WHAT IS A BATTERY?
A battery is a device that stores chemical energy and converts it on demand into electrical energy. It carries out this process through an electrochemical reaction, which is a chemical reaction involving the transfer of electrons.
Batteries have three main parts, each of which plays a different role in the electrochemical reaction: the anode, cathode, and electrolyte. The anode is the "fuel" electrode (or "negative" part), which gives up electrons to the external circuit to create a flow of electrons, otherwise known as electricity. The cathode is the oxidizing electrode (the positive part), which accepts the electrons given up by the anode. The electrolyte carries the electric current between the anode and cathode as ions.
Batteries are made up of one or more basic electrochemical units called cells. Cells are usually connected in a series to increase a battery's voltage. For example, two 1.5V AA cells connected in a series make a 3V battery.
HEVs, PHEVs, and EVs combine batteries with controllers to create a battery system that provides energy to the vehicle. All PHEVs and EVs can rely on the battery system exclusively for power, as long as the battery has a sufficient charge. Some HEVs can rely on battery power alone to drive the vehicle for short distances, while others can only use the battery system to provide assistance to the internal combustion engine.
TYPES OF BATTERIES
Different batteries rely on different chemistries, depending on what they are used for. Advances in these chemistries have allowed larger and more powerful batteries to be used in vehicles and other applications. There are four major types of batteries most commonly used in vehicles:
- Lead-acid batteries are the most prevalent battery chemistry used for vehicle starting and other ancillary power functions such as headlights. Lead-acid batteries can be designed to be high power, and are inexpensive and more reliable compared to the other available chemistries. However, lead-acid batteries have low specific energy (energy per unit mass), perform poorly in cold temperatures, and have a shorter calendar and life cycle than the other batteries when they are put under the same stresses.
- Nickel-metal hydride batteries, which use a different combination of metals than lithium-based batteries, are used in a variety of applications. In particular, most currently-available HEVs use high-voltage nickel-metal hydride batteries. However, nickel-metal hydride batteries still face the challenge of being more expensive than lead-acid batteries.
- Lithium-ion batteries are used in most of today's PHEVs and EVs, as well as some HEVs. A number of consumer electronics, including mobile phones and laptops have used lithium-ion batteries since the 1990s. Lithium-ion batteries' high specific energy, high energy efficiency, and long life make them particularly useful for these applications. The U.S. Department of Energy (DOE) and industry are working together to conduct research and development to reduce these batteries' cost, size, and weight.
- Lithium-polymer batteries are similar to lithium-ion batteries, but they use a solid conducting polymer as an electrolyte instead of the liquid electrolyte in lithium-ion batteries. Lithium-polymer batteries can provide high specific power (power per unit of volume) and are used in some HEVs. Like lithium-ion batteries, researchers are working to reduce their cost and improve performance.
IMPROVING BATTERY TECHNOLOGIES
Advancing battery technologies is critical as DOE works with industry to continue developing HEVs, PHEVs, and EVs. Current battery research and development investigates advanced battery chemistries such as lithium-metal, lithium-air, and lithium-sulfur. Visit the U.S. Department of Energy's Vehicle Technologies Office energy storage pages for more information.
Visit the Alternative Fuels Data Center to learn more about vehicle batteries.