Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering http://pid.sagepub.com/
A review of current automotive battery technology and future prospects Heide Budde-Meiwes, Julia Drillkens, Benedikt Lunz, Jens Muennix, Susanne Rothgang, Julia Kowal and Dirk Uwe Sauer Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering 2013 227: 761 originally published online 19 April 2013 DOI: 10.1177/0954407013485567 The online version of this article can be found at: http://pid.sagepub.com/content/227/5/761
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Original Article
A review of current automotive battery technology and future prospects
Proc IMechE Part D: J Automobile Engineering 227(5) 761–776 Ó IMechE 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954407013485567 pid.sagepub.com
Heide Budde-Meiwes1,2, Julia Drillkens1,2, Benedikt Lunz1,2, Jens Muennix1,2, Susanne Rothgang1,2, Julia Kowal1,2 and Dirk Uwe Sauer1,2,3
Abstract In this article, today’s battery technologies and future options are discussed. Batteries have been one of the main focuses of automotive development in the last years. Technologies that have been in use for a very long time, such as the lead– acid battery, are indispensable but need improvement. New technologies such as the lithium-ion battery are entering the market. Supercapacitors (also known as electrochemical double-layer capacitors) can be used for high-power requirements such as regenerative braking. The variety of vehicles has increased with the introduction of hybrid vehicles, plug-in hybrid vehicles and electric vehicles and, for each type, suitable battery types are being used or under development. Appropriate battery system designs and charging strategies are needed. Battery technologies can be classified according to their energy density, their charge and discharge characteristics, system integration and the costs. Further relevant performance parameters are the calendar lifetime, the cycle lifetime, the low- and high-temperature performances and the safety.
Keywords Battery, lead–acid battery, lithium-ion battery, supercapacitor, battery management system, charging strategy, vehicle requirement, hybrid vehicles, full-electric vehicles
Date received: 6 December 2012; accepted: 13 March 2013
Introduction The idea of combining internal combustion and electrical engines has a long history. J Lohner and F Porsche presented a very early hybrid electric vehicle in 1901. However, the development status of electrical components was not sufficiently advanced and so the concept failed at that time. Even in those early days, another main challenge was to find a battery with an excellent performance1. Despite many concept cars and test fleets with hybrid and full-electric vehicles, it took until 1997 when the Toyota Prius came on the market as the first hybrid electric vehicle built in series production with more than 100,000 vehicles. Modern electrified-vehicle concepts have a wide range of propulsion technologies. Micro-hybrid vehicles with a stop–start function and regenerative braking are in series production now. Further steps are mild- or full-hybrid vehicles, powered with a conventional combustion engine with further additional electrification. These cars are not made for driving purely on electricity beyond some hundreds of metres, but they enable the
shift of the engine operation close to the maximumefficiency region. Full-electric vehicles without combustion engines are presented on the market or under development for the start of production in the coming months and years. A standard starting, lighting and ignition (SLI) battery is a part of all cars, either for cranking the combustion engine in conventional and micro-hybrid vehicles, or for supplying the 12 V power supply system. The SLI battery typically is a lead–acid 1
Electrochemical Energy Conversion and Storage Systems Group, Institute for Power Electronics and Electrical Drives (Institut fu¨r Stromrichtertechnik und Elektrische Antriebe - ISEA), RWTH Aachen University, Germany 2 Ju¨lich Aachen Research Alliance, JARA-Energy, Germany 3 Institute for Power Generation and Storage Systems (PGS), E.ON Energy Resarch Center, RWTH Aachen University, Germany Corresponding author: Heide Budde-Meiwes, Institute for Power Electronics and Electrical Drives (Institut fu¨r Stromrichtertechnik und Elektrische Antriebe - ISEA), RWTH Aachen University, Ja¨gerstrasse 17/19, 52066 Aachen, Germany. Email:
[email protected]
A standard 12 V power supply provides 0.5–1 kWh. Depending on the drive style, compared with a similar car with a conventional engine. b
15–35 (highly dependent on the expected electric driving range) 200–400
a
Battery electric vehicles
25–120
4–10 10–20 15–30 30–100 (highly dependent on the average mileage per trip) 100 0–1 0.25–1 0.7–2.5 4–10 (highly dependent on the expected electric driving range) 2–10 10–20 20–70 20–70 12 (14)–48 48–200 200–400 200–400 Micro-hybrid vehicles Mild-hybrid vehicles Full-hybrid vehicles Plug-in-hybrid vehicles
Additionally usable battery energy content for electrification functionsa (kWh) Battery voltage level (V)
Battery power (kW)
Reduction in fuel consumptionb (%)
Proc IMechE Part D: J Automobile Engineering 227(5)
Vehicles
Table 1. Battery voltage levels, battery powers, additionally usable battery energy contents and fuel reductions for various vehicle types (standard passenger vehicles are assumed in all cases) (based on our own market observations and those of Koehler1, Karden et al.9 and Eckstein and Gies10).
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battery, but concepts with lithium-ion batteries are also under development2,3. The majority of the hybrid electric vehicles on the market today use nickel–metal hydride batteries, but it is expected that lithium-ion batteries will overtake this market segment in a few years4,5. Full-electric vehicles use mainly lithium-ion batteries, but there are also manufacturers which offer electric vehicles with lead–acid or sodium–nickel chloride batteries for niche markets6. A battery for electrified vehicles is always a compromise between a high power performance, a high energy storage capability, a low weight, a small volume, a long lifetime and a low price. New propulsion concepts require improved battery systems. For each propulsion type a suitable battery system is needed. Despite the discussion on technical performance indicators, the price of the battery is the main challenge for market introduction. Cost reduction is achieved by technology improvements but mainly by economy-of-scale effects. Legislation, especially in the EU und the USA, forces car manufacturers to raise fuel efficiency and to lower carbon dioxide (CO2) emissions. In the EU, the range is 160–130 g CO2/km for 65 % of a manufacturer’s fleet in 2012, and it will rise to 100 % by 20157. Emissions of 95 g CO2/km are under discussion for 2020. US legislation requires 172 g CO2/km under the New European Driving Cycle (NEDC) as the fleet average by 20168. This pressure has changed the research focus to new propulsion concepts where the battery system is a central part. It is worth mentioning that even fuel-cell electric vehicles usually have a battery on board for peak power and regenerative braking. For the People’s Republic of China, various suggestions exist, but there are no clear aims on CO2 reduction available. Regarding the continuous smog pollution, with a peak in early 2013, the pressure to develop vehicles with lower exhaust emissions or electric vehicles is rising. Batteries will be the most expensive part in fullelectric vehicles. It is therefore essential for bringing electric cars to the market to select appropriate battery technologies, to size the battery optimally to the customers’ needs and to design optimal batteries with the highest reliability, lifetime and safety.
Classification and requirements for various hybrid vehicles Selection and sizing of a battery system depend on the requirements of the respective vehicles. The term ‘hybrid vehicle’ describes a wide range of vehicle concepts. In the following, various vehicle concepts are classified. This classification scarcely differs between the various car manufacturers. Overviews have been given by Karden et al.9 and Koehler1. Figure 1 compares all types of electrified vehicle with respect to the energy and power demand. Table 1 compares the battery voltage levels, battery powers,
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Plug-In HEV
lifetime. According to a Woehler curve, the number of achievable cycles increases strongly with decreasing depth of discharge. For several batteries, the SOC during operation is of relevance as well. However, it is worth mentioning that most batteries die in vehicles because of calendrical ageing, and hence in rest and parking periods11.
Full HEV
Micro-hybrid electric vehicles
100
Energy (kWh)
EV
10
1
0.1
Mild HEV
Micro HEV
1
10 Power (kW)
100
Figure 1. Energy and power demands on batteries for various types of electrified vehicle (based on our own market observations and those of Koehler1, Karden et al.9 and Eckstein and Gies10). EV: electric vehicle; HEV: hybrid electric vehicle.
100
Typical cycling Typical SOC range
SOC (%)
80 60 40
Plug-in-HEV & EV
Full-HEV
Mild HEV
Micro HEV
0
Convenonal car
20
Figure 2. SOC regions of battery operation in various HEV duties. SOC: state-of-charge; HEV: hybrid electric vehicle; EV: electric vehicle.
additionally usable battery energy contents and possible fuel reductions for the vehicles described in the following sections. Fuel-cell electric vehicles are not mentioned here explicitly. However, the battery sizing and requirements are typically quite similar to those of fullhybrid electric vehicles mentioned in the figures and tables. The used state-of-charge (SOC) region is specific to each vehicle concept (Figure 2). Related to this, a suitable battery type or a combination is chosen. This is of relevance because the number of cycles and the cycle depth are of the highest relevance to the battery
The first step of hybridisation is a micro-hybrid vehicle12,y. Vehicles with a conventional combustion engine are equipped with options for reducing CO2 emissions. The stop–start function which has been available on the market for several years is widely known. The combustion engine is switched off during standing when the engine is in the idle state, e.g. at traffic lights. During engine-off, the battery takes over all electric requirements. In most concepts the engine starts again when the driver presses the clutch. A further function is regenerative braking. In conventional cars, braking energy is lost by heat, while micro-hybrid vehicles use a part of this energy for charging the battery (recuperation) via the existing reverse-operated power generator. Customers do not notice this function during driving (in contrast with the stop–start system), but they can observe that less fuel is needed. The next step is already under development: ‘sailing’. Sailing means ‘engine-off’ during driving at a constant speed, e.g. on highways. The challenge for the battery is a largely increased number of cranking events and a higher number of fullcycle equivalents (owing to the extra use of electrical power during stops)13. Regenerative braking can save fuel only when the dynamic charge acceptance is high, which is a challenge, especially for lead–acid batteries. Charge acceptance strongly depends on the SOC (which is better at low values) and the temperature (which is better at higher values). Furthermore, the battery needs to be capable of partial SOC operation. Additionally, an alternator and a load control system are required in such a vehicle. Operation modes such as sailing require a specifically high level of reliability. Failure on cranking during sailing would be potentially catastrophic. The on-board voltage level is usually 12 V (or 14 V respectively) because changes to the conventional system are kept as small as possible!. In the future a 48 V y
Some researchers do not count micro-hybrid vehicles as hybrid vehicles because they do not support the engine. However, micro-hybrid vehicles extend the traditional functionality of the power system in vehicles significantly and they allow an accountable increase in the fuel economy. ! 12 V and 14 V voltage levels are used synonymously. 14 V is approximately the voltage during charging, and 12 V is the nominal voltage of lead–acid batteries with six cells connected in series. With ‘48 V’, typically a battery with a nominal voltage of 48 V is meant. This is equivalent to either 24 lead–acid batteries or approximately 13 lithium-ion cells connected in series.
764 level system might be added. Micro-hybrid vehicles are state of the art for many vehicle manufacturers. All European car manufacturers offer micro-hybrid vehicles. The batteries are sized to deliver appropriate cranking power but also to deliver sufficient energy, e.g. during long-time parking for the remaining power consumption of various control units in the car (e.g. the alarm, wireless door opening and radio).
Mild-hybrid electric vehicles Mild-hybrid vehicles offer the same functions as microhybrid vehicles. In addition, they provide boosting as an extra function which is the electric motor assist for the drivetrain, especially during acceleration. As in micro-hybrid vehicles, the battery is charged only during driving. This can occur either when the vehicle’s speed is reduced (regenerative braking) by the generator or when recharge energy is provided actively by the combustion engine. For the battery this is a high-power application. The battery is sized in such a way that it meets the power requirements appropriately.
Full-hybrid electric vehicles Full-hybrid vehicles cover all functions of mild-hybrid vehicles. In addition, pure electric driving is possible as well as higher power assist during acceleration and regenerative braking. The battery system is used when a high power is needed or the efficiency of the combustion engine is low. In full-hybrid vehicles the battery is recharged only during driving by the internal-combustion engine (ICE) or during regenerative braking. Therefore the pure electric driving range is very limited (typically below 1 km). However, it is an interesting option for silent cruising in residential areas1.
Plug-in hybrid electric vehicles Plug-in hybrid electric vehicles combine the characteristics of both a full-electric vehicle and a conventional vehicle with an ICE. For plug-in hybrid electric vehicles, charging is carried out during standing times when the car is plugged into the grid. Therefore the pure electric driving range is higher than with full-hybrid vehicles. When the battery is at an SOC level of approximately 20 %, the ICE works as the power source. From this point onwards the car mainly behaves like a full-hybrid vehicle. Generally, two different concepts can be realised to integrate the ICE. The parallel drivetrain design allows the ICE to transfer mechanical power via the gearbox directly to the wheels, and in addition it can recharge the battery to maintain the low SOC level. This is the concept used by full-hybrid vehicles as well; plug-in hybrid vehicles are equipped with a larger battery. Most European car manufacturers are developing
Proc IMechE Part D: J Automobile Engineering 227(5) parallel concepts. A variation of the parallel drivetrain is the power-split drivetrain (also called the series– parallel hybrid drivetrain), developed mainly by Japanese and US car manufacturers. The ICE and two electrical engines (one for powering and one for recuperation) are connected directly to the axle by a planetary gear set. A power-split drivetrain is used in the Toyota Prius14, which is the most successful hybrid vehicle on the market so far. From the battery’s point of view, it does not make any difference whether a parallel or a power-split drivetrain is used. In series drivetrains the ICE is used only for powering the generator which is delivering electric power to the electric machine and for charging the battery. This allows the ICE to be operated in just two or three operation modes (off, on with a low power and on with a high power) and therefore at the optimum efficiency and with the lowest possible emissions. The used SOC range of the battery is larger in a series drivetrain than in a parallel drivetrain. A series drivetrain is used, for example, in the Chevrolet Volt15. It is often said that series hybrid vehicles have a lower overall efficiencyz. In any case, all components of the drivetrain (except for the ICE itself) and the electrical system on board in a series hybrid vehicle are similar to those in full-electric vehicles and fuel-cell electric vehicles. Because of the use of building blocks for the main drivetrain components, series hybrid vehicles therefore represent a very efficient vehicle design. As energy from the grid is needed for recharging the battery, CO2 emissions for electric driving depend on the energy mix (fossil, nuclear or regenerative energy)1 and on the ratio of the electrically travelled miles to the overall travelled miles. Complete equipment with an ICE, an electric motor and a traction battery system, with each of the components designed to be able to drive the car alone, causes higher weight and rising costs. However, plug-in hybrid vehicles are operated with significantly smaller batteries than full-electric vehicles while at the same time having a maximum driving range due to the ICE. Furthermore, mobility statistics clearly show that vehicles can save much gasoline and diesel. If all vehicles were to be plug-in hybrid electric vehicles, having a battery for 30 km of full-electric driving and having the opportunity to obtain charges twice a day (e.g. at home during the night and at work), about two thirds of the total mileage of all vehicles would be electrically driven miles. Thus, small batteries allow a dramatic reduction in the CO2 emissions and will be competitive on a lifecycle cost basis very soon16. Total costs are expected to decrease by 2020 with respect to the vehicle size, horsepower, vehicle taxes, maintenance, initial costs and z
However, the truth is that mechanical engineers prefer parallel hybrid vehicles (because the ICE, gearbox and clutches are masterpieces of mechanical engineering) and electrical engineers prefer series hybrid vehicles (because the topology eliminates all these complex mechanical elements).
Budde-Meiwes et al. efficiency of electric components16. As long as a combustion engine is included, the driving range per day is not limited.
Electric vehicles Electric vehicles are solely electrically driven and operate completely without an ICE. Brake energy recovery is installed as well. The possible driving range of electric vehicles is lower than that of vehicles with conventional combustion engines. As 90 % of all trips are less than 100 km per day, a driving range of 100 km is acceptable for most applications, especially if the full-electric vehicle has been advertised from the beginning as a city or delivery car. It is most suitable for all trips to work (because that distance is well known) and as a second car for families. The ‘second-car market’ in Germany is roughly 25–30 % of the total vehicle market. Larger driving ranges would require a larger battery but, since the costs and the weight would increase as well, equipping electric vehicles with larger batteries does not make sense economically. For an electric driving distance of 500 km, the mass of the battery would rise to 720 kg, assuming 15 kWh per 100 km. An electric driving range of 100 km is available4 with a battery mass of 150 kg and a volume of approximately 100 l. Electric vehicles are locally emission free. The total emissions of today’s electric vehicles are highly dependent on the power-producing energy mixture. Reduction in the CO2 emissions can be achieved when the power is produced from renewable energy sources. For a distance of 100 km, approximately 15 kWh are required. This currently costs approximately 3–4e per 100 km. In comparison, an efficient gasoline engine consumes 5 l per 100 km (moderate driving), amounting to 7–8e. The future development of the costs for electricity cannot be foreseen, especially not under the assumption that a large number of electrically driven cars will roam our streets. The need for machine maintenance is expected to be less for electric vehicles as the electric motor works almost without attrition. Thus, the traction battery system constitutes the main cost factor. However, even with slightly higher initial costs, electric vehicles can compete on the market with lower maintenance and lower fuel costs.
765 machine to support the ICE and to optimise its operations, as well as to recover braking energy or for electronically assisted steering systems. The 12 V system operates the low-power systems such as light, entertainment, control units, etc. Staying below 60 V lowers the requirements on insulation, testing and system layout. Full-hybrid and electric vehicles mostly have a voltage level of around 400 V but there are also systems with a lower voltage and a more modular system layout. Modular battery packs with integrated DC-DC can work with a lower battery voltage level. In general, the voltage level of large-scale automotive battery packs for electric vehicles or full-hybrid applications depends on the system architecture. A mostly parallel connection of cells leads to a relatively low voltage but serves higher current rates. This requires thicker copper bus bars within the pack and, with several individual packs at different voltage levels, a DC-DC converter per pack is mandatory. On the other hand, having more cells and packs in parallel increases the reliability of the system as a single cell fault does not necessarily lead to a complete system failure as would be the case in a strictly serial connection. This is true at least as long as cells do not show internal short circuits. In contrast with the parallel structure, a serial connection of cells leads to a higher voltage and consequently to a lower current. As a result, the copper bars and passive components of the system can be downsized. On the other hand, a voltage measurement of each individual cell in a series connection is necessary as well as a charge-balancing system. Moreover, a strictly serial connection does not offer any redundancy in the case of a single cell fault. It is therefore strongly recommended that all the advantages and the disadvantages of the different battery system layouts are considered carefully. More information on battery pack design can be found in the section ‘battery system design’.
Battery technologies Various characteristics can be used to classify battery technologies. The most important parameters are described as follows. 1.
Voltage levels In vehicle applications, different main voltage levels are used. Micro-hybrid vehicles make use of the conventionally existing power system based on 12 V batteries. The voltage of mild-hybrid vehicles is usually below 200 V. As the threshold value of 60 V is exceeded, these systems have to be treated as high-voltage systems. However, combined 12 V–48 V systems have been widely discussed in recent years. The 48 V system serves high-power loads up to 10 kW using the electric
2.
The gravimetric power density (also called the specific power density) and the volumetric power density are among the most important parameters for hybrid vehicles. High values usually imply a low electrical resistance, resulting in low energy losses and high-power capability. Besides the power, the gravimetric energy density (also called the specific energy density) and the volumetric energy density are important as well. Capacitors can certainly deliver a high power, but they can do so for only a short time, while the energy density of batteries is higher. Figure 3 gives an overview of the specific gravimetric power and energy density of batteries. The theoretical
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Figure 3. Ragone plot of various battery technologies with specification at cell level for automotive applications without lithium– sulphur and metal–air batteries. SuperCap: supercapacitor; Pb: lead; Li-ion: lithium-ion; NiCd: nickel–cadmium; NiMH: nickel–metal hydride; NaNiCl2: sodium–nickel chloride; ZEBRA: Zero Emission Battery Research Activities.
Various battery technologies are presented in the following. An overview on battery technologies and applications for electric vehicles has been given by Bo¨cker et al4 (in German only). A general overview on batteries for hybrid vehicles has been given by Koehler1.
Because of the low energy density, lead–acid batteries are the choice when operating distance and weight are less important and a low price is crucial. This is the case for micro-hybrid vehicles or electric scooters. One drawback is the low energy density, which cannot be significantly improved because of the low theoretical value20. Further issues are the lifetime and the dynamic charge acceptance21, e.g. during regenerative braking. Various developments in the last few years have shown performance characteristics that allow use in mild- and full-hybrid vehicles. Lead–acid batteries are available with a liquid electrolyte or with an electrolyte adsorbed in a glass-fibre mat (typically, an adsorbed glass mat). For stop–start application, carbon is often added to the negative active material to raise the dynamic charge acceptance, which is an up-to-date research topic. The advantages of lead–acid batteries are the low material costs, intrinsic safety and high recycling quotes (with a value of more than 95 %, the highest of all battery technologies). In spite of their high weight, lead– acid batteries are still a promising option also for future development. Today, in every car with a high-voltage battery pack (e.g. electric vehicles) there is still a small lead–acid battery installed as well, e.g. for security reasons, as an SLI battery and to serve the 12 V power system. In the case of an accident, the high-voltage battery is disconnected from the electric system and the lead– acid battery provides power, e.g. for the emergency flasher system.
Lead–acid battery
Nickel–cadmium battery
Lead–acid batteries are by far the cheapest energy storage technology with regard to the raw material costs.
Nickel–cadmium batteries show a slightly higher energy density than lead–acid batteries do and a significantly
3.
4.
5.
6.
gravimetric energy density can be calculated from the main electrochemical reaction. The realistic value of the cell level is usually 25–50 % of the theoretical value. The ratio of the discharged energy to the charged energy is the energy efficiency. Energy losses are transformed into heat and must be removed to avoid overheating of the batteries. The calendar lifetime describes the battery lifetime until failure as long as the battery is not used. Higher temperatures accelerate the ageing process. Many high-performance batteries in standard vehicles will probably die because of calendrical ageing rather than because of the capacity turnover. The cycle lifetime, on the other hand, describes how many cycles the battery can perform until it fails. The cycle lifetime depends on the cycle depth, current rate and average SOC. The capacity turnover is measured in full equivalent cycles. Besides technical aspects, the costs are relevant for choosing a battery system. The current costs are identified in Table 2. However, the costs strongly depend on the specific requirements and the quality of the battery.
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Table 2. Cell voltages and current costs of various battery technologies on a cell level (in particular, batteries at the lower end of the price scale may not fulfil all other requirements of car manufacturers such as the deep-temperature performance, calendar lifetime, cycle lifetime or safety) (based on our own market observations and those of Pavlov17, Bergveld et al.18 and Wenzl19). Technology
Nominal voltage (V)
Costs (e/kWh)
Lead–acid (SLI)
2.0
Nickel–cadmium Nickel–metal hydride
1.2 1.2
Sodium–nickel chloride Lithium-ion (LiCoO2)
2.5 3.6
Lithium polymer
3.7
Lithium-ion (LiFePO4)
3.25
Supercapacitor Lithium carbon capacitors Lithium–sulphur Rechargeable metal–air batteries
2.5–2.8 3.8–4 2.2 1–4 (depending on technology)
25–40 for OEM 100–180 for after-market 200 – 500 for OEM 275 – 550 for OEM 600 for HEVs 350 – 600 200–500 for OEM 400–800 for HEVs 200–500 for OEM 400–800 for HEVs 200–500 for OEM 400–800 for HEVs 10,000–20,000 Early market introduction Early market introduction Early research state
SLI: starting, lighting and ignition: OEM: original equipment manufacturer; HEV: hybrid electric vehicle.
higher power density20, but the specific battery costs are much higher. Security requirements are met in the case of an accident by this battery type, but from the ecological point of view the usage of cadmium is critical. This battery type is technically mature and the possibility of using nickel–cadmium batteries in electric vehicles is similar to that of employing lead–acid batteries. However, it is not expected that nickel–cadmium batteries will play a major role in the future electrification of vehicles considering the energy density and costs4.
Nickel–metal hydride battery Nickel–metal hydride batteries are a further development of nickel–cadmium batteries, with the aim of creating a battery without toxic cadmium but with the advantages of nickel–cadmium batteries. Nickel–metal hydride batteries can be interchanged with nickel– cadmium batteries as the cell voltages, discharge curves and charge curves of both battery types are almost identical. The deep-temperature performance of nickel– metal hydride batteries is rather poor. However, nickel– metal hydride batteries achieve approximately double the energy density that nickel–cadmium batteries do, but it is especially the high power density (greater than 1000 W/kg) and the sufficient lifetime that qualify the technology as the world market leader in hybrid electric vehicles. From the security point of view, the use in vehicles is not critical. Nickel–metal hydride batteries are used in hybrid vehicles by Toyota, Honda, Lexus and many other car manufacturers. Research activity on nickel–metal hydride batteries is relatively low. Those car manufacturers currently using nickel–metal hydride batteries in large quantities for hybridelectric vehicles will probably continue using
this proven and mature technology for several years. However, at present, only a few experts expect to see nickel–metal hydride batteries in full-electric vehicles or in plug-in hybrid electric vehicles in large quantities owing to their high material costs. Nickel–metal hydride batteries do not seem to be competitive with lithium-ion batteries in the long term4,5.
Sodium–nickel chloride battery The sodium–nickel chloride battery (NaNiCl2, Zero Emission Battery Research Activities, ZEBRA) is made of liquid sodium at the operating temperature and a solid ceramic electrolyte which also acts as a separator. Their main disadvantage is the high operating temperature of around 300 °C, which requires good isolation. A cool-down to ambient temperatures leads to a high thermomechanical stress for the ceramic electrolyte (if it breaks, the cell cannot be used any more). Further known problems with this technology are the limited power capability due to the high inner resistance and the self-discharge of 10–15 % per day to maintain the high temperature level. Therefore, ZEBRA batteries are an interesting option only for fleets, e.g. for delivery services in cities. Usage in private vehicles is difficult because of the low daily mileage and therefore the need to compensate thermal losses. At present, the costs for these systems are roughly similar to those of lithiumion batteries. Research on ZEBRA batteries is quite limited. Currently, only two companies are developing them worldwide. They seem to focus more on stationary than on automotive applications. For automotive applications, however, the perspectives for lithium-ion batteries regarding the costs, energy density and handling are better.
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Lithium-ion batteries ‘Lithium-ion batteries’ is an umbrella term for a variety of material combinations used to form batteries. Their characteristics regarding the power, lifetime, low- and high-temperature performances and safety are very dependent on the material combination. The electrode design allows optimisation towards high-power or high-energy cells. According to Figure 3, lithium-ion battery cells achieve the highest gravimetric energy and power densities of all commercially available rechargeable batteries. Very long lifetimes and safety levels can be achieved by using titanate for the negative active material instead of carbon, but this results in significantly lower energy densities. Today’s material combinations for lithium-ion batteries do not offer a mechanism for overcharging that could take in current during overcharge without damaging the battery. This brings the advantage of high Coulomb efficiencies close to 100 %. Consequently, the energy efficiency is also very high compared with other battery technologies. However, because of the missing internal overcharge process, lithium-ion battery cells must each be supervised individually to avoid overcharging of any single cell as this would finally lead to the destruction of the cell. To avoid this danger, the first cell achieving the fully charged state terminates the charging process, and the first cell achieving the end of discharge voltage during discharging terminates the discharge. Series-connected strings of lithium-ion cells therefore depend very much on the worst cell, leading to the necessity for very high quality in the production process to minimise the differences between the capacities, internal resistances and amounts of ageing of the cells. Another advantage of lithium-ion batteries is the high-capacity utilisation even at high current rates. This is why lithium-ion batteries are especially suitable for applications with high currents, e.g. electric vehicles. Today, lithium-ion batteries are offered in two major production lines: high-energy batteries and high-power
(a)
batteries. High-power batteries have very thin active mass layers so that the relative volumes of the metal foils (used as current collectors) and the separator (or electrolyte) are high compared with the volume of the active mass. To reach the same capacity as high-energy batteries, high-power cells need more material, resulting in higher costs. Values for the power density of lithiumion batteries can be gradually shifted between highenergy and high-power cells. Three different cell designs are being developed and produced today: cylindrical, prismatic and pouch (also called ‘coffee-bag’) cells (Figure 4). The long experience in the production of the cylindrical design is a considerable advantage of cylindrical cells. Their container is dense and can withstand some inner pressure (which can arise from side reactions) without deformation. A disadvantage is the low packaging density and the lower heat transport from inner cell parts to the outside. In particular, for cells with a higher capacity, this can lead to higher temperatures in the middle of the cells. However, the relation between the surface and the volume is more efficient, which makes cooling concepts easier. Other problems might be the density of the cells and the swelling when the inner pressure rises. It is predicted that, in the future, pouch cells can achieve lower production costs and a slightly higher energy density as the case foils are lighter than the containers of cylindrical cells. In pouch cells the quality of the welding is an issue even after several years of ongoing development. Prismatic cells are from the construction point of view similar to cylindrical cells. Because of the strength of the container, they are at present preferred for automotive applications by some manufacturers. Today, costs for lithium-ion batteries are still too high for the mass market (see also Table 2). When mass production comes to the market, relevant cost reductions are expected. However, high-power cells probably will remain 50–70 % more expensive than high-energy cells. Packaging and battery management need to be added to obtain total costs, but these costs are also expected to decrease in the future. Unfortunately, cells
(b)
Figure 4. Various cell designs: (a) cylindrical; (b) prismatic; (c) pouch.
(c)
Budde-Meiwes et al. from consumer applications (mobiles, laptops, etc.), which are already in the mass production status, do not fulfil the demands of automotive applications, especially with respect to the lifetime and safety. The availability of lithium, when the number of electric vehicles increases, is under controversial discussion. However, according to the current state of knowledge, there is sufficient lithium available to equip all vehicles in the world with large lithium-ion batteries. For now this is sufficient but it also means that a recycling system for retrieval needs to be established. Lithium sources are limited to few countries with a core area in South America. The operators of lithium mines need early planning reliability to raise the production capability to avoid price maxima for lithium.
769 Typically, one electrode is a supercapacitor electrode and the other is a battery electrode in commercial cells using an organic electrolyte. Lithium–carbon capacitors allow higher cell voltages24 of up to 4 V and, as the stored energy is proportional to the squared voltage, the energy density can be significantly increased in comparison with that of supercapacitors. Lithium–carbon capacitors also possess a higher power density than batteries do. With respect to the cycle lifetime, lithium– carbon capacitors are limited by the battery electrode although the cycle lifetime is still better than that of batteries. The commercialisation of lithium capacitors has just started24.
Supercapacitors and lithium–carbon capacitors
Future technologies: lithium–sulphur and metal–air batteries
Supercapacitors (also known as electrochemical double-layer capacitors, and often abbreviated to supercaps or ultracaps) can store and deliver energy very quickly at high current rates for a short time. They are therefore specifically suitable for boosting and regenerative braking. Supercapacitors have a very high power density and outperform nearly all battery technologies with respect to the cycle lifetime. However, the energy density of supercapacitors is much lower than that of batteries, and they can therefore be used only as buffers within a system together with one or more types of energy storage. If lithium-ion batteries with a high power density are used in an electric vehicle, there is no need for further power storage. However, if the battery technology has a lower power density, hybridisation with supercapacitors makes sense. Supercapacitors are especially an option for pulse power9. Commercial supercapacitors usually consist of two activated carbon electrodes in an organic electrolyte. As the energy is stored purely electrostatically, no chemical reaction occurs during charging and discharging respectively, and therefore charging and discharging are highly reversible22. This leads to a very long cycle lifetime. More than 500,000 full cycles have been reported23. Supercapacitors are commercially available in many different designs, predominantly in a cylindrical or a prismatic shape. However, today’s costs of supercapacitors are in the range of 15,000e/kWh of storage capacity. Even though the costs for the stored energy are high, the costs for the available power are as low as 15e/kW or even less. Lithium–carbon capacitors are so-called ‘hybrid capacitors’, a combination of a supercapacitor and a lithium-ion battery with the aim of uniting the advantageous characteristics of both types of energy storage. Their power density and cycle lifetime are higher than those of lithium-ion batteries and their energy density is a factor of up to 2 higher than the energy density of supercapacitors24.
For energy densities above those of todays’ lithium-ion batteries, metallic lithium is being used as the active material at the anode. One of those material systems is lithium–sulphur. The anode is metallic lithium and the cathode is a mix of elemental sulphur (S8 rings) with carbon. Both lithium and sulphur are dissolved in the electrolyte and the lithium ions react with the polysulphides, which change their composition during discharge. During charge, these processes run the other way round. The carbon is needed for the conductivity to compensate for the low electrical conductivity of sulphur. If the carbon is added in the form of carbon nanotubes, there is a good path for electric conductivity and also a structure for the sulphur, which can be used to obtain handleable cathodes. With this concept the theoretical energy density is more than 2500 Wh/kg; in reality, energy densities of 500–600 Wh/kg can be achieved25,26. Some cells are designed for low-temperature applications. These cells still work even at temperatures below 230 °C so that it is possible to use them in automotive applications27. Another advantage of these cells is their high efficiency in terms of sulphur usage, which is above 80 %. Their charge–discharge efficiency is quite good as well but the number of cycles and stability are poor. Another approach to reach a high energy density is to use the oxygen of the atmosphere as the active material on the cathode. This means that the anode is a pure metal which is oxidised with oxygen. Because oxygen is available everywhere, only the metal has to be stored in the battery. Therefore the energy density increases dramatically owing to the low weight of the stored material in the battery. However, the battery weight increases with discharging (and thus the energy density decreases) because oxygen is incorporated in the form of metal oxides. Between the metal anode, e.g. lithium, and the porous membrane, which is the contact with the atmosphere, there is an electrolyte which transports the dissolved lithium cations from the anode to the membrane where it reacts with the oxygen. Because of this membrane, this battery technology is not that of a classical
770 battery but a combination of the technologies of a battery and a fuel cell. A lithum–air battery, for example, has a theoretical energy density of 13 kWh/kg without the oxygen, which is comparable with that of fuel (typically also characterized without the oxygen needed for burning it). However, besides the possible high energy density, there are still great challenges in metal–air systems such as the number of cycles, the safety and the lifetime. Those parameters are massively influenced by the ambient air which also contains water, carbon dioxide and nitrogen. All these components cause side reactions with the active materials, which lead to poor reversibility of the reactions and binding of the active components26.
Combination of the battery technology and hybrid vehicles As a summary, Table 3 gives a short overview of suitable battery technologies for hybrid vehicles. Limitations by the batteries are outlined.
Battery system design The development of cell technologies is important, but the suitable system integration into the powertrain and the vehicle is essential for the performance and security. In the following, several system aspects are discussed, such as the battery safety, which requires a battery management system and thermal management. Further hybrid battery systems are also discussed. Battery safety is an important aspect as avoiding accidents with batteries is vital for market introduction and costumer assurance. Battery management systems are responsible for the reliability of the system and a long lifetime. An essential part is the thermal management, especially for lithium-ion and high-temperature batteries. A way to combine the advantages of different battery technologies is to use two or more battery packs for different requirements. Such a system is called a ‘hybrid battery system’.
Battery safety Battery safety is a main feature for the successful introduction of hybrid and electric vehicles to the market with high customer acceptance. The need for safety systems depends on the chosen technology, and the realisation of safety systems can produce significant costs. This topic is particularly important when lithium-ion batteries are used as the management of lithium batteries is of higher complexity than that of lead–acid batteries, for example, and the chemistry is more reactive. Therefore, the following comments focus in particular on lithium-ion batteries. Safety precautions are divided into passive and active systems. Passive safety takes place at a cell level
Proc IMechE Part D: J Automobile Engineering 227(5) as well as at a system level. At a cell level, the choice of cell chemistry directly influences the safety. LiFePO4 for instance is a relatively intrinsically safe material compared with lithium–metal-oxide cathode materials using cobalt, nickel or manganese. An even higher safety level can be achieved if lithium titanate (Li4Ti5O12) is used instead of carbon for the negative electrode (anode). The higher the passive safety of a cell, the lower is the cell voltage and the lower is the energy density. Figure 5 shows this for various material combinations with lithium. Moreover, safety components such as ceramic separators, positive-temperaturecoefficient (PTC) elements to limit the cell current, or flame-retardant additives can enhance cell safety28. At a pack level, passive safety can be ensured, for example, by overpressure vents in the case of venting cells. An 18650 cell emits around 2.5 l of gas in the case of a thermal runaway29, which can be scaled up to the size of a battery pack. However, it must be considered in the dimensioning that not all cells will vent at the same time but possibly in a chain reaction. Additionally, the pack should be protected against crash impact by the housing and by fuses against being short circuited. In a recently completed research project, a battery pack design was demonstrated that allows crash energy to be absorbed and at the same time safeguards the individual cells from destruction30. Thus, in the future, it is likely that batteries can be designed in a way that they can even be placed in the crash zone of a vehicle. Active safety includes individual voltage control of each cell in a series connection, exact charge and discharge control management, as well as temperature control and cooling systems. The measured values have to be made plausible and a redundancy for faulty measurements should be given. Even though the technically safe state of the battery is to open the switches in the case of a fault, this is only the ultimate option as the actual driving situation could be potentially more crucial than the fault itself, e.g. on railway tracks or when the active moment distribution is activated. In general, the battery management system will not open the switches by itself unless there is at least one independent battery pack that ensures the operability of the vehicle. This again is an advantage of modular battery systems. To evaluate the safety risks, batteries are tested on the basis of electrical impacts (e.g. overcharge or short circuit), thermal impacts (e.g. external heat source) and mechanical impacts (e.g. nail penetration or crash). These tests depend on regulations regarding the transport31 as well as the usage of batteries in vehicles32. Moreover, the functional safety has to be considered in the system design, as given in ISO/DIS 26262-9: 200933.
Battery management system Battery management systems secure safety aspects as well as a long-lasting lifetime. Battery monitoring, diagnostics, electrical and thermal management as well as
–
–
+
o
–
Nickel–metal hydride
Sodium–nickel chloride
Lithium-ion
Supercapacitor
Lithium–air
+ cycle lifetime o only in a hybrid battery system – energy density – probably no high power
+ energy and power density + cycle lifetime – costs o for safe operation, a highly sophisticated battery management system is needed
– heat loss – low power
– low deep-temperature performance
+ cheap, safe – charge acceptance
+ , suitable; o, neutral; –, not suitable.
+
Micro-hybrid
–
o
+
–
o
o
cheap, safe charge acceptance power density sufficient lifetime limited cost reduction
+ cycle lifetime o only in a hybrid battery system – energy density – probably no high power
+ energy and power density + cycle lifetime – costs o for safe operation, a highly sophisticated battery management system is needed
– heat loss – low power
+ – – + –
Mild-hybrid
Limitations for the following hybrid types
Lead–acid
Battery technology
Table 3. Overview of suitable battery technologies and limitations for hybrid vehicles.
–
o
+
–
+
– mature sufficient lifetime power density limited cost reduction heat loss low power
+ cycle lifetime o only in a hybrid system – energy density – probably no high power
+ power density o for safe operation, a highly sophisticated battery management system is needed
+ + + – – –
– power density
Full-hybrid
o
–
+
–
o
–
o only if cost per kWh is less than cost for Liion – as hybrid system
– energy density – costs
+ cycle lifetime o costs o for safe operation, a highly sophisticated battery management system is needed
– power limited in small batteries – too high heat loss
– costs – weight for larger batteries
– weight, power density
Plug-in-hybrid
+
–
+
o
–
–
+ potential for high mileage o open questions: safety, lifetime, lowtemperature performance
o fleet operation or significant daily mileage – heat loss for standard use profiles + energy density + range \ 150 km o if range . 300 km, then competition with a fuel cell o for safe operation, a highly sophisticated battery management system is needed – energy density – costs
– weight, range o urban delivery, light trucks – costs – energy density
Battery electric
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Negative
Positive Thermal instability
C6 / LiMeO2 (Me = Co, Ni, Mn, Al)
Safe
3.6 V (fully charged)
C6 / LiFePO4 0V
1V
2V
3V
Safe
2.7 V (fully charged)
LTO / LiMeO2
4V
Potential versus Li/Li+ (V)
Thermal instability
Lithium plating
4.2 V (fully charged)
LTO / LiFePO4
Safe
Safe
2.1 V (fully charged)
Figure 5. Electrode voltages for several material combinations with lithium. LTO: lithium titanate (Li4Ti5O12).
cell balancing are typical functions of such a system. Battery management systems monitor the battery pack’s SOC and state-of-health (ageing) and update its power capability. This information is used by the vehicle management, e.g. for the overall energy management. The battery management also provides the vehicle management with information on how to operate the battery to achieve the longest possible lifetime34. Cell management is most important for the lithiumion technology as this technology does not imply any intrinsic overcharge mechanism. In addition, the SOC of each cell differs over time caused by the production and temperature variations between the cells. Charge equalisation systems are thus required to balance cells to a similar level. Different SOCs between the cells can also be avoided by a high production standard with equal qualities for all cells.
Thermal management Thermal management regulates the temperatures within a battery to be as constant as possible at a defined level. Deep temperatures are avoided because the cells usually have a reduced power capability at low temperatures; high temperatures, on the other hand, reduce the lifetime. In particular, for lithium-ion batteries, the maximum charge current must be limited at low temperatures to avoid risky states in the battery and accelerated ageing due to an effect called lithium plating. Moreover, safety issues of lithium-ion battery packs arise because of potential overheating. Under normal operating and ambient temperature conditions, battery systems can be easily controlled in the range35 20–55 °C. Regarding stressful conditions, such as a fast charge at a high cell temperature or a high ambient temperature, heat generation might increase tremendously
and eventually lead to a so-called thermal runaway. An initial event triggers an exothermal reaction. The reaction causes heat, which further accelerates the exothermal reaction. This process can eventually lead to explosion or fire and therefore has to be prevented by all means. The most important factors for efficient thermal management are the battery pack design and the cooling system. As a rule of thumb, a 10 K temperature increase reduces the lifetime by a factor of 2. Today, various cooling systems are considered ranging from air cooling via water cooling to phase-changing cooling liquids. In the future, phase-change materials (PCMs) as packaging material systems are also conceivable but have so far not been realised beyond research activities. PCMs can help to limit the temperature gradient, thus requiring less active cooling. For all cooling systems the number of temperature sensors needed, as well as their distribution within the pack, has to be considered. It is necessary to detect overheating of any cell within the pack with a high reliability. The thermal contacts of the cell to the cooling system depend on the chosen cell type (cylindrical, prismatic or pouch bag) and can be realised, for example, by cooling the bottom of the cell, the mantle or the contacts. On the one hand, the objective of temperature management is to operate the battery in an optimum temperature range (25–40 °C during driving, and as low as possible during parking periods). On the other hand, the temperature gradient between the cells within a battery pack should be as low as possible. Because in a series connection of lithium-ion batteries the worst cell determines the total performance of the battery string, and because of the high impact of temperature on the lifetime, a minimised temperature gradient is a key factor to the desired performance of the whole battery system. At the same time, the thermal
Budde-Meiwes et al. management system should be as lightweight and as efficient as possible. The latter means a low hydraulic resistance and a low pressure drop.
Hybrid batteries As can be seen in Figure 3, the energy and power densities of batteries vary in a wide range. However, a high power density usually does not accompany a high energy density. Therefore, a combination of two technologies (which would have a high power density and would also be superior with respect to the energy density) to use both advantages seems to be perfectly reasonable36. A combination often discussed in literature is supercapacitors with high-energy cells. While the power density of supercapacitors is superior to those of most other batteries, the energy density is by far the lowest. On the contrary, high-energy cells have a low power density owing to the slow mass transport processes during charging and discharging. For hybridisation, two general concepts are considered24. The first approach favours an internal hybridisation (within each cell) which can be realised either in a parallel connection or in a series connection. In the parallel connection, both electrodes contain an electrochemical capacitor as well as battery materials and are therefore called bi-material electrodes. In the series connection, the device consists of a battery electrode and an electrochemical capacitor electrode. The second approach is an external combination of a supercapacitor and a rechargeable battery. For the external approach as well, both series connection and parallel connection are feasible. By realising the battery as a modular system with a joint intermediate DC circuit, it is also possible to combine different cell technologies at different voltage levels5. This gives the prospect of also combining, for example, lithium-ion cells with different behaviours to fulfil ideally the load profile of the specific application. Today, there are also direct parallel connections of various technologies (e.g. lead–acid and lithium) without a DC-DC. converter under development, driven by cost savings for the DC-DC. converter.5 The idea is that the lithium system compensates the limited dynamic charge acceptance of the lead–acid battery37 and also unloads the lead–acid batteries from micro-cycling. As lithium-ion batteries have cycle lifetimes one to two orders of magnitude higher than those of lead–acid batteries, this leads to longer overall lifetimes. On the other hand, lead–acid batteries can provide the necessary energy reserve at low costs. However, the voltage levels of the two technologies must fit each other and therefore the number of cells connected in series from each technology must correspond properly. Another variation of hybrid batteries is the modular battery system approach. There are various advantages in using several smaller battery packs in a vehicle. The packs contain different cells to fulfil different requirements such as a high power for a high engine
773 power or a high energy density for long driving ranges, but the cells could also be similar. In combination with individual DC-DC converters for each battery pack, the reliability can be increased, a battery voltage level below 60 V can be used to lower safety risks due to high-voltage systems, and vehicle designers can obtain more flexibility in positioning the batteries in the vehicle.
Charging concepts Charging concepts in general can be classified into standard charging, fast charging and battery exchange. Different concepts are introduced and their effects on the battery system design are discussed in the following. The vehicle-to-grid (V2G) concept enables electric vehicles to become active elements of the electricity grid. The potential of this technology is shown using the example of cost-minimised battery charging.
Standard charge Standard charging refers to charging at the standard power outlet (e.g. 3.7 kW, single phase and 230 V in Germany) with an on-board battery charger. Recharging a fully depleted battery takes about 4–5 h for an electric vehicle (100 km range) and can be achieved without installing a costly charging infrastructure. According to Bo¨cker et al.4, this charging concept will be dominating in the future as ‘overnight’ or ‘during the day’ recharging is sufficient in most cases. Onboard battery chargers have a high potential for cost reductions as they can be manufactured in very large quantities (one universal on-board charger for different types of car). The effect on the battery system design is very limited as the charging power is below the power during driving. This means that the driving operation is dominating with regard to the choice of cells and the cooling system. Charging concepts can influence the lifetime of the battery. The standard charge is most preserving.
Fast charge In contrast with standard charging, the vehicle battery can be recharged in 10–20 min with fast charging. The typical charging power is 50–100 kW and a public or semipublic charging infrastructure has to be built as it is not cost efficient to supply each car with its own fastcharging station. Normally, off-board battery chargers with a DC connection to the car are used. Because of the external supply of the DC voltage, the communication interface of the charging station and the battery management system have to be standardised. Two common interfaces used today are CHAdeMO (up to 62.5 kW charging power)38 and COMBO-2 (up to 100 kW charging power)39.
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To guarantee fast-charging capability, the battery system has to be designed for high constant charging powers. Typically, the loads during charging are higher than during driving operation. The design of the cooling system has to be regarded carefully as losses in the range of 5 kW and more can occur in the battery pack. While air cooling might be sufficient for the driving operation, liquid cooling becomes necessary for the fast-charging capability. The need to use high-power battery cells and a more advanced cooling system raises the costs of the battery system.
Battery exchange Battery exchange stations are an option to extend the range of electric vehicles and to overcome the ‘range anxiety’ of users. The concept is mainly promoted by the company ‘Better Place’40. The idea is to exchange the discharged vehicle battery fully automatically in a battery swap station in approximately 5 min. There are several reasons why this technology is hardly introducible in the mass market. 1.
2.
3.
Battery packs of different car manufactures would have to be standardised with regard to connector systems, communication protocols and mounting positions in the car. Battery exchange is only needed very rarely as the battery range is sufficient for most trips. However, during holiday times there might be many car users who would need to exchange their vehicles’ batteries, resulting in the need to establish battery exchange stations in sufficient quantities. To supply fully charged battery packs throughout the year, a large quantity of spare battery packs has to be stored in the exchange stations which raises the investment in expensive batteries.
However, battery exchange stations for buses in public transport systems or for taxis can be an interesting solution. In these applications, standstill times have to be minimised and the mileage of these vehicles cannot be covered by one battery charge per day. Furthermore, these exchange stations would be used more frequently than those for standard vehicles.
Vehicle-to-grid concepts The V2G concept is commonly referred to the feeding of power from the vehicle battery into the electricity grid. Using bidirectional battery chargers makes it possible to take part in electricity trading, to supply balancing power or to support the grid in times with low renewable power in-feed. Investigations of the influence of bidirectional charging in plug-in hybrid electric vehicles indicate that the current lithium-ion battery technology can meet both driving and electricity trading demands at the same time41. The increased number of cycles due to bidirectional charging does not decrease
the calendar battery lifetime because the total energy throughput capability is well beyond the needs for driving. The typical number of equivalent full cycles in a plug-in hybrid electric vehicle application is around 1500 without the V2G concept and around twice that with the use of the V2G concept, which is far below the cycle capability of current lithium-ion batteries (around 5000 equivalent full cycles if not cycled with 100 % depth-of-discharge cycles). The results obtained by Lunz et al.41 furthermore showed that the V2G concept offers the potential to increase the battery lifetime by using intelligent charging strategies compared with the reference case of uncontrolled charging (immediately recharging after arrival). That is because calendrical ageing can be decreased by lowering the SOC of lithium-ion batteries during standstill periods. The combination of cost reductions due to lifetime extension and decreased costs for electricity purchase add up to a saving potential of around 30 % compared with the reference case. The V2G concept would not make sense with, for example, lead–acid batteries in electric vehicles. The cycle lifetime of lead–acid batteries is significantly lower and therefore a battery would die earlier with an additional V2G concept. In this case the owner has to replace the battery earlier and so the full battery costs must be accounted for.
Summary Modern cars are built with a wide range of propulsion concepts. Lowest hybridisation starts with the stop– start function and regenerative braking (micro-hybrid vehicles). Boosting during acceleration (mild-hybrid vehicles) and driving short distances electrically (fullhybrid vehicles) are the next steps. Plug-in hybrid vehicles still have an ICE but are mainly charged by the grid. Electric vehicles are supported only by battery power without an ICE. The voltage levels are different for different vehicle concepts. Micro-hybrid vehicles have a voltage level of 14 V, like conventional vehicles. For higher hybridisation levels and electric vehicles, the voltage level can be as high as 600 V, depending on the battery pack design. A suitable battery technology including the battery pack design, battery management and charging strategy needs to be identified for each car concept. The lifetime of the battery, the related calendar lifetime and the capacity turnover are crucial for the costs. Each battery technology has its special characteristics that need to be considered when choosing a suitable battery for vehicles. Lead–acid batteries are over 100 years old and still the workhorse in the automotive industry. Their main problem is the low charge acceptance, which becomes more important when CO2 reduction must be achieved. However, at present and probably also in the future, micro-hybrid vehicles will still be built with lead–acid batteries for economic
Budde-Meiwes et al. reasons, possibly in combination with high-power storage such as supercapacitors or high-power lithium-ion batteries. Nickel–cadmium batteries have been displaced from the market because of their poisonous cadmium content. The first hybrids on the market were powered with nickel–metal hydride batteries because of their high energy density (approximately double the energy density of lead–acid batteries) and for safety reasons. Sodium–nickel chloride batteries need a temperature of approximately 300 °C, which has to be maintained at all times. Their self-discharge is therefore high and a daily recharge is necessary. Thus, the application of this battery is only economic if it is used at least once a day. It is expected that lithium-ion batteries will take over the market in a wide range as cost reductions for this expensive technology can be expected soon as a result of mass production. Highpower and high-energy cells are available in various housings. As there is no overcharge mechanism, battery monitoring is most important for this technology for security reasons. Supercapacitors constitute yet another technology and provide the highest power density. In automotive applications, they can be used as additional storage for high-power functions, e.g. for regenerative braking or boosting. Lithium–sulphur and metal–air batteries are under research and promise a higher energy density. In introducing more hybrids to the market, battery safety is essential. There are active as well as passive safety systems. Passive safety means, for example, an intrinsically safe battery chemistry or the safe construction of the battery pack. An example of an active safety system is single-cell voltage control. Battery management gives information about the battery to the vehicle control units that choose the driving strategy. Singlecell monitoring is specifically important for lithium-ion cells as they do not have an intrinsic overcharge mechanism. SOC, state-of-health, cell balancing, electric management and thermal management are all covered by the battery management system. Thermal management keeps the temperature within a battery pack as constant as possible. Today, cooling systems with air, water or a cryogenic agent are used. Charging strategies are highly related to the customers’ use of the vehicles. Overnight charging for several hours is most common and feasible through every house connection to the grid. For fast charging (10–20 min), standards for building the respective infrastructure are needed. Battery exchange concepts are not necessary for daily travel and need many spare batteries for long distances, e.g. during holiday times; the costs for their implementation will therefore be immense. V2G concepts, where the battery of the car is used not only for driving but also for grid stability, are under development. The battery will be used more often, but amortisation of the battery can be quicker. It can be concluded that each vehicle concept needs its own suitable battery. As each battery technology has its own characteristics, the technology has to be chosen
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