High voltage battery packs are required due to the power required to move a car. A 50hp engine[1] needs ~37kW which at 5 volts, that is 7,500 amps of current,

High voltage battery packs are required due to the power required to move a car. A 50hp engine[1] needs ~37kW which at 5 volts, that is 7,500 amps of current, which requires comically large bus bars (~200x10mm) and conversion electronics. Even at 48 volts[2], you still need ~780 amps, which still requires very large conductors. At 400 volts, for the same motor, the wiring only needs to be sized for 100 amps, which is reasonably practical (think wire found on welders). [1]: Let assume that 50hp is the max required steady state output for a motor in a passenger car, and lets ignore any peak/dynamic loads and HVAC as this is napkin engineering [2]: 48v is generally accepted as the highest voltage before you really need to know what you are doing, though at these power levels, you still should know what you are doing as things like to weld, melt, become bad motors and/or explode.