A battery’s charging and discharging rate is determined by its C rating. This rating measures the amount of current a battery draws when charging and discharging. Typically, batteries are rated at a 1C rating, which means that a fully charged 10Ah battery should be able to deliver 10 amps of current in one hour. If discharged at a 0.5C rate, the same 10Ah battery will provide 5 amps of current in two hours; discharged at a 2C rate, it will provide 20 amps of current in 30 minutes. It is critical to understand the C-rating of a battery because charging and discharging currents affect the availability of stored energy.
Battery C-Rate Chart
The chart below shows the C-rates of various batteries and their corresponding usage times. It is important to note that while discharging at different C rates should theoretically consume the same amount of energy, internal energy may actually be lost. Some energy may be converted to heat at higher C rates, reducing capacity by 5% or more.
Manufacturers typically set alkaline and lead-acid batteries at a lower discharge rate of 0.05C, or 20 hours of discharge, to provide a reasonably accurate capacity reading. Even when discharged at this slow rate, lead-acid batteries have a hard time reaching 100% capacity because these batteries are usually overrated. If discharged at a higher C rate than specified, the manufacturer will provide capacity compensation to adjust for the difference.
Calculating the C rating of a battery
The C rating of a battery is determined by the time rate of charging or discharging. Changing the C-rate affects the time required for these processes. The C-rate charging or discharging time varies with the rating: 1C equals 60 minutes, 0.5C equals 120 minutes, and 2C equals 30 minutes.
Formula:
- t= time
- Cr = C rate
- t = 1 / Cr (hour)
- t = 60 minutes / Cr (minutes)
Example:
- 0.5C multiplier Example:
- 2300mAh Battery
- 2300mAh / 1000 = 2.3Ah
- 0.5C x 2.3Ah = 1.15 amps available
- 1 / 0.5C = 2 hours
- 60 / 0.5C = 120 minutes
- 2C Rate Example:
- 2300mAh battery
- 2300mAh / 1000 = 2.3Ah
- 2C x 2.3Ah = 4.6 Amps Available
- 1 / 2C = 0.5 hours
- 60 / 2C = 30 minutes
- 30C Rate Example:
- 2300mAh Battery
- 2300mAh / 1000 = 2.3Ah
- 30C x 2.3Ah = 69 amps available
- 60 / 30C = 2 minutes
Output Formula:
- Er= Rated Energy (Ah)
- Cr = C rate
- I= Charging or Discharging Current (Amps)
- * I = Cr*Er
- Cr = I / Er
Finding the C Rating of a Battery
Smaller batteries are usually labeled with a 1C or 1-hour discharge rate. For example, if a battery is labeled with a 1-hour discharge rate of 3000 mAh, the 1C discharge rate would be 3000 mAh. The C discharge rate is usually labeled on the battery’s label or data sheet. Different chemistries have different C rates; for example, lead-acid batteries typically have lower discharge rates, such as 0.05C or 20-hour rates. The chemistry and design of the battery determine its maximum C rate. Lithium batteries can withstand higher discharge C rates than chemistries such as alkaline batteries. Contact the manufacturer if the C rate is not on the label or data sheet.
Applications Requiring High C-Rates
Many modern applications require high C-rate batteries, including industrial and consumer devices such as RC models, drones, robots, and car starters. These applications need to provide a powerful burst of energy in a short period of time.
Keheng starting power supplies typically require discharge rates of up to 80 C. In the RC industry, high-rate discharge batteries are available at up to 50 C discharge rates. Some batteries on the market claim to have higher C rates based on a maximum pulse discharge rate, which requires the battery to be fully discharged within a few seconds. However, most applications do not require such high C rates.
If you need help selecting the right battery for your application, please consult a Keheng application engineer for expert advice.