Abstract: Disclosed embodiments involve a rechargeable lithium-ion battery module assembly for use as an auxiliary power unit (APU), particularly in commercial trucks. Battery module assembly is recharged through the semi-trailer truck's alternator during engine operation. Battery module assembly has active voltage control capabilities to reduce charge time. Each battery array has two collector plate printed circuit board assemblies (PCBA) and two banks of lithium iron phosphate (LFP) battery cells. Individual battery cells are wire bonded to the collector plate PCBs, one of such PCBs incorporates a battery management system to monitor the electrical parameters and state of charge of the battery cells in the system. Battery cells are thermally coupled to an aluminum enclosure with a thermal gap filling material. Using different chemistries for the APU and the starting battery of the commercial truck, and methods of sequential charge and discharge cycles of each, without any other discrete device.