In Chap. 2 , an improved boundary modulation strategy is proposed for the semi dual active bridge (SDAB) DC-DC converter, but the modulation is suitable only for the middle and high power applications under certain voltage conversion gains. It is not suitable to be used for low power in facing wider voltage gains employing fixed switching frequency control strategy. So in order to meet the full load range operation requirement for semi-DAB DC-DC converter working as a battery charger, a multi-mode control strategy is presented in this chapter. At heavy load, it works with single phase shifted control. At light load, it works with PWM modulation and variable switching frequency. At very light load conditions, it works with PWM modulation and constant switching frequency mode. Light load conversion efficiency can be improved significantly by using the proposed multi-mode control strategy. Detailed loss breakdown is made and compared with different switching frequencies under light load conditions. The implementation of the control strategy was given and seamless transition can be obtained among three operating modes according to the load power. The effectiveness of the control strategy was validated by the experimental results of 1.6 kW hardware prototype.