What’s C-rate?
The C-rate is a unit of declared current value used to estimate and/or specify the expected life time of a battery under variable charge/discharge conditions. The charge&discharge current of a battery is measured in C-rate.
Most portable batteries are rated at 1C. This means that a 1000mAh battery will supply 1000mA for one hour if discharged at a rate of 1C. The same battery discharged at 0.5C can supply 500mA for 2hours. At 2C rate, a 1000mAh battery can deliver 2000mA for 30 minutes. 1C is usually referred to as a one-hour discharge; 0.5C for two hours, and 0.1C for 10 hours.
The capacity of a battery usually measured with a battery analyzer. If the analyzer’s capacity reading is displayed as a percentage of nominal rating, it will show 100% if a 1000mAh battery can supply this current for one hour. If the battery cut off by delivering only 30 minutes, it will show 50%. New batteries sometimes provide more than 100% capacity. When discharging a battery by using a battery analyzer that allows setting different discharge C-rates, higher capacity readings are observed if the battery is discharged at a lower C-rate and vice versa.
By discharging a 1000mAh battery at 2C or 2000mA, the analyzer achieves full capacity within 30 minutes. In theory, the capacity reading should be the same as it would be with a slower discharge, since the same energy is dispensed, just for a shorter period of time. Capacity readings may drop as low as 95% due to internal energy loss and voltage drop causing the battery to reach the low end voltage cutoff voltage more quickly.
Same battery to be discharged by 0.5C or 500mA over two hours may increase the capacity reading to about 105%. The differences in capacity readings at different C-rates is related to the internal resistance of the battery.
In order to calculate the load current value by the charge/discharge rate, it can be obtained by;
∴ C-Rate (C) = Charge or Discharge Current (A) / Rated Capacity of Battery
Also, expected available time of the battery on given discharge capacity can be obtained by;
∴ Used hour of the battery = Discharge capacity (Ah) / Discharge current (A)
Discharge Capability of high power Lithium cell.
[Example] In High Power products, rated capacity of SLPB11043140H model is 4.8Ah. A Lithium-ion NMC cell.
1. What is 1C discharge current condition at this model?
∴ Charge (or discharge) Current (A) = Rated capacity of the battery * C-rate = 4.8 * 1(C) = 4.8 A
It’s means the battery is available for 1 hour by this current discharge condition.
2. The discharge current value under 20C discharge condition is 4.8(A)*20(C)=96A This battery reveals the excellent performance even if the battery discharges 20C discharge condition. The following is the available time of the battery when the capacity of battery shows 4.15Ah
∴ Used hours (h) = Discharged capacity(Ah) / Applied current(A) = 4.15(Ah) / 96(A) ≒ 0.043hours ≒ 2.6 minutes with 96A
It means the battery can be use for 2.6minute (0.043h) with load current of 96A
