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Battery Technology
Brief History
In 1990, the auto industry was forced into establishing an electric passenger car market when California’s Air Resources Board (CARB) implemented a Zero Emission Vehicle (ZEV) program: 2% of the vehicles produced for sale in California had to be ZEVs, increasing to 5% in 2001 and 10 percent in 2003. By 1996, CARB backed down on the 1998 deadline for the program, and in 2001, the program relaxed its standards to include “partial” zero emission vehicles (PZEV). The CARB attempted to set a minimum quota for the use of electric cars, but this was withdrawn after complaints by auto manufacturers that the quotas were economically unfeasible due to a lack of consumer demand. However, many believe this complaint to be unwarranted due to the claim that there were thousands waiting to purchase or lease electric cars from companies such as General Motors, Ford, and Chrysler in which these companies refused to meet that demand despite their production capability. Others note that the original electric car leases were at reduced cost and the program could not be expected to draw the high volumes required without selling or renting the cars at a financial loss. Since the California program was designed by the CARB to reduce air pollution and not to promote electric vehicles, the zero emissions requirement in California was replaced by a combination requirement of a tiny number of zero-emissions vehicles (to promote research and development) and a much larger number of partial zero-emissions vehicles (PZEVs), which is an administrative designation for an super ultra low emissions vehicle (SULEV), which emits polution of about ten percent of that of an ordinary low emissions vehicle when in opration and is also certified for zero evaporative emissions at all times.
Based on further changes agreed upon in 2003, the ZEV program is scheduled to restart in 2005 with a set of complicated rules and tables which allow carmakers to use low-speed, low-range electric cars, hybrids, full function electric cars and ultimately fuel cells to pass prescribed standards and quantities up through 2017. These ZEV mandates could significantly increase the number of hybrids on the road.
With the advent of the electric car came the need for new battery technologies to make them a viable alternative in the modern automotive market.
Introduction
Production of purely electric vehicles for commercial use has fallen by the wayside. General Motors, Ford, Honda and Toyota have all discontinued their electric vehicle (EV) programs - despite growing concern for the environment and over ever-increasing fuel costs, the market for EVs never really flourished. Evolving from these early electric vehicles, today's hybrid vehicles use electricity stored in batteries to assist the gasoline engine and to completely power the vehicle while idling or at consistent low speeds. Because these hybrids also use a gas engine, the battery is substantially smaller than those that were used in the purely electric vehicles. Vehicles such as the Toyota Prius, Honda Civic Hybrid and Ford Escape Hybrid include drive-train management systems that automatically decide when to use the batteries or internal-combustion engine.
However, improvements in battery technology may one day resurrect EVs by extending their driving range. Electric vehicle advocates and engineers are now looking at gas-electric hybrids, which, unlike current hybrid offerings, could be plugged in to provide a greater capacity for running purely on electric power. A 'plug-in' hybrid electric vehicle (PHEV) is a hybrid which has additional battery capacity and the ability to be recharged from an external electrical outlet. The vehicle can be used for short trips of moderate speed without needing the internal combustion engine (ICE) component of the vehicle, thereby saving fuel costs.
Battery Types
Lead Acid
The lead acid battery in a conventional car contains enough energy to drive a small electric motor; these batteries are designed to deliver a burst of current for a short period of time only. Otherwise, the battery is only needed to support accessories such as the radio, lighting, power windows, etc. while the engine is not running. A hybrid vehicle uses a conventional lead acid battery for all the same reasons that a conventional car does; however, a hybrid also has a rechargeable deep cycle battery. The difference is that hybrid vehicles use electric motors to provide some portion of their driving force, and therefore need a great deal of stored electrical energy. And unlike gasoline engines, electric motors can be greater than 90 percent efficient at using that electrical energy.
Lead acid batteries were found to have too many limitations, making their continued use impractical. Because they are so heavy, it is unreasonable to add more or larger units in order to cope with the higher electrical demands of hybrid vehicles. They are also relatively slow charging, and do not lend themselves to deep cycling - a full discharge causes extra strain, and each cycle robs the battery of a small amount of capacity. They also pose more environmental concerns regarding proper disposal/recycling.
Lead-free alternatives, such as nickel metal hydride and lithium ion batteries, are already on the market in electric and electric-hybrid vehicles, and offer several advantages over conventional lead acid starter batteries. Performance and environmental benefits include:
- Higher energy (power) density
- Reduced weight/volume
- Longer battery life
- Improved fuel economy due to lighter weight and higher energy capacity
- Less material used, lower toxicity, and potentially recyclable
Nickel Metal Hydride
A nickel metal hydride battery (abbreviated NiMH) is a type of rechargeable battery similar to a nickel-cadmium (NiCd) battery, but has a hydride absorbing alloy for the anode instead of cadmium, which is an environmental hazard; therefore, it is less detrimental to the environment. Nickel metal hydride batteries are lightweight, have a longer shelf life, and produce more energy than lead acid batteries. For example: the first generation GM EV1s used lead-acid batteries in 1996, and a second generation batch with nickel metal hydride batteries in 1999. The "Gen I" cars got 55 to 95 miles (90 to 150 km) per charge with the lead acid batteries, while "Gen II" cars got an improved 75 to 150 miles (120 to 240 km) per charge with nickel metal hydride batteries.
Most - not all - current hybrids have a rechargeable NiMH battery as an integral part of their hybrid system, to assist in fuel savings and lower emissions. Applications of NiMH type batteries include hybrid vehicles such as the Honda Insight and Toyota Prius. NiMH batteries are a major step up from the lead acid variety; however, while more powerful than lead acid batteries, they have not provided the long-term cost benefits that were hoped for - the power of nickel batteries comes from the raw material, which is getting more expensive due to increased demand.
Lithium Ion
Lithium ion batteries (sometimes abbreviated Li-Ion) are a type of rechargeable battery commonly used in consumer electronics. They are currently one of the most popular types of battery, with one of the best energy-to-weight ratios, no memory effect and a slow loss of charge when not in use. Lithium ion battery applications have the potential of eclipsing the NiMH battery in hybrid vehicles [1]; compared to a lithium ion battery, the NiMH battery's volumetric energy density (amount of potential energy stored in the battery) is lower and self-discharge is higher. Lithium ion batteries are smaller, lighter, and have fewer volatile gases than NiMH batteries. The production cost of these lighter, higher-capacity lithium batteries is gradually decreasing as the technology matures and production volumes increase through mass processing, which can scale to the high volumes required for the rapidly growing hybrid market - without a corresponding jump in price. However, they are not currently scaled for use in hybrid vehicle applications - while they have potential cost-saving attributes, they can be dangerous if mistreated, and, because they are less durable, may have a shorter lifespan compared to other battery types.
Lithium Ion Polymer
Lithium ion polymer batteries, or more commonly lithium polymer batteries (Abbreviated Li-Poly or LiPo) are rechargeable batteries which have technologically evolved from lithium ion batteries. Ultimately, the lithium salt electrolyte is not held in an organic solvent like in the proven lithium ion design, but in a solid polymer composite such as polyacrylonitrile. There are many advantages of this design over the classic lithium ion design, including the fact that the solid polymer electrolyte is not flammable (unlike the organic solvent that the Li-Ion cell uses); thus, these batteries are less hazardous if mistreated.
Zinc-air
Zinc-air batteries, also called "zinc-air fuel cells" are a non-rechargeable electro-chemical battery powered by the oxidation of zinc with oxygen from the air. These batteries have very high energy densities and are relatively inexpensive to produce. Zinc-air batteries have properties of fuel cells as well as batteries: the zinc is the fuel; the rate of the reaction can be controlled by controlling the air flow; and used zinc/electrolyte paste can be removed from the cell and replaced with fresh paste. Research is being conducted in powering electric vehicles with zinc-air batteries.
Both Li-Poly and Zinc-air batteries have demonstrated energy densities high enough to deliver range and recharge times comparable to conventional vehicles.
The Future
As hybrids become more refined, battery life, capacity and energy density will improve and the combustion engine will be used less (particular with PHEVs). At some point it may become economic for hybrids to be sold without their ICE, finally leading to Battery Electric Vehicles (BEVs) being commonplace.
External Links
http://www.hybridcars.com/battery-comparison.html
http://www.evworld.com/view.cfm?section=article&storyid=1042
