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This paper proposes a method to implement gain-scheduled adaptive PI control of wireless charging series-series resonant converter duty cycle control at fixed switching frequency operation. The method is applicable for both voltage mode battery charging control and current mode battery charging control. The gain scheduled PI controller look-up tables are established based on the optimized frequency domain control loop design in the entire equivalent load resistance variation range for the full battery charging cycle including both current mode charging operation and voltage mode charging operation for the voltage mode control. The same gain-scheduled PI control look-up tables are applied for the current mode battery charging control with additional loop gain applied for battery equivalent load resistance variation compensation. In both voltage mode and current mode control scheme, the DC input voltage magnitude variation and the non-linear gain of duty cycle command are both compensated so that the control system performance will not be affected by operation condition changes of DC input voltage and duty cycle level. The proposed gain scheduled PI control ensures that uniformed control loop bandwidth and satisfactory dynamic responses are achieved with sufficient stability margins in the wide range of equivalent load resistance change required for battery charging profile optimization.