Electric Vehicle Batteries
Battery technology is very important to the feasibility of electric cars and has progressed over time. Plug-in cars manufactured in 2010 or later typically use lithium ion batteries. Vehicles built before 2010 may have used lead acid, nickel metal hydride or nickel cadmium batteries.
A number of characteristics affect battery perfomance and use:
- energy density (the amount of useful energy stored by the battery per unit of weight)
- power density (the rate at which energy is converted into work, per unit of weight)
- expected life
- environmental impacts
Lead acid batteries are commonly used to provide start-up or backup power in gasoline- and diesel-powered vehicles. In addition, lead acid batteries have often been used in many special-purpose vehicles, including fork-lifts, low-speed utility vehicles and golf carts. Some do-it-yourself conversion kits for electric vehicles also use lead-acid batteries.
Lead acid batteries are well understood, highly recycled and relatively inexpensive. They are comparatively heavy because they contain lead, which is toxic, and sulfuric acid, which is a hazardous material. Lead acid batteries also emit hydrogen gas while being charged, which creates a fire and explosion hazard unless adequate ventilation is provided.
Economic incentives and regulatory constraints ensure that 99 percent of lead acid batteries are recycled.
Nickel cadmium batteries, once commonly used to power consumer electronics and power tools, have largely been supplanted by nickel metal hydride and lithium ion batteries. Some homemade electric vehicles may still use nickel cadmium batteries.
Cadmium is toxic, and proper disposal or recycling of these batteries is important. However, smaller incentives and fewer regulatory constraints mean that fewer of these batteries are recycled than lead acids.
Nickel metal hydride batteries are used in many hybrid electric vehicles, such as the Toyota Prius, Honda Insight and Ford Escape. They are also used in many consumer electronics and power tools. These batteries have good power density but lower energy density and efficiency compared to lithium ion batteries.
Lithium ion batteries have replaced nickel cadmium and nickel metal hydride batteries in most consumer electronics products that use rechargeable batteries. Lithium ions provide higher energy and power densities and better energy efficiency than earlier battery systems. This makes them the battery of choice for many plug-in vehicles planned by major automakers.
Lithium ion varieties
Within the lithium ion family there are a number of different varieties, depending on how they are packaged and which chemical compounds they use. Packaging formats include cylindrical, prismatic, button and pouch. Generally the prismatic format is thought to be ideal for vehicles, while other formats are used in many consumer electronics.
Chemistries include lithium cobalt oxide, lithium nickel cobalt aluminum, lithium iron phosphate, lithium nickel cobalt manganese, lithium manganese spinel and lithium titanate.
For hybrid and electric vehicles, use of the lithium-manganese-spinel, lithium-iron-phosphate and lithium-titanate chemistries are leading candidates. Because they are mass-produced for consumer electronics, lithium ion batteries now cost less than their competitors, especially for the cylindrical format with lithium-cobalt-oxide chemistry.
Taking advantage of this, the Tesla company uses thousands of lithium-cobalt-oxide cylindrical batteries in its battery-electric sports car. This approach requires a greater investment in the battery management system to ensure safety and battery life.
Lithium ion recycling
Recycling of lithium ion batteries is increasing in the United States. Plus, automotive manufacturers and utilities, including TVA, are exploring ways to re-use batteries that are too depleted for vehicular use but still have enough life remaining for other uses. One application might be to use these partially depleted batteries to smooth out the variability of distributed generation sources like solar panels.