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A conventional vehicle uses a lead acid battery to start the engine and power auxillary loads (lighting, electronics, etc.). Hybrid electric, electric, and fuel cell vehicles (HEVs, EVs, and FCVs) have conventional lead acid batteries, but also have propulsion batteries, which are constructed quite differently—they are built for high power, high energy, and long cycle lives. Some low-voltage hybrid vehicles use advanced lead acid batteries, known as valve regulated lead acid (VRLA) batteries.

How a Battery Works

A battery is a portable energy source with three basic components-an anode (the negative part), a cathode (the positive part), and an electrolyte (a liquid solution that aids in the flow of energy). As current is drawn from the battery, electrons start to flow from the anode through the electrolyte and back to the cathode in the battery cell. Diagram 1 depicts a battery module and a battery cell.

Schematic of a battery module and a battery cell.

This flow of energy from the battery to power the vehicle results in a decrease in voltage (energy level). In other words, as a battery is used or discharged over time, the voltage drops as the anode and cathode undergo electrochemical changes.

HEVs, EVs, and FCVs also use electric motors, which are powered by the batteries to assist the engines. In addition, many hybrids use energy generated from deceleration through regenerative braking to recharge the batteries, which ultimately increases overall efficiency. (Learn more about how hybrid vehicles work). In conventional vehicles, energy from deceleration is wasted as it dissipates.

A hybrid may also use an ultracapacitor (alone or with a battery) to extend the life of its battery system because it is better suited to capturing high power from regenerative braking and releasing it for initial acceleration. An energy management system decides when to use the motor and the engine and when to store electricity in the battery pack for future use.

Most propulsion batteries for full hybrid vehicles are made of nickel-metal hydride (NiMH), rather than lead. (Some 42-Volt hybrid vehicles use VRLA batteries.) A NiMH battery can hold twice as much energy as a lead battery, has a longer life cycle, and requires no maintenance. And its materials are less toxic than those in a regular car battery. Learn about other types of batteries, such as lead-acid, nickel-metal hydride, lithium ion, lithium ion polymer, and nickel-cadmium.

A few production hybrids with advanced batteries have been introduced in the market. However, technical challenges remain for demonstrating battery technology that has an acceptable combination of power, energy efficiency, and life for high-volume production vehicles.