How to measure total cell's capacity fade?

[cyasing]

I tried these three ways of doing it with default model and parameters and SPM:

1. capacity_fade_val[i] = (solution.summary_variables["Measured capacity [A.h]"][0] - solution.summary_variables["Measured capacity [A.h]"][-1]) / solution.summary_variables["Measured capacity [A.h]"][0]
2. capacity_fade_val[i] = (param["Nominal cell capacity [A.h]"] -(p+n)) / param["Nominal cell capacity [A.h]"]
   where p and n are: p = solution.summary_variables['Positive electrode capacity [A.h]'][-1]
n = solution.summary_variables['Negative electrode capacity [A.h]'][-1]
3. capacity_fade_val[i] = (param["Nominal cell capacity [A.h]"] - solution.summary_variables['Total capacity lost to side reactions [A.h]'][-1]) / param["Nominal cell capacity [A.h]"]
   but 1 and 2 result in negative values with a discharge (till 3.3 V) and charge (till 4.1 V) experiment, while 3 results in all 1's, i.e. without any losses. What should I do?

[RuiheLi]

Hi, some ageing mechanisms must be enabled to make the cell degrade, for example: spm = pybamm.lithium_ion.SPM({"SEI": "ec reaction limited"}), you can check a notebook here. Also you would need to extract a discharge step rather than the whole solution. I think the first one is closer to the right one, though I personally prefer the variable "Discharge capacity [A.h]"

[cyasing]

Of course, I know about the options list. It's quite strange though that the capacity does not act normally like other variables.