The variable C
Even though C is printed in some way on every lipo we buy, many people do not fully understand what it is or how to figure it out. Simply put C is a variable just like X from your algebra days. It is tied to a capacity of a lipo and as such can be used to figure things like charge rates and maximum continuous discharge rates.
Since C is tied to the capacity of the pack, it can be used to estimate charge times, discharge times and stress. For example if a pack is discharged at 1C, it will be totally discharged in 1 hour. It will also be charged in 1 hour when fully discharged. At 2C those time are cut roughly in half and at 3C those times will be cut down to roughly 1/3rd.
Figuring out what C is for a particular pack is simple, just take the capacity of the pack in mAh, divide it by 1000 to get Ah and then drop the "h", that is C.
For example C for a 2200mAh 3s pack is 2.2A. Start by dividing 2200mAh by 1000 to get 2.2Ah. Then drop the "h" to get C=2.2A. It is as simple as that but here are a few other examples
850mAh 2s 15C lipo
C = .85A
5000 6s 35C/45C lipo
C = 5A
2500 12s 25C/50C lipo
C = 2.5A
Important concept: Note how the number of cells in each pack, as well as other ratings, have no impact on C. Only the capacity is used to determine C.
Now that we know how to find C, we can use it to figure out important numbers like the 1C charge rate for a pack or how many amps a pack can output continuously. So take the first example pack, a 2200mAh 3s pack. What is the 1C charge rate? We just multiply the numbers to find out.
The 1C charge rate for a 2200mAh 3s pack
1C = 1 * C
1C = 1 * (2.2A) = 2.2A
Next the 2C charge rate for the same pack
2C = 2 * (2.2A) = 4.4A
Lastly the 5C charge rate for the same pack
5C = 5 * (2.2A) = 11A
Now that we know C and how to calculate the charge rate for the 2200 3s pack, lets expand on the uses for C and figure out the maximum continuous discharge rating, or maximum amps. Say the above 2200mAh pack is rated at 25C/40C. This means that the continuous discharge rating of the pack is 25C and the burst rating is 40C.
25C = 25 * C = 25 * (2.2A) = 55A
40C = 40 * C = 40 * (2.2A) = 88A
For repetition sake, lets do the same exercise above with a different pack. This time lets use a 6s 5000mAh 20C/30C pack. First the 1C, 2C and 5C charge rates
1C = 1 * (5.0A) = 5A
2C = 2 * (5.0A) = 10A
5C = 5 * (5.0A) = 25A
Now for the max discharge ratings, both 20C continuous and 30C burst.
20C = 20 * C = 20 * (5.0A) = 100A
30C = 30 * C = 30 * (5.0A) = 150A