Relation between C-rate and current used for discharging the cell.
I have a slightly different view. As I understand it there is a "nominal capacity" of a cell, for example 10 Ah. The C rate is defined as the current to discharge the nominal capacity in 1 hour, so it would be 10 A. The C/2 rate is 5 A. The 2C rate is 20 A. However, it is not likely that the 2C rate will discharge the cell in one half hour. Probably the capacity realized at the 2C rate will be less than 10 Ah. So we cannot say that 20 A will discharge the cell in 1/2 hour. Rather it is a defined rate, not a result of an experiment. Correct me if I am wrong about this. Thank you.
C-rate is the current which we use how fast the cell is charging or discharging. 1C means to allow the cell to charge/discharge in 1 hour. 0.5C means to allow the cell to charge/discharge in 2 hour. 2C mean half hour and so on.
I have a slightly different view. As I understand it there is a "nominal capacity" of a cell, for example 10 Ah. The C rate is defined as the current to discharge the nominal capacity in 1 hour, so it would be 10 A. The C/2 rate is 5 A. The 2C rate is 20 A. However, it is not likely that the 2C rate will discharge the cell in one half hour. Probably the capacity realized at the 2C rate will be less than 10 Ah. So we cannot say that 20 A will discharge the cell in 1/2 hour. Rather it is a defined rate, not a result of an experiment. Correct me if I am wrong about this. Thank you.
The C-rate is an ideal calculation based upon the nominal capacity as John has pointed out. Ghulam Ali is slightly incorrect because C-rate is applied to 1/2 the cell cycle most typically the discharge as John said.
I do not agree with any of the answers. Firstly, what do you mean by nominal capacity? Why do you say 10 mA is nominal? The capacity of a cell depends on the charge-discharge current if the testing is done galvanostatically. Therefore, you will not get the same value if you test the cell at different current values.
To get C rate, ( I am referring to a Li-ion cell) first of all, you have to calculate the theoretical specific capacity of the cathode material. Then you will be able to calculate the theoretical specific capacity of your cell (this is dependent on the amount of the cathode material you use in your cell). This will give the theoretical current value to charge or discharge the cell in 1 h which means to say that this is the amount of current to use if you want to charge-discharge the cell at 1 C rate.
I agree with N. Kam. The C-rate is based upon the theoretical capacity and the active loading. I would point out that the C-rate typically discharges less than the time desired due to the number of kinetic limitations in Li-ion cells particularly in research-type cells such as coin or pouch cells but less so in industry manufactured cells.
However, I think that the way battery manufacturers calculate their batteries is not using the way scientists do the calculations. They normally rate their batteries in Ah (ampere hour) which actually does not tells us much at all. They don't even say at what current values are the rating done at. As I have mentioned, the current values does affect the capacity of the batteries.
I suppose, for a practical battery, we can say that 1 C rate will be the current value to charge or discharge a battery in 1 h. Which means to say that in the lab, we have to try out the charge-discharge process of the first cycle for different current values and take the current value that produces a discharge of 1 h as 1 C rate. However, the voltage range complicates this too. Batteries carge-discharge at different capacities for differet voltage ranges!!! So, the situation is quite, quite complex.
A reasonable testing scheme for 1 C rate, I suppose, is to obtain the current value for a discharge of the battery in one hour at the operating volatge of 4.2 to 2.5 V (1st cycle only). Therefore, you have to test the batteries at different current values to know the practical 1 C rate for your particular cathode material. Thus, you will then be able to calculate the practical C-rate for all other current values.
Dear John, Ali, Christopher, Gualm Ali , Kamarulzaman.
thanks a lot for valuable comments... sincerely
Dear all,
I want to test coin cell lithium ion battery using EC Lab software. But as a beginner, i don't know how to test it using that software. I have assembled the cell using LiFePO4 as cathode material. As u mentioned in one of your answer, the theoretical capacity of the LiFeP4 based lithium ion battery is 170 mAh/g. Now what current i must use for 0.1 C rate? and will the voltage range be 4.2 and 2.5 for charging and discharging?
Kindly reply me
I need your help desperately
Thanks and regards
Usaid Farooqui
This may be useful:
http://web.mit.edu/evt/summary_battery_specifications.pdf
Could any one tell me how to calculate C rate based on cathode (LFP) active material. I
what is the rest time in charging and discharging of a battery and why it is required any evidence?
John Dunning
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