For the mobility gap of hydrogenated micro-crystalline silicon (μc-Si:H) a value near 1.1 eV is commonly found, similar to the bandgap of crystalline silicon. However, in other studies mobility gap values have been reported to be in the range of 1.48-1.59 eV. Indeed, for accurate modeling of μc-Si:H solar cells it is paramount that key parameters like the mobility gap are accurately determined. In this work we will discuss a method to determine the mobility gap of μc-Si:H using the dark current activation energy of μc-Si:H pin devices, and apply this method to μc-Si:H solar cells with varying crystalline volume fraction. We found the mobility gap is around 1.2 eV to 1.26 eV for μc-Si:H solar cells with a crystalline volume fraction between 50 % and 70 %. For a highly crystalline solar cell we found a mobility gap of 1.07 eV.