The electrochemistry dependent variable voltage of the battery, the volume and weight limitations in electric vehicles (EVs) pose a few challenges regarding higher power density, higher efficiency, wider and higher voltage range, and bidirectional capability of the auxiliary power modules (APM). The APM enables power transfer from the high voltage (HV) traction battery to the low voltage (LV) loads. Furthermore, two-winding based converter topologies lead to high current stress on the LV side. To address the aforementioned issues, this paper firstly proposes a multiwinding CLLC converter topology using GaN HEMT on the HV side and Si MOSFET on the LV side, and secondly considers the variation of battery charge with respect to voltage in the design optimization. Utilizing a multiwinding transformer with parallel outputs distributes the current stress and improve thermal management on the LV side, while increasing power density and efficiency. Optimization for the multiwinding transformer and converter will be carried out using the new approach that considers the electrochemistry of the battery to weight the operation points of the converter. Experiments verify the proposed multiwinding CLLC converter and an efficiency of 96.6 % and power density of 1.36 kW/L including cold plate, microcontroller and protection circuit are achieved.