A transfer unit includes a rotatable static-dissipative member with a time-varying electrical property. A second member selectively transfers toner to or from the static-dissipative member. A power source selectively produces an electrostatic transfer field between the static-dissipative member and the second member, so that toner is transferred between the static-dissipative member and the second member. A charger spaced apart from the static-dissipative member selectively deposits charge thereon. A control system successively drives a plurality of different selected voltages or currents through the charger and measures a plurality of respective resulting charger currents or voltages. It uses the selected voltages or currents and the respective charger currents or voltages to automatically estimate a variation in the electrical property. It then causes the power source to produce an electric transfer field that transfers toner and compensates for the estimated variation.