The relaxation time of charge transfer and the contact time are two of the parameters controlling charge separation when ice particles collide with one another or with liquid water droplets. For maximum efficiency of charge separation, the relaxation time should be of the same order as the contact time. The relaxation time of charge transport in ice was systematically determined as a function of impurity content and temperature. The contact time for ice/ice collisions was estimated from theory. Depending on the impurity species and their concentration, the relaxation time may be increased or decreased by orders of magnitude with respect to pure ice. Only particles of the order of 10-2 m radius seem to have contact times sufficiently long to give appreciable charge separation by ion transport in collisions between pure or slightly doped (0.1 ppm chloride) ice particles in a dry environment. Charge separation mechanisms involving liquid water and those operating by the transfer of electrons from surface states largely relax these conditions. These results have implications for cloud electrification models involving collision mechanisms.