What is the battery charge and discharge rate C-rate and how to understand it?

‌Definition of charge and discharge rate C

By definition, the charge and discharge rate is numerically equal to the charge and discharge current/rated capacity, C=I/Q. The unit of current is ampere (A), and the unit of capacity is ampere-hour (unit: Ah). For example, a battery with a capacity of 10Ah is discharged with 10A, and its discharge rate is 1C. Similarly, a battery with a capacity of 10Ah is discharged with 5A, which is called 0.5C discharge.

Explanation of charge and discharge rate

What is the effect of discharge current on capacity?

It is worth noting that for the same battery, the capacity obtained by using different discharge currents is different. For the same battery, generally, the larger the current, the smaller the total capacity of electricity that can be discharged. For example: Assuming there is a battery with a capacity of 1000mAh, if it is discharged at a current of 1C (1000mA), the battery can discharge 1000mAh of electricity in 1 hour. If it is discharged at a larger current, such as 3C (3000mA) or 4C (4000mA), the chemical reaction speed inside the battery will be accelerated, but the efficiency of the battery chemical reaction inside the battery will decrease, resulting in a decrease in the total capacity of the battery. This is because when discharging at a high rate, the heat generated by the chemical reaction inside the battery and the resistance inside the battery will increase, resulting in an increase in the loss of electrical energy, which reduces the total capacity of the battery. In actual use, it is necessary to select a suitable current according to the charge and discharge rate and use conditions of the battery to ensure that the battery can meet the required power and energy requirements without causing damage to the battery.

Relationship between system maximum power P, system capacity E, and charge/discharge rate C

From the perspective of charge/discharge time, for two batteries of the same capacity, the battery with a larger charge/discharge rate takes a shorter time to charge from 0% to 100%, and vice versa. Taking charging as an example, it is like using a faucet to fill a pool with water. If the water flow is small (the current is small, 1C), it takes a longer time to fill the pool with water (the battery's power). If the water flow is large (the current is large, 2C), it takes a shorter time to fill the pool with water (the battery's power).

If we multiply the numerator and denominator of the charge and discharge rate formula C=I/Q by the rated voltage, we get C=UI/UQ=P/E numerically, that is, C=power/energy. (The unit of power is watts, and the unit of energy is watt-hours). Usually, when talking about the scale of an energy storage system, it is expressed in the form of "system maximum power/system capacity", where the system capacity reflects the theoretical maximum energy storage capacity of the energy storage system. For example, a certain energy storage power station is 1MW/2MWh, where 1MW refers to the maximum charge and discharge power of the system, and 2MWh refers to the system capacity of the power station. It can be seen that if the power is discharged at a rated power of 1MW, the power station will be fully discharged in 2 hours, and its configuration is 0.5C. When the system maximum power is relatively large relative to the system capacity parameter, such as 2MW/1MWh, it is called a power-type energy storage system. On the contrary, for example, 1MW/2MWh, it is called an energy-type energy storage system. When designing, it should be adapted according to the actual application scenario.