An automotive battery system, comprising: - a lead-acid battery electrically coupled to a first bus, wherein the lead-acid battery is configured to supply first 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, wherein the lithium-ion battery is configured to: - capture and store electrical energy generated by a regenerative braking system when the vehicle brakes, the regenerative braking system coupled to the second bus; and - supply second electrical power to the second bus using the electrical energy captured from the regenerative braking system; and - a DC/DC converter electrically coupled between the first bus and the second bus, wherein the DC/DC converter is configured to control division of the second electrical power into: - a first portion of the second electrical power supplied to an electrical system via the second bus; and - a second portion of the second electrical power to charge the lead-acid battery, wherein the DC/DC converter is configured to selectively connect and disconnect the first bus and the second bus, wherein the vehicle is configured to disable the internal combustion engine when idle and key on, and wherein the automotive battery system is configured to subsequently supply third electrical power to the starter to warm crank the internal combustion engine, characterized in that: - the lead-acid battery is configured to supply a third portion of the third electrical power and the lithium-ion battery is configured to supply a fourth portion of the third electrical power when state of charge of the lithium-ion battery is greater than a first threshold or state of charge of the lead-acid battery is less than a second threshold; and - the lead-acid battery is configured to supply the third electrical power by itself when state of charge of the lead-acid battery is not less than the second threshold. - a lead-acid battery electrically coupled to a first bus, wherein the lead-acid battery is configured to supply first 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, wherein the lithium-ion battery is configured to: - capture and store electrical energy generated by a regenerative braking system when the vehicle brakes, the regenerative braking system coupled to the second bus; and - supply second electrical power to the second bus using the electrical energy captured from the regenerative braking system; and - capture and store electrical energy generated by a regenerative braking system when the vehicle brakes, the regenerative braking system coupled to the second bus; and - supply second electrical power to the second bus using the electrical energy captured from the regenerative braking system; and - a DC/DC converter electrically coupled between the first bus and the second bus, wherein the DC/DC converter is configured to control division of the second electrical power into: - a first portion of the second electrical power supplied to an electrical system via the second bus; and - a second portion of the second electrical power to charge the lead-acid battery, wherein the DC/DC converter is configured to selectively connect and disconnect the first bus and the second bus, wherein the vehicle is configured to disable the internal combustion engine when idle and key on, and wherein the automotive battery system is configured to subsequently supply third electrical power to the starter to warm crank the internal combustion engine, characterized in that: - a first portion of the second electrical power supplied to an electrical system via the second bus; and - a second portion of the second electrical power to charge the lead-acid battery, - the lead-acid battery is configured to supply a third portion of the third electrical power and the lithium-ion battery is configured to supply a fourth portion of the third electrical power when state of charge of the lithium-ion battery is greater than a first threshold or state of charge of the lead-acid battery is less than a second threshold; and - the lead-acid battery is configured to supply the third electrical power by itself when state of charge of the lead-acid battery is not less than the second threshold.