We fabricated few-layer graphene FETs by mechanical exfoliation and standard microfabrication techniques. We employed a Li-TFSI based ion gel to induce carrier densities as high as ≈6e14 e−/cm2 in the devices' channel. We found a strong asymmetry in the sheet conductance and mobility doping dependences between electron and hole doping. We combined the experimental results with ab initio DFT calculations to obtain the average scattering lifetime of the charge carriers. We found that the increase in the carrier density and an unexpected increase in the density of charged scattering centers compete in determining the scattering lifetime.