How is active material volume fraction calculated from porosity in bpx file?

[dion-w]

Hello everyone,
I'm using a bpx file to start pybamm. I wanted to check whether the exchanged capacity between electrodes (i call it reversible electrode capacity for now) is the same, that means that the same amount of capacity leaves one electrode and enters the other. I tried to calculate it with the volume, the electrode area and the active material volume fraction as 1-porosity as the porosity is the only value that i get from the bpx file. This though gets me different reversible electrode capacities. I checked the exchanged number of moles and these were the same. I simulated a C/100 discharge with the bpx-file and saw that the active material volume fraction and the porosity of each electrode don't add up to one. They only add up to 0.94. I tried another bpx file, but here porosity and active material fraction also don't add up to one or even go above 1.

Could anyone tell me how the active material volume fraction is calculated from the porosity when using a bpx file?

The bpx file i used is attached.
nmc_pouch_cell_BPX.json

[dion-w]

For bpx files, the active material volume fraction is calculated back from the specific surface area via "specific surface area = 3 * active material volume fraction / particle radius".