An automotive battery system that includes a lead-acid battery electrically coupled to a first bus, in which the lead-acid battery supplies electrical power to a starter via the first bus to cold crank an internal combustion engine of a vehicle; a lithium-ion battery electrically coupled to a second bus, in which the lithium-ion battery captures and stores electrical energy generated by a regenerative braking system when the vehicle brakes and supplies electrical power to the second bus using the electrical energy captured from the regenerative braking system such that a first portion of the second electrical power is supplied to an electrical system; and a DC/DC converter electrically coupled between the first bus and the second bus, in which the DC/DC converter controls supply of a second portion of the second electrical power to charge the lead-acid battery.