Here is the standard that I have come up with to rate cells/packs.
All C rate testing will be done on the cells used to make a pack. For 2S2P packs testing will be done on a 1S1P cell. This allows me to be able to do higher rate discharges and keep the Watt Hour down so my testing loads don't need to support as much Watt Hour.
When taking a 1S1P cell and adding it to another cell in parallel to make a 2S2P cell the IR drops in half this means the cell will be able to withstand double the amp load if testing were to be done as 2S2P.
Cell temperature and ambient temperature is critical to get consistent and accurate results. In my testing an increase of 3 degrees Celsius resulted in a drop of 0.20milliohms.
The charge rate is also very important as this will have an impact on the cell temperature.
All cells will be tested at 22 degrees Celsius and will be charged at 2C.
For a cell to be claimed at a certain C rate it will need to be able to handle the full C rate discharge and have a linear drop in the voltage curve. This means the voltage can drop but not rise again due to excessive heat in the cell lowering the IR thus increasing the voltage.
You can see the first graph of a 3250mAh cell used in a 2S Hardcase pack. Using my standard this cell is very close to a 30C cell but there is a bit of drop in the voltage then rise. This cell is most likely a 25 to 28C cell. You can see that it can handle 40C but has a major drop in voltage then as it gets hot due to the excessive stress the voltage rises again. The capacity retention is only 70% at 40C.
The second graph is another 3250mAh at 25C. This is the graph of the cell tested in the video below.
Typically factories will claim C rate based on the capacity retention. I feel this is not an accurate method as I have seen cells stay above 90% but have the sag due to the amp load being to high.
As my testing is a work in progress I hope to post my results of each cell I test and also post the cells Internal Resistance value as once we know the IR value of a cell we can know it's C rate base on my standard.
This should help clean up the market and I hope factories and resellers will adopt my standard but I think this will be a big challenge as many know that C rates help sell packs.
Here is a video showing how I do C rate testing.