Traction Battery

An electric vehicle battery (EVB, also known as a traction battery) is a rechargeable battery used to power the electric motors of a battery electric vehicle (BEV) or hybrid electric vehicle (HEV). Typically lithium-ion batteries, they are specifically designed for high electric charge (or energy) capacity.

Electric vehicle batteries differ from starting, lighting, and ignition (SLI) batteries as they are designed to give power over sustained periods of time and are deep-cycle batteries. Batteries for electric vehicles are characterized by their relatively high power-to-weight ratio, specific energy and energy density; smaller, lighter batteries are desirable because they reduce the weight of the vehicle and therefore 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.

The most common battery type in modern electric vehicles are lithium-ion and lithium polymer, because of their high energy density compared to their weight. Other types of rechargeable batteries used in electric vehicles include lead–acid ("flooded", deep-cycle, and valve regulated lead acid), nickel-cadmium, nickel–metal hydride, and, less commonly, zinc–air, and sodium nickel chloride ("zebra") 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 kilowatt-hours (kWh).

Since the late 1990s, advances in lithium-ion battery technology have been driven by demands from portable electronics, laptop computers, mobile phones, and power tools. The BEV and HEV marketplace has reaped the benefits of these advances both in performance and energy density. Unlike earlier battery chemistries, notably nickel-cadmium, lithium-ion batteries can be discharged and recharged daily and at any state of charge.

The battery pack makes up a significant cost of a BEV or a HEV. , the cost of electric vehicle batteries has fallen 87% since 2010 on a per kilowatt-hour basis. As of 2018, vehicles with over of all-electric range, such as the Tesla Model S, have been commercialized and are now available in numerous vehicle segments.

In terms of operating costs, the price of electricity to run a BEV is a small fraction of the cost of fuel for equivalent internal combustion engines, reflecting higher energy efficiency.

Electric vehicle battery types

Lead-acid

Flooded lead-acid batteries are the oldest, cheapest, and, in the past, most common vehicle batteries available. There are two main types of lead-acid batteries: automobile engine starter batteries, and deep cycle batteries. Automobile engine starter batteries are designed to use a small percentage of their capacity to provide high charge rates to start the engine, while deep cycle batteries are used to provide continuous electricity to run electric vehicles like forklifts or golf carts. Deep cycle batteries are also used as the auxiliary batteries in recreational vehicles, but they require different, multi-stage charging.(discussing damage caused by sulfation due to discharge below 50%) No lead acid battery should be discharged below 50% of its capacity, as it shortens the battery's life. Flooded batteries require inspection of electrolyte levels and occasional replacement of water, which gases away during the normal charging cycle.

Previously, most electric vehicles used lead-acid batteries due to their mature technology, high availability, and low cost, with the notable exception of some early BEVs, such as the Detroit Electric which used a nickel–iron battery. 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 specific energy than petroleum fuels—in this case, 30–50 W⋅h/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%.

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.

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 have a specific energy of 30–80 W⋅h/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 in the surviving first-generation NiMH Toyota RAV4 EVs that still operate well after 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 A⋅h 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

The sodium nickel chloride or "Zebra" battery uses a molten sodium chloroaluminate (NaAlCl₄) salt as the electrolyte. A relatively mature technology, the Zebra battery has a specific energy of 120 W⋅h/kg. Since the battery must be heated for use, cold weather does not strongly affect its operation except for increasing heating costs. They have been used in several EVs such as the Modec commercial vehicle. Zebra batteries can last for a few thousand charge cycles and are nontoxic. The downsides to the Zebra battery include poor specific power (<300 W/kg) and the requirement of having to heat the electrolyte to about , which wastes some energy, presents difficulties in long-term storage of charge, and is potentially a hazard.

Lithium-ion

lithium-ion (and the mechanistically similar lithium polymer) batteries, were initially developed and commercialized for use in laptops and consumer electronics. With their high energy density and long cycle life they have become the leading battery type for use in EVs. The first commercialized lithium-ion chemistry was a lithium cobalt oxide cathode and a graphite anode first demonstrated by N. Godshall in 1979, and by John Goodenough, and Akira Yoshino shortly thereafter. The downside of traditional lithium-ion batteries include sensitivity to temperature, low temperature power performance, and performance degradation with age. Due to the volatility of organic electrolytes, the presence of highly oxidized metal oxides, and the thermal instability of the anode SEI layer, traditional lithium-ion batteries pose a fire safety risk if punctured or charged improperly. These early cells did not accept or supply charge when extremely cold, and so heaters can be necessary in some climates to warm them. The maturity of this technology is moderate. The Tesla Roadster (2008) and other cars produced by the company used a modified form of traditional lithium-ion "laptop battery" cells.

Recent EVs are using new variations on lithium-ion chemistry that sacrifice specific energy and specific power to provide fire resistance, environmental friendliness, rapid charging (as quickly as a few minutes), and longer lifespans. These variants (phosphates, titanates, spinels, etc.) have been shown to have a much longer lifetime, with A123 types using lithium iron phosphate lasting at least more than 10 years and more than 7000 charge/discharge cycles, and LG Chem expecting their lithium-manganese spinel batteries to last up to 40 years.

Much work is being done on lithium-ion batteries in the lab. Lithium vanadium oxide has already made its way into the Subaru prototype G4e, doubling energy density. Silicon nanowires, silicon nanoparticles, and tin nanoparticles promise several times the energy density in the anode, while composite and superlattice cathodes also promise significant density improvements.

New data has shown that exposure to heat and the use of fast charging promote the degradation of Li-ion batteries more than age and actual use, and that the average electric vehicle battery will retain 90% of its initial capacity after six years and six months of service. For example, the battery in a Nissan Leaf will degrade twice as fast as the battery in a Tesla, because the Leaf does not have an active cooling system for its battery.

Battery capacity

Non–plug-in hybrid cars have battery capacities between 0.65 kW⋅h (2012 Honda Civic Hybrid) and 1.8 kW⋅h (2001 Toyota Prius).

Plug-in hybrid cars have battery capacities between 4.4 kW⋅h (2012 Toyota Prius Plug-in Hybrid) and 40.6  kW⋅h (Li Auto One).

All-electric cars have battery capacities between 6.0 kW⋅h (2012 Renault Twizy) and 212.7 kW⋅h (2022 GMC Hummer EV).

Battery cost

In 2010, scientists at the Technical University of Denmark paid US$10,000 for a certified EV battery with 25 kWh capacity (i.e. US$400/kWh), with no rebates or surcharges. Two out of 15 battery producers could supply the necessary technical documents about quality and fire safety. In 2010 it was estimated that at most 10 years would pass before the battery price would come down to one-third.

According to a 2010 study, by the United States National Research Council, the cost of a lithium-ion battery pack was about /kWh of usable energy, and considering that a PHEV-10 requires about 2.0 kWh and a PHEV-40 about 8 kWh, the manufacturer cost of the battery pack for a PHEV-10 is around and it goes up to for a PHEV-40. The MIT Technology Review estimated the cost of automotive battery packs to be between to per kilowatt hour by 2020. A 2013 study by the American Council for an Energy-Efficient Economy reported that battery costs came down from /kWh in 2007 to /kWh in 2012. The U.S. Department of Energy has set cost targets for its sponsored battery research of /kWh in 2015 and /kWh by 2022. Cost reductions through advances in battery technology and higher production volumes will allow plug-in electric vehicles to be more competitive with conventional internal combustion engine vehicles. In 2016, the world had a Li-ion production capacity of 41.57 GW⋅h.

The actual costs for cells are subject to much debate and speculation as most EV manufacturers refuse to discuss this topic in detail. However, in October 2015, car maker GM revealed at their annual Global Business Conference that they expected a price of /kWh for Li-ion cells entering 2016, substantially lower than other analysts' cost estimates. GM also expects a cost of /kWh by the end of 2021.

According to a study published in February 2016 by Bloomberg New Energy Finance (BNEF), battery prices fell 65% since 2010, and 35% just in 2015, reaching /kWh. The study concludes that battery costs are on a trajectory to make electric vehicles without government subsidies as affordable as internal combustion engine cars in most countries by 2022. BNEF projects that by 2040, long-range electric cars will cost less than expressed in 2016 dollars. BNEF expects electric car battery costs to be well below /kWh by 2030, and to fall further thereafter as new chemistries become available. ''See embedded video.''

;Battery cost estimate comparison

EV parity

In 2010, battery professor Poul Norby stated that he believed that lithium batteries will need to double their specific energy and bring down the price from US$500 (2010) to US$100 per kWh capacity in order to make an impact on petrol cars. Citigroup indicates US$230/kWh.

Toyota Prius 2012 plug-in's official page declare of range and a battery capacity of 5.2 kWh with a ratio of /kWh, while the Addax (2015 model) utility vehicle already reaches 110 kilometres (68.5 mi) or a ratio of 7.5 kilometers (4.6 mi)/kWh.

Battery electric cars have an energy consumption between /kWh (85 MPGe) and /kWh (135 MPGe).

United States Secretary of Energy Steven Chu

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