Electric vehicle battery

For the starting, lighting and ignition system battery of an automobile, see Automotive battery.
A Mitsubishi i-MiEV having its batteries installed
The Tesla Roadster, which began shipping to customers in 2008, uses Li-Ion batteries to achieve up to 244 miles (393 km) per charge while also capable of going from 0 to 60 miles per hour (0 to 97 km/h) in under 4 seconds.

An electric vehicle battery (EVB) or traction battery is a battery used to power the propulsion of battery electric vehicles (BEVs). Vehicle batteries are usually a secondary (rechargeable) battery. Traction batteries are used in forklifts, electric Golf carts, riding floor scrubbers, electric motorcycles, full-size electric cars, trucks, and vans, and other electric vehicles.

Electric vehicle batteries differ from starting, lighting, and ignition (SLI) batteries because they are designed to give power over sustained periods of time. Deep cycle batteries are used instead of SLI batteries for these applications. Traction batteries must be designed with a high ampere-hour capacity. Batteries for electric vehicles are characterized by their relatively high power-to-weight ratio, energy to weight ratio and energy density; smaller, lighter batteries reduce the weight of the vehicle and improve its performance. Compared to liquid fuels, most current battery technologies have much lower specific energy; and this often impacts the maximum all-electric range of the vehicles. However, metal-air batteries have high specific energy because the cathode is provided by the surrounding oxygen in the air. Rechargeable batteries used in electric vehicles include lead-acid ("flooded", Deep cycle, and VRLA), NiCd, nickel metal hydride, lithium ion, Li-ion polymer, and, less commonly, zinc-air and molten salt batteries. The amount of electricity (i.e. electric charge) stored in batteries is measured in ampere hours or in coulombs, with the total energy often measured in watt hours.

The battery makes up a substantial cost of BEVs, which unlike for fossil fueled cars, profoundly manifests itself as a price of range. In the case of the MiEV 2012 model, the price tag and advertised range is close to proportional between two versions with a different battery, [1] giving the (false) impression that the battery makes up close to 100% of the cost (95% for the higher priced version). However, some of the price difference comes from extra features in the higher priced version, plus an unknown price premium, making such a retail price comparison a very bad indicator of actual cost of battery capacity, but nevertheless serves to quantify battery capacity as a premium feature. The few electric cars with over 500 km of range (including Tesla Model S with the 85 kWh battery), are firmly in the luxury segment, as of 2015. Since the late 1990s, advances in battery technology have been driven by demands for portable electronics, like laptop computers and mobile phones. The BEV marketplace has reaped the benefits of these advances. However, Mitsubishi ascribes the price reduction of its 2012 model MiEV, compared to the 2011 model, to "a dramatic reduction in the cost of batteries". [1] The cost of electric vehicle batteries has been reduced by more than 35% from 2008 to 2014. [2]

Rechargeable traction batteries are routinely used all day, and fast–charged all night. Forklifts, for instance, are usually discharged and recharged every 24 hours of the work week.

The predicted market for automobile traction batteries is over $37 billion in 2020. [3]

On an energy basis, the price of electricity to run an EV is a small fraction of the cost of liquid fuel needed to produce an equivalent amount of energy ( energy efficiency). The cost of replacing the batteries dominates the operating costs. [4]

Battery types

Main article: rechargeable battery
Old: Banks of conventional lead-acid car batteries are still commonly used for EV propulsion
Cylindrical cell (18650) prior to assembly. Several thousand of them ( lithium ion) form the Tesla Model S battery (see Gigafactory).
Lithium ion battery monitoring electronics (over- and discharge protection)

In 2015, the most used battery type for electric vehicles is Lithium-ion battery. For example: cars Nissan Leaf, Tesla Model S, Renault Zoe, BMW i3, BYD e6, Tesla Model X and more; battery electric bus: BYD ebus

Lead-acid

Main article: lead–acid battery

Flooded lead-acid batteries are the cheapest and in past most common traction batteries available. There are two main types of lead-acid batteries: automobile engine starter batteries, and deep cycle batteries. Automobile alternators are designed to provide starter batteries high charge rates for fast charges, while deep cycle batteries used for electric vehicles like forklifts or golf carts, and as the auxiliary house batteries in RV's, require different multi-stage charging. [5] No lead acid battery should be discharged below 50% of its capacity, as it shortens the battery's life. [5] Flooded batteries require inspection of electrolyte level and occasional replacement of water which gases away during the normal charging cycle.

Traditionally, most electric vehicles have used lead-acid batteries due to their mature technology, high availability, and low cost (exception: some early EVs, such as the Detroit Electric, used a nickel–iron battery.) Like all batteries, these have an environmental impact through their construction, use, disposal or recycling. On the upside, vehicle battery recycling rates top 95% in the United States. Deep-cycle lead batteries are expensive and have a shorter life than the vehicle itself, typically needing replacement every 3 years.

Lead-acid batteries in EV applications end up being a significant (25–50%) portion of the final vehicle mass. Like all batteries, they have significantly lower energy density than petroleum fuels—in this case, 30–40 Wh/kg. While the difference isn't as extreme as it first appears due to the lighter drive-train in an EV, even the best batteries tend to lead to higher masses when applied to vehicles with a normal range. The efficiency (70–75%) and storage capacity of the current generation of common deep cycle lead acid batteries decreases with lower temperatures, and diverting power to run a heating coil reduces efficiency and range by up to 40%.[ citation needed] Recent advances in battery efficiency, capacity, materials, safety, toxicity and durability are likely to allow these superior characteristics to be applied in car-sized EVs.

Charging and operation of batteries typically results in the emission of hydrogen, oxygen and sulfur, which are naturally occurring and normally harmless if properly vented. Early Citicar owners discovered that, if not vented properly, unpleasant sulfur smells would leak into the cabin immediately after charging.

Lead-acid batteries powered such early-modern EVs as the original versions of the EV1 and the RAV4 EV.

Nickel metal hydride

Nickel-metal hydride batteries are now considered a relatively mature technology. While less efficient (60–70%) in charging and discharging than even lead-acid, they boast an energy density of 30–80 Wh/kg, far higher than lead-acid. When used properly, nickel-metal hydride batteries can have exceptionally long lives, as has been demonstrated in their use in hybrid cars and surviving NiMH RAV4 EVs that still operate well after 100,000 miles (160,000 km) and over a decade of service. Downsides include the poor efficiency, high self-discharge, very finicky charge cycles, and poor performance in cold weather.

GM Ovonic produced the NiMH battery used in the second generation EV-1, and Cobasys makes a nearly identical battery (ten 1.2 V 85 Ah NiMH cells in series in contrast with eleven cells for Ovonic battery). This worked very well in the EV-1. Patent encumbrance has limited the use of these batteries in recent years.

Zebra

Main article: molten salt battery

The sodium or "zebra" battery uses a molten chloroaluminate sodium (NaAlCl4) as the electrolyte. This chemistry is also occasionally referred to as "hot salt". A relatively mature technology, the Zebra battery boasts an energy density of 120Wh/kg and reasonable series resistance. Since the battery must be heated for use, cold weather doesn't strongly affect its operation except for in increasing heating costs. They have been used in several EVs. Zebras can last for a few thousand charge cycles and are nontoxic. The downsides to the Zebra battery include poor power density (<300 W/kg) and the requirement of having to heat the electrolyte to about 270 °C (520 °F), which wastes some energy and presents difficulties in long-term storage of charge.

Zebra batteries have been used in the Modec commercial vehicle since it entered production in 2006.

Lithium ion

Lithium-ion (and similar lithium polymer) batteries, widely known via their use in laptops and consumer electronics, dominate the most recent group of EVs in development. The traditional lithium-ion chemistry involves a lithium cobalt oxide cathode and a graphite anode. This yields cells with an impressive 200+ Wh/kg energy density [6] and good power density, and 80 to 90% charge/discharge efficiency. The downsides of traditional lithium-ion batteries include short cycle lives (hundreds to a few thousand charge cycles) and significant degradation with age. The cathode is also somewhat toxic. Also, traditional lithium-ion batteries can pose a fire safety risk if punctured or charged improperly. [7] These laptop cells don't accept or supply charge when cold, and so heaters can be necessary in some climates to warm them. The maturity of this technology is moderate. The Tesla Roadster uses "blades" of traditional lithium-ion "laptop battery" cells that can be replaced individually as needed.

Most other EVs are utilizing new variations on lithium-ion chemistry that sacrifice energy and power density to provide fire resistance, environmental friendliness, very rapid charges (as low as a few minutes), and very long lifespans. These variants (phosphates, titanates, spinels, etc.) have been shown to have a much longer lifetime, with A123 expecting their lithium iron phosphate batteries to last for at least 10+ years and 7000+ charge cycles, [8] and LG Chem expecting their lithium- manganese spinel batteries to last up to 40 years.[ citation needed]

Much work is being done on lithium ion batteries in the lab. [9] Lithium vanadium oxide has already made its way into the Subaru prototype G4e, doubling energy density. Silicon nanowires, [10] [11] silicon nanoparticles, [12] and tin nanoparticles [13] [14] promise several times the energy density in the anode, while composite [15] [16] and superlattice [17] cathodes also promise significant density improvements.