The most distinctive characteristic of an energy storage system is that it includes an energy storage medium—batteries. One of the key performance indicators of batteries is their charge and discharge speed or capability, often expressed in specifications as a “***C” parameter, such as “0.2C,” “0.3C,” “1C,” or “2C.” In industrial and commercial energy storage systems, the most common specification is “0.5C.” But why is 0.5C the most frequently used?

1. What is “C”?

"C" is the abbreviation for Coulomb, the unit of electric charge. This concept was first introduced by French physicist Charles-Augustin de Coulomb and is defined as the quantity of electric charge that passes through the cross-sectional area of a conductor in one second.

In the context of energy storage batteries, "C" represents the charge and discharge rate, which indicates the size of the charge and discharge current. A 1C charge and discharge rate means that the battery can discharge its entire capacity within one hour, while a 2C rate means that the battery can discharge its entire capacity in half an hour.

2. How is “C” calculated or derived?

The "C" rate is a conceptual parameter, unlike current (A) or voltage (V), which are fixed values. For example, if an electrical circuit has a current of 1A, no matter what equipment is used for measurement, the current value will remain at 1A.

For a 1C charge and discharge rate, however, it also depends on the specific capacity of the battery. For a battery with a capacity of 1Ah, its 1C charge and discharge current would be 1A. For a battery with a capacity of 2Ah, its 1C charge and discharge current would be 2A, and so on.

Therefore, the formula is:

Battery Charge/Discharge Rate (C) = Battery Charge/Discharge Current ÷ Battery Rated Capacity

For example, a 1000mAh battery, at 0.2C, would have a charge/discharge current of 200mA (1000mAh × 0.2), while at 1C, it would have a charge/discharge current of 1000mA (1000mAh × 1).

Using the "C" rate makes it easy to compare the charge and discharge capabilities of two batteries with the same total capacity under the same conditions. For instance, if two batteries both have a capacity of 1Ah, but Battery 1 has a 3C rate, meaning it can charge/discharge at 3A, while Battery 2 can only charge/discharge at 0.5C or 0.5A, it intuitively shows that Battery 1 has a much better instantaneous charge/discharge capability (burst power).

3. Why 0.5C?

It’s essential to understand the impact of the C rate on the battery:

• Excessive polarization and increased internal resistance: A higher charge/discharge rate increases the growth rate of internal polarization and resistance, leading to reduced energy storage capacity.
• Loss of active materials and Li+: A higher charge/discharge rate speeds up the loss of active materials and Li+, resulting in capacity degradation.
• Electrolyte consumption: A higher charge/discharge rate increases electrolyte consumption, further affecting battery lifespan.
•  

Typical lithium metal batteries use manganese dioxide (MnO2) as the positive electrode material, metallic lithium or lithium alloy as the negative electrode material, and a specialized non-aqueous electrolyte solution.

For lithium-ion batteries:

• Discharge Reaction: Li+ + MnO₂ → LiMnO₂
• Charge Reaction: LiCoO₂ + 6C → Li(1-x)CoO₂ + LixC6

Setting the charge/discharge rate too high can negatively impact battery lifespan, so it should not be set too high. However, a rate that is too low, such as 0.1C, 0.2C, or 0.3C—commonly seen in lead-acid batteries—means low charging current and slower speed. Although this better protects the battery, it is not ideal for industrial and commercial energy storage projects where profit comes from peak-valley arbitrage based on time-of-use rates set by the power grid. A lower C rate reduces the kWh charged or discharged during the same time period, thereby lowering daily profit and extending the payback period.

In summary, choosing a 0.5C charge/discharge rate balances charge/discharge capacity, battery lifespan protection, and compatibility with peak-valley periods. For example, for a 209kWh or 215kWh single cabinet system paired with a 100kW PCS (Power Conversion System), the battery can be fully charged or discharged in 2 hours, aligning with the length of peak and valley periods defined by various grid companies. This ensures that charging and discharging can occur within the specified time frame without wasting power or time and achieves the expected return on investment. Therefore, a 0.5C rate is reasonable.