This paper makes an effort to define an optimal electrical architecture for a 12V BSG system for a mid- sized diesel engine, with the help of functional validation tests and specific test cases developed by the authors. The first few sections of the paper explain various modes of a 12V BSG hybrid operations namely-ESS, torque assist and recuperation on a reference vehicle. Set of test cases have been developed to explain 12V BSG behavior as a DC motor during ESS and torque- assist mode of operation and as a 3-phase generator during regenerative coasting mode. In the final section, an optimized electrical architecture of 12V BSG system is proposed for mid-sized diesel engine with manual transmission, based on objective measurement of test cases and a Pugh Comparison matrix of various feasible architectures.