A composite battery cell includes a plurality of electricity supply elements connected to each other in series/parallel to form the electricity supply element groups. The electricity supply element groups are connected to each other in parallel/series and packed to form the battery cell with high capacity and high voltage. Each electricity supply element is an independent module and the electrolyte system does not circulate therebetween. There only have charges transferred rather than electrochemical reactions between the adjacent electricity supply elements. Therefore, the electrolyte decomposition would not occur result from the high voltage caused by connecting in series. Both series and parallel connection are made within the package of the battery cell to achieve high capacity and high voltage.