What is Battery C Rate & How to Calculate C-Rate?

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What is Battery C Rate?

Battery experts usually use the C rate to measure the speed of battery charge and discharge. However, this concept is often confusing for beginners and people in other fields.

What is C-Rate for Batteries?
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Charging at 1C means that the battery can be fully charged from 0% to 100% within 1 hour, and vice versa.

Charging at 2C means that the battery can be fully charged from 0% to 100% within 0.5 hours, and vice versa.

Taking charging as an example, it is like filling water into a pool with a faucet. When the water flow is small (low current, 1C), it takes a long time to fill the pool with water (like battery charge). When the large water flow (like large current, 2C), it takes a shorter time to fill the pool with water (battery charge).

What is Battery C Rating & How to Calculate C Rate

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 the battery with 10Ah capacity is discharged with 10A, and its discharge rate is 1C. By analogy, if a 10Ah battery discharge with 20A current, the used capacity is discharged in 2 hours, which is called 0.5C discharge.

Calculator:

Multiply the numerator and denominator of the above formula C=I/Q by the rated voltage at the same time, then numerically C=UI/UQ=P/E, namely C=power (power)/energy (energy). (The unit of power is Watt W, the unit of energy is watt hour Wh). Usually, it will be expressed in the way of “power/energy” when we talk about the size of an energy storage system. For example, 1MW/2MWh of an energy storage power station. Here 1MW refers to the charging and discharging power, and 2MWh refers to the energy of the power station. As can be seen, if the discharge is carried out with the rated power of 1MW, the power of the power station will be discharged in 2 hours, and its configuration is 0.5C.

Shorthand method: The corresponding relationship between C and battery charging and discharging time is a reciprocal relationship:

  • 2C —> 0.5 hours
  • 1C —> 1 hour
  • 0.5C —> 2 hours

How to Calculate C-Rate?

The charge-discharge rate refers to the current value required for the battery to release its rated capacity within the specified time, and the value is equal to the multiple of the rated capacity of the battery, usually represented by the letter C. Battery discharge C rate, 1C, 2C, 0.2C is the battery discharge rate: a measure that indicates the speed of discharge. The capacity of the battery is discharged in 1 hour, which is called 1C discharge; in 5 hours, it is called 1/5=0.2C discharge. Generally, the capacity of the battery can be detected by different discharge currents. For 24AH battery, 2C discharge current 48A, 0.5C discharge current is 12A.

C rate Formula one:

  • Current ÷ Capacity = Rate
  • Capacity ÷ Current = Time

Example: It is known that the capacity of the K60 power battery is: 129Ah. 1 hour charging, it is called 1C, and the pile charging current of fast charging is required 129A, 129÷129=1 hour to fully charge. If the charging current of the slow charging pile is 12.9A, the charging The magnification is 1/10=0.1C (12.9÷129=0.1), 129÷12.9=10 hours to fully charge.

Reality: Even with fast charging, few people set it to 1C in terms of strategy, and the temperature rise of the battery must be considered. Another strategy is, after the battery reaches a voltage platform, the current will drop, especially the current at the end charging terminal will be smaller. For example, some lithium iron phosphate batteries When the battery is at 3.1V, the charging rate is 0.8C, and when it reaches 3.4V, the charging rate drops to 0.5C. Depends on charging policy Therefore, if the strategy is not clear, the charging time cannot be determined.

C rate Formula two:

  • Capacity ÷ Current = Time × Speed = Endurance
  • Capacity × power = remaining capacity

Example: The battery life is 300 kilometers at a constant speed, and the discharge current is 40A during driving. In theory, what the driving speed of the K60 car is ?

129÷40=3.225    300÷3.225=93.023

Reality: Because the above are theoretical values, the reality is more cruel. The first charging current decrease when the total voltage reaches the upper limit. The second SOC is an estimated value, and the high current charging in the early stage will bring high total voltage and SOC. SOC 100 dose not mean fully charged. Now among the management system manufacturers that can be ranked among the top three in China, the error of the first place is also plus or minus 5%, that is, within a calculated deviation of 10%. Therefore, people from Lithium ion battery manufacturers will not use SOC to judge the charging capacity of the car. According to the total voltage and discharge curve, the SOC has a certain positive and negative deviation. The same car can be charged to 50% with fast charging and slow charging. Charged to 50%, tested under the same secondary environment and road speed, the battery life is definitely much worse.

C rate Formula three:

  • Energy density (Wh/L) = capacity × platform voltage ÷ volume

Example:

The known battery has a diameter of 18mm and a height of 65mm. The capacity is 2200mah, and the voltage is 3.7V.

Capacity × voltage: 2200mah=22ah, 22×3.7=81.4 (wh)

Volume: 3.14×9×65=1836.9 (square millimeter) 1836.9÷10000=0.18369 (liter)

81.4÷0.18369=443.137895 (wh/L) 1 kilowatt-hour=1 degree of electricity=1000 watt-hours (wh)

Read more: About PV Power Calculation Formula

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