should have high fire resistance and provide thermal insulation to keep batteries within a suitable temperature range (typically 20°C to 23°C).

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