Electric vehicle battery

As a traction battery ( also referred to synonymously as a traction battery, traction battery or battery cycles ) is called an interconnection of individual cells or batteries as energy storage blocks to drive electric vehicles. Since they are exposed to the cyclic loading and unloading processes, use, come exclusively accumulators (secondary elements ) are used.

• 8.1 Lead-acid batteries as a traction battery
• 8.2 Nickel-cadmium storage batteries as a traction battery
• 8.3 Nickel -metal hydride batteries as a traction battery
• 8.4 Thermal batteries as traction battery
• 8.5 Lithium- ion batteries as a traction battery

History

After the electricity was used in the early 19th century for message transmission, the foundations for an electric motor drive were known and developed the electric motor operational by 1837/1838. From Gaston Plante to 1859, the lead-acid battery, the ( lead-acid battery ) was developed as a first rechargeable battery.

Six of these cells with a nominal voltage of 2 volts and spirally wound lead plates formed in 1881 Trouvé Gustave Trouvé Tricycle of the first traction battery (nominal voltage 12 volts) to power the autonomous electric vehicle without rails or cable binding. Was regulated only by closing or opening of the circuit. However, had the " Trouvé Tricycle " nor the cranks of serving as the base tricycle.

A few months later, the electric tricycle of Ayrton & Perry 1882 was the move not only without cranks and with electric lighting, but also with improved traction battery. The 10 lead cells stored at a nominal voltage of 20 volts and 1.5 kWh could be individually switched on and off, what a performance and speed regulation enabled. While the heavy traction battery was placed as low as possible, so as to improve stability and handling With the first vehicles.

But while the battery cells were placed open to the first vehicles, one (from 1888) the traction battery already built in the first electric cars in special housing or they are disguised. The Accumulatoren Factory Tudor 's system Bushes & Müller OHG (now known as VARTA ) introduced in 1888 as the first company in Germany lead-acid batteries industrially produced. The flakes electric cars from 1888 used probably the first vehicle in these batteries. For rail, the Wittfeld - Akkumulatortriebwagen was operated with these batteries. The energy density at that time was 27 Wh / kg and the lifetime reached up to 25 years.

With the developed around 1900 nickel-iron battery ( Thomas Edison ) and developed by the Sweden Waldemar Jungner nickel cadmium battery cells were alternative chemistries for traction batteries available. The NiFe battery is demonstrably used in different cars and has a very long life. Jay Leno has in the United States a Baker Electric, in which the nickel-iron batteries are still functional after nearly 100 years. Henry Ford developed the Model T Ford as an electric car. He had 150,000 nickel-iron accumulators ordered from Edison, as his department for electric cars went up in flames.

The invention of the electric starter, with the aid of a starter battery, the engine without any physical effort could be started by the, initiated the decline of the first golden age of the electric car, as a result stagnated and the accumulator and battery development. It was not until 1990 by the CARB legislation in California, the automotive manufacturers should be forced to gradually zero-emission vehicles (Zero Emission Vehicle) offer, the Akkumulatorforschung again received strong impulses.

For example, while in the first traction batteries of the GM EV1 still available, inexpensive lead-acid batteries were used, (26 blocks with a total capacity of 16.3 kWh and a nominal voltage of 312 volts), were the series production of the Stanford R. Ovshinsky in the second embodiment developed nickel metal hydride storage batteries used. The traction battery was there permanently installed in a central tunnel in the vehicle floor, which contributed to a high level of crash safety and very good handling characteristics.

While the sodium - sulfur battery for the BMW E1 or announced for Hotzenblitz zinc -bromine battery never recaptured series production for the " Mercedes A- Class electric" enhanced sodium - nickel-chloride cell helped ( ZEBRA battery ) the vehicle not only a practical range of over 200 km, but also to applications in military and aerospace. Also interesting is the compact block arrangement that allowed the installation of the entire traction battery in one piece from the bottom and also contributed to high safety for automotive use in this vehicle.

Even the basics of cell chemistry for lithium- ion batteries were placed in this period. However, the automotive industry stopped after the CARB laws easing these activities so that lithium - ion batteries obtained during the 21st century as traction batteries importance. Today, among the different variants as the hope for significant improvements in performance and weight capacity.

Physical and technical characteristics

Compared to portable batteries or Konsumerzellen the cells of a traction battery have a much higher capacity. Moreover, they are from different manufacturers in different designs, some designed and manufactured to customer requirements. Standardized sizes do not exist. Are usual both round cells, in which the electrodes are made of iron and cup-shaped, for example, products of A123 Systems, as well as prismatic cells with plate-shaped electrode array, for example, cells of Winston Battery Company.

It uses high-current solid, deep cycle accumulator systems that are able to dispense electric power depending on driving condition or to absorb and withstand many charge-discharge cycles. Unlike starting batteries lead- traction batteries, for example, be discharged up to 80% deep special design of the lead grid and separators, without taking damage.

During blocks for lead car starter batteries for 12 V or 24 V capacity 36-80 ampere hours (Ah) have to be connected together for forklift cells with capacities from 100 to 1000 Ah to operating voltages of, for example 24 to 96 volts for up to several hundred volts to achieve electric cars. The sizes are accordingly sometimes considerably larger. Higher voltages and to reduce the currents flowing to reduce, inter alia, the ohmic losses in the pipes and the thermal losses when charging and discharging as well as reduce the weight (cable).

By serial interconnection of individual cells results in the driving voltage and the traction voltage. By increasing the size of the cells or by parallel connection of the memory cells and current-carrying capacity can be increased. The product of the traction voltage (V) and the electric charge / electrical capacity of the individual cells / cells in parallel (Ah ) yields the energy content of the traction battery.

Requirements for use in vehicles

The mobile application of traction batteries due to higher security requirements as compared to stationary use. So above all, the safety of mechanical effects must be demonstrated. This is achieved by using secure cell chemistries (eg, lithium iron phosphate batteries ), often with poorer electrical characteristics, the safe structural design of housing in the vehicle ( for example, crash- tested battery trays in the subsoil ), or a combination of both methods. How strong is the influence of the security requirements for traction batteries, the delayed start of production of the Opel Ampera can be traced on the example. Reason was the ( only several weeks) after a crash test caught fire traction battery of the identical model Chevrolet Volt.

Different requirements for all-electric and hybrid vehicles

As all-electric vehicles save only electrical energy for propulsion, special high-energy cells are used to minimize space and weight for the amount of energy required. Due to the necessary capacity of the battery (cell or module size) the current carrying capacity of the cells is given for the unloading and loading operations normally. The load is applied evenly and longer than in hybrid vehicles.

In hybrid electric vehicles, the main part of the driving power is usually stored as chemical energy. The traction battery has a much smaller capacity. It stores electrical energy for locomotion and takes recuperation of regenerative braking on. For high-current cells are used, which can realize the necessary (often short-term ) high current load with good efficiency and the required service life despite lower capacity.

Rated capacity, load capacity, the manufacturer

The rated capacity is the manufacturer under specified criteria assured, removable amount of energy. With capacity comparisons, it is important to consider these criteria. How an accumulator with the data 12 V/60 Ah C3 has a higher capacity than a battery of the same size with marking C5 or C20. The specification Cx characterizes the discharge duration for the specified capacity in hours. At C3, 60 Ah can be taken in three hours more evenly discharge, so it higher currents are possible than with C5 or C20, which is important for use as a traction battery, since the currents often are, in practice, these measured currents ( See also C- rate and Peukert equation).

For heavy-duty lithium - ion batteries, the maximum current load has prevailed in relation to capacity. In this case, it means, for example, for a cell of 3.2 V at 100 Ah standard discharge at 0.5C ( 0.5 or CA), that the capacity is determined with a discharge current of 50 A. Usual capacity data at 0.5 C or 1 C, the permissible continuous load quite 3 C or more (in the example at 3 C so 300 A), the short-term load significantly more (in this case 20 CA, ie 2000 A ) can be.

Increasingly, the capacity of a traction battery is indicated not in ampere-hours of the individual cells, but in watt-hours. As well as different types are comparable, since the voltage flows with. Starter batteries are on an energy content from 496.8 to 960 Wh, traction batteries for forklifts on 4800-28800 Wh and for the Toyota Prius II to 1,310 Wh

Influences on the usable capacity

In traction mode can not be used the entire rated capacity. On the one hand, the usable capacity to sink to the specified end voltage at high extracted currents smaller (see Peukert effect), on the other hand determined at serial interconnections the cell / cell block with the smallest capacity, the usable capacity without damaging deep discharge.

The cells have a traction battery manufacturing tolerances as well as through the use influences whatever differences in the capacity and power output ( internal resistance ) on. As a result, the cells are subjected to different loads during operation, there is a divergence, which reduces the useful capacity of the entire battery. While the capacity of the best cells can be exploited never entirely, the weak cells are regularly overloaded, over-discharged or overcharged. Also to minimize these effects and to avoid, are used in modern traction batteries balancer and battery management systems. Even lower temperatures reduce the ability of the traction battery to deliver high currents and enhance the Peukert effect, as generally reduces the mobility of the electrons. To counteract this effect, and since different battery technologies at low temperatures become unusable traction batteries are often equipped with an additional heating. This assumes either during connection to the power supply, the temperature, or heated themselves from their energy content This and additional consumers such as electric room heater or air conditioner will reduce the winter range, although the usable energy content of the traction battery is also available during the winter.

The discharge of the battery cells is in favor of life often by the battery management system (BMS ), usually limited to 60-80% of the rated capacity. Especially in consumption calculations and comparisons on various traction batteries these circumstances must be considered. This " usable capacity " is from the automaker rarely shown, but described as a usable range of the rated capacity. So the Chevrolet Volt and Opel Ampera a usable battery window of 30-80 % is specified, these are (in favor of durability), only 50% of the rated capacity of 16 kWh.

Life and cycle stability

Plug in America has among drivers of the Tesla Roadster, a survey carried out on the lifetime of the installed batteries. It was found that after 100,000 miles = 160,000 km, the batteries still had a remaining capacity of 80 to 85 percent. This was regardless of climate zone in which the vehicle was moved. The Tesla Roadster was built between 2008 and 2012 and sold. Today's battery technology is likely to be more advanced and probably perform better.

Lithium iron phosphate batteries, which are used as traction batteries according to the manufacturer reach more than 5000 cycles at respective depth of discharge of 70% ( Depth of Discharge, DOD). Assuming 300 charge cycles per year for a car ( or about one load operation per day), it might be sufficient for the car's life, especially rare in a day the full capacity of a battery is used and run shallower charge cycles generally results in a longer lifetime ( s lithium iron phosphate battery ).

Used in solar batteries lithium -ion batteries have partially to a very high cycle stability of more than 10 000 charge and discharge cycles and a long life of up to 20 years.

For its 85 kWh battery pack in the Tesla Model S Tesla are 8 year warranty with unlimited mileage.

Examples of use

Traction batteries sealed lead acid batteries are used as counterweights for staple goods in electric forklift trucks, to place the latter with the help of the counterweights in a position to be able to transport a certain (larger) physical mass. Also in automated guided vehicle systems for flat applications, they are still used. The heavy weight and the strong temperature dependence are detrimental from differences in altitude and slopes and in the winter. Therefore they are less suitable for use in electric bicycles, electric scooters and in electric vehicles.

In modern electric bikes / Pedelecs come from space and weight reasons, almost exclusively based on lithium batteries for use. Initially used lead-acid batteries have not been successful.

In electric scooters are as traction batteries variety of battery systems in use. Again, the lead-acid battery NiCd has been deprecated, as a proven and lithium-based batteries as powerful.

When used in hybrid vehicles like the Toyota Prius or the Honda Civic IMA (2012 ) Traction batteries are currently being used by the type nickel - metal -hydride battery voltages of several 100 volt and 10 amp-hours. The capacity constraint is obtained from patent law provisions severely restrict the production and further development. New developments are usually equipped with traction batteries based on lithium.

In solar vehicles exclusively modern high-performance batteries are used, lithium-base for weight and volume reasons. The world's largest solar car, the catamaran Tûranor PlanetSolar, has currently at 1.13 MWh also the world 's largest lithium traction battery. The cells are derived from the Thuringian cell producers Gaia accumulators GmbH.

In electric cars today come (3/ 2014) almost only lithium - ion batteries for use (see Tesla Model S, BMW i3, Renault Zoe, Nissan Leaf, Volkswagen E -Up! Etc.).

Environmental aspects

Traction batteries are considerably higher than the portable batteries or Konsumerbatterien from single cells, both in size (capacity) as well as in the number of individual cells (voltage). Therefore, they contain larger quantities of individual commodities, so that after the use of a return to the cycle of materials (recycling) is economically and ecologically reasonable and necessary. For starter batteries and traction batteries, lead-acid battery as therefore a battery deposit of 7.50 € / piece was introduced in Germany with the battery regulation. The return rate is over 90%.

For advanced lithium - ion batteries such deposit solution does not exist yet. However, it is expected that the recovery, especially the lithium contained, is economically viable even for traction battery. Due to the elaborate lithium extraction and high energy use in the production of the batteries was often assumed that lithium batteries degrade the LCA of electric cars significantly. Recent studies have shown that the ecological rucksack is much lower than previously thought

Price history

Prices for traction batteries are intended for the available and inexpensive starting supply materials only to a small extent by the raw materials. While in single and small series production of traction batteries 2008-2010 prices of some significantly more than 500 € / kWh were called nominal capacity, the prices for the first gefertigeten in series traction batteries in 2012 were already at 280-350 € / kWh in 2013 amounted Li -Tec the Price 200 € / kWh and is looking for partners to implement the cost-effective mass production. Another price drop to below 150 € / kWh is to be expected (see also battery price development). Causes are incipient mass production, which significantly reduce the unit costs through better technologies and economies of scale. However, the declining production costs not be passed on to customers, since only a small price pressure exists in this market, especially in Germany, by the few offers.

The cells of the traction battery of the Mitsubishi i MiEV 16kWh for about 150km range therefore cost about 2013 € 3200 still in the making.

Material-specific classification and practical hints

Conventionally, the traction batteries from the materials which are used for galvanic cells, differentiated. Because of the many different systems only general recommendations can be given. Reference point should always be the respective manufacturer's recommendations, including the possibility of a different, yet their use should be examined in order to counter a possible planned obsolescence and the cost increase (cost / km over the useful life ).

Lead-acid batteries as a traction battery

Lead-acid battery systems were in Germany so far, the traction battery most commonly used types. Despite superficial similarities, they differ in the structure and use of starter batteries, as they are optimized for higher energy density and longer cycle stability, starter batteries, however, at high power density (brief high current output ).

In order to maximize the lifetime, the load and depth of discharge should be kept low, which is often difficult to implement constructive is (ratio capacity to the required performance ). It is generally recommended timely boost traction lead-acid batteries after each use with high currents and not deep discharge as possible (flat cycles) A small depth of discharge of only 30 % of the rated capacity can multiply the life. BMS are hardly available, a practical application is purely for the BADICHEQ system ( Diagnostic Battery & Charge equalization ) in Hotzenblitz known. A charge equalization can be realized but not with PowerCheq balancers between cell blocks between the individual cells. An operation in winter temperatures is hardly possible without heating. Also the Charging in cold battery may only be made with lower currents and higher capital losses. It should be stored in a fully charged state at low positive temperatures, temporal control and recharging because of the high self-discharge necessary.

Nickel - cadmium batteries as a traction battery

Nickel -cadmium battery systems have become widespread, as they are very sturdy and durable. In Europe, they are made as wet cells primarily by the company and juice were also used in various French electric cars. However, they contain the toxic cadmium. Although the pan-European ban on NiCd batteries are still excludes traction, they are increasingly being replaced by newer technologies, especially lithium-based. Also NiCd batteries suffer the reversible memory effect, which requires a full discharge and targeted equalization charge / overcharge at intervals to maintain capacity. Generally NiCd batteries are cycled deeper and therefore not reloaded after each use. They are used as a robust and even at low temperatures.

Nickel - metal hydride storage batteries as the traction battery

The nickel -metal hydride battery was due to its high energy density successfully used as a traction battery (example: General Motors EV1 ), but prevented patent law sanctions a manufacturing high capacity cells (more than 10 Ah ) and thus a greater dissemination and development. Therefore, no BMS and difficult matching chargers are available in the traction area, while NiMH batteries are standard in the consumer sector. When use is strong overloading to avoid, as it accelerates aging by heating and exothermic reactions (thermal runaway ) is possible, which can cause a fire hazard. The charge termination by delta peak should be combined with at least one thermal shutdown. The best performance is achieved at about 25 ° C, the lifetime can be> 10 years with appropriate use ( see Toyota Prius ).

Thermal batteries as traction battery

Also, thermal batteries such as the Zebra battery be successfully used as Traktionsakkumulator. It is particularly suitable for regular or continuous use since then the system-related energy losses are negligible. The advantage lies in the full winter conditions, as the ambient temperature does not affect the high operating temperatures and high operational reliability. Dropout cells are low and decrease although the capacity, but not prevent its operation.

Lithium- ion batteries as a traction battery

Lithium -ion battery systems are currently (2012 ) the latest version under the traction batteries. You will be certified for the future a great potential for development. In electric cars today come (3/ 2014) almost only lithium - ion batteries for use (see Tesla Model S, BMW i3, Renault Zoe, Nissan Leaf, Volkswagen E -Up! Etc.).

Since lithium - ion battery is the generic term for a variety of different material combinations with a variety of properties, only general guidance can be given to optimize the use and efficiency:

• The optimal operating range is vary with an average depth of discharge in the wide operating range in which the voltages only slightly to the rated voltage. Frequent reloading, shallow cycles are recommended. Permanent full-charging is equally as deep discharge unfavorable for life. After charging the traction battery should be used. A longer storage when not in use should not be more than about 95 % state of charge.

• Both at the bottom as at the upper cell voltage limit set a processes that reduce the life of the battery, or destroy them. Therefore, the limit voltages are to be avoided, both during charging and during discharging, corresponding control electronics ( balancer / BMS) is generally mandatory

• While LiFePO4 batteries (see Lithium iron phosphate battery ) are quite unempfimdlich against freezing temperatures, especially during the discharge, other battery (LiPo ) are destroyed by frost. The best performance of LiFePO4 obtained at 25-35 ° C, but higher temperatures intensify the creeping loss of capacity due to aging.

It is important to follow the treatment instructions by the manufacturer to achieve optimum performance and long service life.