Electrical System Analysis Of Hybrid Electric Vehicle
Introduction
The trend to save the environment for future generations while at the same time maintain our current lifestyle has proved to be a constant struggle. One of the most discussed and debated issue of modern time is the increased use of petroleum based products for automobiles. Cars are considered consumer goods. Automobiles are run using an internal combustion (IC) engine that burn hydrocarbons to generate energy that helps move the vehicle. Currently, the two most commonly used hydrocarbons are gasoline and diesel.
The growing dependence on imported oil, along with a heightened concern about the environment, has led to our increased interest in electric cars as an alternative to traditional gas-powered automobiles. Battery systems for electric vehicles are improving, but with their limited range of travel, they are still not feasible for most people. In addition, we believe that the average person making the decision to purchase environmentally friendly vehicles would demand that those vehicles be comfortable, attractive, convenient, and affordable to purchase and maintain. Newly available automotive technology, known as hybrid electric vehicles (HEV), appears to meet these requirements. Hybrid power systems were conceived as a way to compensate for the shortfall in battery technology (Office of Transportation Technologies, HEV program). Hybrid electric vehicles recharge as it is driven, get approximately double the miles per gallon of gas than current vehicles (Toyota, technology) and can be refueled at any gas station. Each hybrid vehicle will produce thousands fewer pounds of pollutants than the vehicles currently on the road. According to Department of Energy estimates, a hybrid car driven 12,000 miles per year will cut carbon dioxide emissions by 4,700 pounds over its predecessor, says the National Resources Defense Council article on earth smart cars.
Hybrids will allow drivers to get between 20 and 30 miles per gallon more than standard automobiles. With this kind of savings, it won’t take long to make up the additional cost of the hybrid. Hybrids save on gas in a number of ways. All hybrids shut off the gas engine automatically when the car is stopped. The engine turns back on when the driver presses the gas pedal. The gas engine will also come on to start charging the batteries when they become low on power.
Typically, when a consumer buys products to help the environment the consumer pays more. Hybrids are a refreshing exception where the consumer actually saves money by doing something good for the environment. Not only does fuel efficiency save the drive money, burning less gasoline means that there is less pollution causing emissions released into the atmosphere. There is also a lower level of carbon dioxide, a major contributor to global warming, released into the atmosphere.
HEVs are growing leaders in transportation technology development. Hybrids have great potential for growth in improving the automotive industry, while also reducing serious resource consumption, reliance on foreign oil, air pollution, and traffic congestion. The hybrid’s complexity, and the fact that some of the best storage and conversion systems have yet to be fully developed, ignites varied opinions on hybrids’ ultimate impact in the marketplace.
In conclusion, hybrid cars are better than traditional gasoline powered vehicles, however they still have problems. Currently hybrid cars seem to be the best solution in combating the devastating global effects of exhaust emissions. With lower emissions and improved fuel economy, hybrids are a great way to travel. However, these lightweight cars are more vulnerable to traffic fatalities and still give off some emissions. They also accelerate at a slower pace than conventional vehicles. Hybrids have a short battery life, and their parts are expensive and not easily accessible, but hopefully as hybrids become more popular, this will change.
Overview
Hybrid Systems
The hybrid system is the wave of the future. In its simplest form, a hybrid system combines the best operating characteristics of an internal combustion engine and an electric motor. More sophisticated hybrid systems recover energy otherwise lost to heat in the breaks and use it to supplement the power of its fuel-burning engine. These sophisticated techniques allow the hybrid system to achieve superior fuel efficiency. On continuum that is hybrid technology, we typically break things down into full or strong, mild and micro hybrids which are also known as simply stop-start engine hybrid. A mild hybrid relies on the internal combustion engine to provide constant power for moving the vehicle but is incapable of propelling the vehicle alone. Full hybrids use a gasoline engine as the primary source of power like solar, fuel cells, battery etc; and electric motor provides additional power when needed. All these have been in chapter 1.
Battery Technology
Central to the discussion regarding the relative merits of the various hybrids is the big box that stores the energy to propel the electric motor—the battery. The battery is responsible for 25 – 75% of the increased weight, volume, and cost associated with the various hybrid configurations. Today most hybrid car batteries are Nickel metal hydride or Lithium-ion; both are regarded as more environment friendly then lead-based batteries which constitutes the bulk of car batteries today. The Lithium-ion battery has attracted attention due to its potential for use in hybrid electric vehicles. In addition its smaller size and lighter weight, Lithium-ion batteries deliver performance that helps to protect the environment with features such as improved charge efficiency without memory effect. The battery industry is currently working on the development of better performing and more sophisticated technology that costs less. In chapter 2, all the battery aspects are elaborated.
Electrical and Thermal management
The battery charger is a bidirectional ac-dc converter, recharge mode is ac to dc conversion and inverter mode is dc to ac conversion. To implement the plug-in function a single phase bidirectional ac-dc converter interfacing with the grid is essential. A dc-dc converter balances the voltage between the electric motor and the energy storage device, in a hybrid, boosting or reducing the voltage as necessary, which provides more of the energy under braking and under acceleration.
Regenerative braking means capturing the vehicles momentum (kinetic energy) and turning it into electricity that recharges the on board battery as the vehicle is slowing down or stopping. The super capacitor is an electro-chemical capacitor that has usually high energy density compared to common capacitors. These can quickly store large amount of electricity and discharge the electricity on demand to batteries.
The thermal management system delivers a battery pack an optimum average temperature with only small variations between the modules and within the pack. An ideal thermal management system should be able to maintain the desired uniform temperature in pack by rejecting heat in hot climates, adding heat in cold climates and providing ventilation if the battery generates potentially hazardous gases. The entire chapter 3 is about electrical and thermal management of the hybrid system.
Chapter 01
Hybrid System
1.1 Stop-Starts Hybrids
A stop-start hybrid is the simplest kind, but this minimal technology may become the most common within a few years.
Figure 01: Start-stop hybrid
It is composed simply of an energy storage device—like a battery—and a beefed-up starter-motor that can also act as a generator.
Stop-Startsystems are also called idle-stop—because it puts an end to burning fuel and emitting pollutants when a conventional car would be idling.
In practice, the car’s engine control unit shuts off the engine when the car slows down or comes to a stop. As soon as the driver puts in the clutch, moves the shift lever, or accelerates, the battery powers the starter motor, which quickly switches on the engine
Start-stop systems are the lowest-cost hybrid alternative, but if fitted to large numbers of cars, they could substantially reduce fuel consumption and air pollution from idling vehicles, especially in crowded city centers.
1.2 Mild Hybrid
A mild hybrid is a type of gasoline-electric hybrid that uses an internal combustion engine to power the vehicle at all times. An electric motor is incorporated only as a power booster of sorts, as a starter-generator, or both. While some mild hybrids use an electric drive motor to provide a gasoline engine with extra power, it cannot ever propel the vehicle on its own. Mild hybrids save fuel by shutting engine power off under most circumstances when the vehicle is stopped, braking, or coasting. The engine restarts seamlessly and efficiently. Electric accessories like the radio or GPS navigation continue to function with the engine off.
How They Are Different
All mild hybrids are less expensive than full hybrid systems because they require less sophisticated components and less battery power. Some, but not all, mild hybrids use regenerative braking to recharge the battery. Different mild hybrid configurations exist including Integrated Starter-Generator (ISG) and Belt Alternator Starter (BAS) systems.
The basic premise of the mild hybrid is same as the strong hybrid. An electric motor/generator operates in parallel with the internal combustion engine to provide additional drive torque as well as regenerative braking. The primary difference lies in the power and energy capacity of the electrical side of the system.
Figure 02: Next generation GM hybrid system
The GM system uses what is essentially a beefed up alternator and modified belt drive system to provide some additional drive torque to the engine as well as re-start it. During deceleration the mild hybrid system can also provide some regenerative braking capability.
Benefits of Mild Hybrids
Mainly to get some of the benefits of a hybrid system at a significantly lower cost and weight Mild hybrids typically have a much smaller battery than a strong hybrid and a smaller, weaker motor/generator. Mild hybrids do provide a modest improvement in fuel efficiency of 10 to 15 percent because they’re not burning gas when stopped. While a mild hybrid system can’t drive the vehicle on electricity alone, it still provides benefits. Like direct injection and turbo charging, it allows the automaker to downsize the base engine while maintaining the same performance level. The combination of the reduced peak output of the engine and eliminating engine idle can contribute fuel consumption savings of up to 15 percent in urban driving and 8-10 percent overall. Because they are much less costly than full hybrid systems, a greater number of drivers are likely to choose mild hybrids and realize better fuel efficiency than would otherwise be the case. This easy entry into the world of hybrids will serve to familiarize drivers with hybrid technology and potentially encourage drivers to choose a full hybrid for their next vehicle.
1.3 Full Hybrid
A hybrid vehicle classification in which the arrangement of the electric drive motor, the internal combustion engine and battery system allow the hybrid vehicle to be powered solely by the electric motor under certain operating conditions—generally low speed maneuvering and light cruising. When additional power is needed, the engine kicks in and both power plants work together to propel the vehicle.
In addition, full hybrids can use the electric motor as the sole source of propulsion for low-speed, low-acceleration driving, such as in stop-and-go traffic or for backing up. This electric-only driving mode can further increase fuel efficiency under some driving conditions.
Figure 03: full hybrid vehicle
Starting: When a full hybrid vehicle is initially started, the battery typically powers all accessories.
The gasoline engine only starts if the battery needs to be charged or the accessories require more power than available from the battery.
Figure 04: initially started full hybrid vehicle
The battery stores energy generated from the gasoline engine or during regenerative braking from the electric motor. Since the battery powers the vehicle at low speed, it is larger and holds much more energy than batteries used to start conventional vehicles.
The gasoline engine in a hybrid is much like those in conventional vehicles, except that it is usually much smaller and more efficient.
The electric motor powers the vehicle at low speed and assists the gasoline engine when additional power is required. It also otherwise converts wasted energy from braking into electricity and stores it in the battery.
The generator converts mechanical energy from the engine into electricity which can be used by the electric motor or stored in the battery. It is also used to start the gasoline engine instantly when needed.
The power split device is a gear box connecting the gasoline engine, generator and electric motor. It allows the engine and motor to power the car independently or in tandem and allows the gasoline engine to charge the batteries or provide power to the wheels as needed.
Low speed: For initial acceleration and slow-speed driving, as well as reverse, the electric motor uses electricity from the battery to power the vehicle.If the battery needs to be recharged, the generator starts the engine and converts energy from the engine into electricity, which is stored in the battery.
Figure 05: converting energy from the engine into electricity
Cruising part 1: At speeds above mid-range, both the engine and electric motor are used to propel the vehicle. The gasoline engine provides power to the drive-train directly and to the electric motor via the generator.
Figure 06: the engine and electric motor for propel the vehicle
Cruising Part 2: The generator can also convert energy from the engine into electricity and send it to the battery for storage.
Figure 07: converting energy from the engine into electricity
Passing Part 1: During heavy accelerating or when additional power is needed, the gasoline engine and electric motor are both used to propel the vehicle.
Figure 08: During heavy acceleration the gasoline engine and electric motor for propel the vehicle
Passing Part 2: Additional electricity from the battery may be used to power the electric motor.
Figure 09: battery for power the electric motor
Braking Part 1: Regenerative braking converts otherwise wasted energy from braking into electricity and stores it in the battery.In regenerative braking, the electric motor is reversed so that, instead of using electricity to turn the wheels, the rotating wheels turn the motor and create electricity. Using energy from the wheels to turn the motor slows the vehicle down.
Figure 10: reversing electric motor during the regenerative braking
Braking Part 2: If additional stopping power is needed, conventional friction brakes (e.g., disc brakes) are also applied automatically.
Figure 11: Need of additional stopping power
Stopped: When the vehicle is stopped, such as at a red light, the gasoline engine and electric motor shut off automatically so that energy is not wasted in idling. All other systems, including the electric air conditioning, continue to run.
Figure 12: shut off gasoline engine and electric motor
1.3.1 Power Sources
(a) Solar
Renewable energy sources are being used all over the world. For example, wind energy, solar power, Hybrid cars and using methane gas for stove fuel. These are all excellent examples of renewable energy as they are all affordable and convenient as well as being efficient. Solar power is an excellent energy resource. Cars can now be solely power by solar panels, creating huge possibilities. If solar energy was used worldwide, along with other renewable sources, the world would be cleaner and more pleasurable to live in, knowing that the greenhouse gases are under control.
Solar Powered Cars or Solar Cars are first step towards developing nature friendly vehicles with the help of solar power. Solar Cars also gain energy from Sun. Hundreds of photo voltaic cells are used by these cars to convert sunlight into electricity. Each cell is capable of producing about one half volt of Electricity
Electrical systems in Solar Cars are designed so as to allow for variations in sunlight. The motor and battery of car is charged by energy from sun. This charged battery is used when Sun is hidden by a dense cloud. It takes lot of efforts and meticulous plan to design such Solar powered vehicles because if all energy is diverted towards driving then nothing will be kept in reserve and car will come to halt once when it is taken out on cloudy day. Engine of solar car slows down if too much energy is diverted to the battery. So Solar Powered Cars are designed so as to make it more efficient to run at top speed.
Solar powered vehicles deliver high performance due to their extreme lightness of weight and excellent aerodynamics. But that is also one of the big drawbacks of these vehicles because any vehicle which is designed to ferry passenger in their routine life is heavier and less aerodynamic in nature so that they can achieve more speeds. So it would be better not to design cars which are purely powered by solar energy. The average speed achieved by Solar Powered Cars is 37 miles per hour.
(b) Fuel cells
A fuel cell is an electricity generation system that does not convert the chemical energy of fuel into heat by combustion, but electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. Such a fuel cell can be applied to the supply of electric power for small-sized electrical/electronic devices such as portable devices, as well as to the supply of electric power for industry, homes, and vehicles.
At present, the most preferred fuel cell for a vehicle is a polymer electrolyte membrane fuel cell (PEMFC), also called a proton exchange membrane fuel cell, that preferably comprises: a membrane electrode assembly (MEA) including a polymer electrolyte membrane (PEM) for transporting hydrogen ions and an electrode catalyst layer, in which an electrochemical reaction takes place, disposed on both sides of the PEM; a gas diffusion layer (GDL) for uniformly diffusing reactant gases and transmitting generated electricity; a gasket and a sealing member for maintaining air tightness of the reactant gases and coolant and providing an appropriate bonding pressure; and a bipolar plate for transferring the reactant gases and coolant.
In the fuel cell having the above-described configuration, hydrogen as a preferred fuel and oxygen (air) as a preferred oxidizing agent are supplied to an anode and a cathode through flow fields of the bipolar plate, respectively. The hydrogen is suitably supplied to the anode (also called a “fuel electrode”, “hydrogen electrode”, and “oxidation electrode”) and the oxygen (air) is suitably supplied to the cathode (also called an “air electrode”, “oxygen electrode”, and “reduction electrode”). The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H + ) and electrons (e ? ) by catalyst of the electrode catalyst layer preferably provided on both sides of the electrolyte membrane. At this time, only the hydrogen ions are selectively transmitted to the cathode through the electrolyte membrane, which is preferably a cation exchange membrane and, at the same time, the electrons are transmitted to the anode through the GDL and the bipolar plate, which are conductors. At the cathode, the hydrogen ions supplied through the electrolyte membrane and the electrons transmitted through the bipolar plate meet the oxygen in the air supplied to the cathode by an air supplier and cause a reaction that produces water. Due to the movement of hydrogen ions occurring at this time, the flow of electrons through an external conducting wire occurs, and thus a current is suitably generated.
If the fuel cell is used as the only power source of an electric vehicle, the fuel cell powers all loads of the vehicle, which results in performance deterioration during operation where the efficiency of the fuel cell is low. Moreover, during high speed operation where a high voltage is required, a sufficient voltage required by a drive motor is not supplied due to a rapid decrease in output voltage, thus decreasing acceleration performance. Furthermore, if a sudden load is applied to the vehicle, the output voltage of the fuel cell suddenly drops and sufficient power is not supplied to the drive motor, thus decreasing vehicle performance and accordingly, a sudden change in load imposes a heavy burden on the fuel cell since electricity is generated by an electrochemical reaction. In addition, since the fuel cell preferably has unidirectional output characteristics, it is impossible to recover energy from the drive motor during braking of the vehicle, thus decreasing the efficiency of the vehicle system.
Accordingly, a fuel cell hybrid vehicle has been developed. Exemplary fuel cell hybrid vehicles include large vehicles, such as a bus, as well as small vehicles that are preferably equipped with storage means such as a high voltage battery or a super capacitor as an auxiliary power source for suitably providing the power required for driving the motor in addition to the fuel cell as a main power source. At present, a fuel cell-storage means hybrid vehicle that does not employ a power converter has been studied, and the fuel cell-storage means hybrid vehicle has high fuel efficiency (e.g. high regenerative braking, high efficiency of super capacitor, and without the use of the power converter), an increase in durability of the fuel cell, suitably high reliability control, and the like.
In the hybrid vehicle in which the fuel cell and the storage means are preferably directly connected, the fuel cell continuously outputs power at a suitably constant level during driving. If electric power is suitably sufficient, the storage means is charged with surplus power, whereas, if the electric power is insufficient, the storage means supplies the insufficient power to drive the vehicle.
The driving mode of the hybrid vehicle including the fuel cell as the main power source and the super capacitor (or a high voltage battery which is a secondary battery) as the auxiliary power source preferably includes an electric vehicle (EV) mode in which the motor is driven only by the power of the fuel cell, a hybrid electric vehicle (HEV) mode in which the motor is driven by the fuel cell and the super capacitor at the same time, and a regenerative braking (RB) mode in which the super capacitor is charged.
However, in fuel cell-super capacitor hybrid vehicle the super capacitor is automatically charged by the fuel cell, which thus restricts the regenerative braking. Accordingly, stopping the operation of the fuel cell during low power operation and during regenerative braking will overcome this restriction. Moreover, it is possible to improve the fuel efficiency by restricting the use of the fuel cell during low power operation where the efficiency of the fuel cell is low.
According to the present invention, the air and hydrogen supply is suitably cut off in the low efficiency region of the fuel cell, and the fuel cell voltage drops by consuming residual oxygen and hydrogen, thus stopping the operation of the fuel cell (EV mode or regenerative braking mode). If the conditions for restarting the fuel cell are suitably satisfied, in which the voltage of the storage means (super capacitor or battery) is below a predetermined reference voltage or the load required by the vehicle is above a reference load, the air and hydrogen supply is restarted to restart the fuel cell (HEV mode). As a result, the present invention has the following effects.
1. Since the operation of the fuel cell BOP components (especially, the air blower) is preferably stopped during low power operation where the efficiency of the fuel cell is low, it is possible to suitably improve the fuel efficiency and the efficiency of the fuel cell system.
2. Since the automatic charge from the fuel cell to the super capacitor is prevented and thereby an increase in the voltage of the super capacitor is suitably prevented, the amount of regenerative braking is increased, thus improving the fuel efficiency.
3. It is possible to suitably improve the durability of the fuel cell by reducing the open circuit voltage (OCV).
4. It is possible to prevent the deterioration of the fuel cell stack, which occurs when oxygen is introduced into the anode, without a loss of improvement in fuel efficiency, and further improve the durability of the fuel cell stack by the control process of the non-power generation region, in which, if the air supply to the fuel cell stack is suitably cut off as the fuel cell stop mode is started, preferably, the hydrogen supply is not immediately cut off and, if the fuel cell voltage is eliminated after maintaining the anode pressure at an optimal level through the hydrogen supply, the load device for voltage elimination is driven and the hydrogen supply is cut off.
5. Since the voltage unexpectedly generated in the fuel cell stack can be immediately or substantially immediately eliminated by driving the load device for voltage elimination along with the hydrogen supply cut off, it is possible to prevent the deterioration of the fuel cell stack and improve the durability of the fuel cell stack.
(c) Battery
A hybrid car basically uses the hybrid car battery for its electrical power, which can make the car run. These hybrid batteries are used in the hybrid car as the second power source. The hybrid battery was previously used in electric cars where there was no other power source except the hybrid batteries. Unfortunately, those hybrids weren’t very practical due to the short distance they could go on battery power only.
Hybrid Cars use a rechargeable energy storage system to supplement fossil fuel energy for vehicle propulsion. Hybrid engines are smaller and more efficient than traditional fuel engines. Some hybrid vehicles use regenerative braking to generate electricity while travelling. The term “Hybrid Vehicle” can also refer to a vehicle engine that uses a combination of different fuels such as petroleum and ethanol.
The most popular hybrid batteries that are being developed are Li-ion, more formally known as Lithium ion batteries. This hybrid battery will soon be used in a lot of hybrid cars as more and more car manufacturers are using the Li-ion battery as a way to power up these hybrid vehicles. The Lithium ion batteries are seen as the power source for the next generation cars and other hybrid vehicles. These Lithium ion batteries cost a lot because they are not only rechargeable but they also last for a long period of time. With the help of these hybrid batteries, hybrid cars will be able to drive well over 100,000 miles before replacement.
These recycling batteries are very important for any hybrid or electric car. The main reason for this is the fact that the electric or the hybrid car should be able to run several miles on electricity after which it needs to be recharged again to ensure again that the car is running. If there is no option for recycling batteries, then it would be a terrible loss considering the fact that the batteries need to be changed again and again – every time the power in the battery is drained. Thus, rechargeable batteries are very important to ensure that the battery can be charged again and again and used for a long period of time.
Once a customer can buy a plug-in hybrid electric car, pre-equipped with a hybrid car battery, it will likely be recommended that they charge the hybrid battery cell/pack to its fullest before driving it. The Li-ion battery should be fully charged to ensure that it gives power to its full capacity. Also, the batteries will be built to last for the whole life of the car. So, most of the time, it won’t be necessary to change the hybrid car battery at all, but sometimes the Lithium ion batteries will become damaged due to some external factors and might need to be changed. This situation is very rare and most of the time, the hybrid batteries will not require a change at all. When the batteries need to be changed, the replacement costs are a little high. The life time of a hybrid car battery as said earlier will be up to the life of the car and only in some cases, the life of the battery will reduce.
In such cases when the life of the batteries is over, then it is necessary to take care of the disposal of the hybrid batteries. The disposal should be done properly to ensure that the batteries do not cause any leakage and do not pollute the environment. The hybrid cars are being manufactured more and more each and every day because of the demands for these cars are increasing. Thus, with the help of these hybrid batteries, one is able to save on fuel and one also contributes in reducing pollution.
Chapter: 02
Battery of Hybrid Vehicles
2.1 Battery Technology
Of all recent futuristic technology, the hybrid battery takes the cake as far as most complex. Hybrid cars rely extensively on their batteries, more so than other types of vehicles because they provide power for one of the car’s two main engines. With that being said, there are many types of batteries out there and still more are being developed every day.
Hybrid cars are powered by two types of technologies working together at once: the electric motor and the internal combustion (gasoline) engine. These engines work together to make the car extremely fuel efficient, often turning off the combustion engine when not in use such as at a red light and using regenerative brake technology to recharge the battery for the electric motor. Because the usage of the battery itself is so extensive, it becomes absolutely necessary to charge the battery at all times when not in use. The type of batteries used in a hybrid car must also have a higher capacity than regular batteries.
Like all batteries, hybrid batteries have two electrodes (which collect or emit an electric charge) that sit in an ion-rich solution called the electrolyte. (An ion, by the way, is an atom or group of atoms with an electrical charge.)
The electrodes are typically very close, so a polymer film, called a separator, prevents them from touching, which would create a short circuit. An on-off switch in whatever device is powered by the battery- phone or laptop, bridges the cell’s electrodes to generate power. That’s when the electrochemical reaction begins.
Ionized elements in one electrode are in a chemical state where they are easily attracted to combine with other molecules, emitting electrons (energy) in the process. Those elements are tugged through the electrolyte and the separator toward the opposing electrode. The ions of the negative electrode (anode) give up electrons; the positive ions coming toward the anode accept them. The electrons released during this process travel through the external circuit (e.g. phone), producing a flow of charge in the opposite direction to the flow of ions. During recharge, current is forced into the cell, reversing the process.
2.1.1 Early Revolution
Nickel-Cadmium Batteries
Fortunately for us and for the environment, hybrid cars do not use the typically problematic Nickel-Cadmium batteries, which you most commonly see as rechargeable batteries in small devices such as cell phones, digital cameras and remote-controlled toys. These batteries contain lead, which is highly toxic, harmful to the environment, and difficult to recycle. They also have a small energy capacity, which makes them inappropriate for the heavy-duty usage needed to run a hybrid car. These types of batteries can be found under the hood of almost every conventional gasoline-run vehicle, the image of which comes to mind when picturing what’s under the hood of a typical car.
NiMH Batteries (Nickel Metal-Hydride)
Currently, this type of battery is being used in hybrid cars. Like its Nickel-Cadmium counterpart, this battery uses the same chemical nickel oxyhydroxide (NiOH) to help it hold a charge. But unlike the Nickel-Cadmium batteries, which uses cadmium for the negative electrode, this type of battery uses a Nickel alloy called Nickel Metal-Hydride for the negative electrode. Due to the absence of Cadmium, which is considered environmentally toxic, this type of battery is more “green.” It is also safer to use and has a higher capacity than traditional Nickel-Cadmium batteries. The downside of this battery is that it still does not have the high-capacity to run a hybrid’s sophisticated electric motor without being charged as much as possible. It is also more expensive than most batteries due to the cost of Nickel. This is currently what keeps the cost of a hybrid car at a premium.
Lithium Batteries
Next on the list of up-and-coming hybrid vehicle technology, the Lithium battery is currently powering small handheld devices such as laptops and cell phones. Lithium batteries have a higher capacity than other types of batteries, and they are also made more cheaply. Lithium batteries could enable hybrid cars to go much longer distances without using a single drop of gasoline, for distances anywhere from 50-100 miles. Although most hybrids are not plug-in models, this would be required with the current technology if these types of batteries were to be used. Another problem with these batteries is that they use cobalt in their formulas, which tends to explode. Automakers are scrambling to find an alternative to cobalt which would provide the same amount of power.
Lithium Ion Battery – For Next Generation Hybrids Cars
Lithium ion (or Li-ion) batteries are important because they have a higher energy density the amount of energy they hold by weight, or by volume than any other type. The rule of thumb is that Li-ion cells hold roughly twice as much energy per pound as do the previous generation of advanced batteries, nickel-metal-hydride (NiMH) which are used in all current hybrids including the Toyota Prius. NiMH, in turn, holds about twice the energy per pound of the conventional lead-acid (PbA) 12-Volt battery that powers your car’s starter motor. It’s Li-ion’s ability to carry so much energy that makes electric cars possible.
2.1.2 Cathode Contenders
Cobalt Dioxide
Cobalt Dioxide is the most popular choice today for small cells (those in your mobile phone or laptop). It’s been on the market for 15 years, so it’s proven and its costs are known, though like nickel, cobalt is pricey. Cobalt is more reactive than nickel or manganese, meaning it offers high electrical potential when paired with graphite anodes, giving higher voltage. It has the highest energy density but when fully charged, it is the most prone to oxidation (fire) caused by internal shorts. This can lead to thermal runaway, where one cell causes its neighbors to combust, igniting the whole pack almost instantly. Also, the internal impedance of a cobalt cell, the extent to which it “pushes back” against an alternating current, increases not just with use but with time as well. That means an unused five year old cobalt cell holds less energy than a brand-new one.
Cobalt dioxide cells are manufactured by dozens of Japanese, South Korean, and Chinese companies, but only Tesla Motors uses them in an electric car. Their pack uses sensors, cell isolation, and liquid cooling to ensure that any energy released if a cell shorts out can’t ignite any of its neighbors.
Nickel-cobalt-manganese (NCM)
Nickel-cobalt-manganese (NCM) is somewhat easier to make. Manganese is cheaper than cobalt, but it dissolves slightly in electrolytes which gives it a shorter life. Substituting nickel and manganese for some of the cobalt lets manufacturers tune the cell either for higher power (voltage) or for greater energy density, though not both at the same time. NCM remains susceptible to thermal runaway, though less so than cobalt dioxide. Its long-term durability is still unclear, and nickel and manganese are both still expensive now. Manufacturers include Hitachi, Panasonic, and Sanyo.
Nickel-cobalt-aluminum (NCA)
Nickel-cobalt-aluminum (NCA) is similar to NCM, with lower-cost aluminum replacing the manganese. Companies that make NCA cells include Toyota and Johnson Controls Saft, a joint venture between a Milwaukee automotive supplier and a French battery firm.
Oxide spinel (MnO Manganese)
Manganese oxide spinel (MnO) offers higher power at a lower cost than cobalt, because its three-dimensional crystalline structure provides more surface area, permitting better ion flow between electrodes. But the drawback is a much lower energy density. GS Yuasa, LG Chem, NEC-Lamilion Energy, and Samsung offer cells with such cathodes; LG Chem is one of two companies competing to have its cells used in the Chevrolet Volt.
Iron phosphate (FePo)
Iron phosphate (FePo) might be the most promising new cathode, thanks to its stability and safety. The compound is inexpensive, and because the bonds between the iron, phosphate, and oxygen atoms are far stronger than those between cobalt and oxygen atoms, the oxygen is much harder to detach when overcharged. So if it fails, it can do so without overheating. Unfortunately, iron phosphate cells work at a lower voltage than cobalt, so more of them must be chained together to provide enough power to turn a motor. A123 Systems which is competing for the Volt contract as well uses nanostructures in their FePo cathodes, which it says produces better power and longer life. Other manufacturers include Gaia and Valence Technology.
2.1.3 Recently Used Batteries
Toyota Prius Hybrid Battery
The battery pack of the second generation Toyota Prius consists of 28 Panasonic prismatic nickel metal hydride modules each containing six 1.2 volt cells connected in series to produce a nominal voltage of 201.6 volts. The total number of cells is 168, compared with 228 cells packaged in 38 modules in the first generation Prius. The pack is positioned behind the back seat.
The weight of the complete battery pack is 53.3 kg. The discharge power capability of the Prius pack is about 20 kW at 50 percent state-of-charge. The power capability increases with higher temperatures and decreases at lower temperatures. The Prius has a computer that’s solely dedicated to keeping the Prius battery at the optimum temperature and optimum charge level. The Prius supplies conditioned air from the cabin as thermal management for cooling the batteries. The air is drawn by a 12-volt blower installed above the driver’s side rear tire well.
Highlander Hybrid Battery Toyota
The nickel metal hydride battery used in Highlander Hybrid and the Lexus RX 400h is packaged in a newly developed metal battery casing. The 240 cells can deliver high voltage of 288 volts but the motor-generators units can operate on variable voltage anywhere from 280 volts to 650 volts. The battery pack supplies 288 volts, but the boost converter, a part of the inverter above the transaxle, changes this to 500 volts. This battery pack provides 40 percent more power than the Prius battery, despite being 18 percent smaller.
Each of the modules has its own monitoring and cooling control system. The cooling performance reduces efficiency losses due to excessive heat, ensuring that the battery can supply required electric power to the motors at all times. The battery-monitoring unit manages discharge and recharging by the generator and motors to keep the charge level constant while the car is running. The battery pack is stowed under the rear seats.
Ford Escape Hybrid Battery
The Ford Escape Hybrid’s battery pack, made by Sanyo, consists of 250 individual nickel metal hydride cells. As with other hybrid battery packs, the cells are similar in shape to a size D flashlight battery. Each individual battery cell, contained in a stainless steel case, is 1.3 volts. The cells are welded and wrapped together in groups of five to form a module. There are 50 modules in the battery pack. The total voltage of the battery pack is 330 volts.
Honda Insight Battery
The Honda Insight’s battery pack, made up of 120 Panasonic 1.2-volt nickel metal hydride D cells is capable of 100A discharge, and 50A charge rates. The system limits the usable capacity to 4ah to extend battery life. Total battery pack output is 144 volts. The batteries are located under the cargo compartment floor, along with the Honda Integrated Motor Assist’s power control unit. Honda used technology developed for its EV Plus electric car for the original development of the Insight’s battery system.
2.2 Battery Capacity
The valve regulated lead acid-battery (VRLA) is a maintenance-free lead acid battery operating on the principle known as “sealed, recombination,” wherein all the electrolyte is stored in absorptive glass mats (AGM) separators. The battery must remain sealed for its entire operating life and, to achieve maximum cycle life, must be properly recharged to prevent any excessive overcharge. Excessive overcharge results in excessive gas pressure build-up inside the battery, which is relieved by the opening of the pressure relief valve (typically set at 1.5 psi ± 0.5 psi). Every time the valve opens, water vapor is lost, which in turn reduces battery life.
The battery has been developed from extruded lead onto glass-fiber filaments that are woven into grids (mats) for use as electrode plates. This process provides the desirable crystal structure of lead oxide (PbO2) active material. The battery must be maintained, however, under optimal driving conditions.
The USABC has outlined the performance requirements for VRLA batteries for the near term and the next few years, especially for use in electric vehicle applications. VRLA battery provides up to 95 Whr/L of energy, while the requirements are to increase the energy density to 135 Whr/L over the next few years. This increase in the energy density means that there has to be a significant increase in the battery capacity.
The useful available capacity of the battery (in Ahr) is dependent on the discharge current. This relationship can be expressed in the form
* t = K
where I is the discharge current in A, t (0.1 < t < 3) is the duration of the discharge in hours and n and K are constants for a particular battery type.
Figure 13: The estimated Peukert plot at 80°F for an 80Ahr battery
For example, an 80 Ahr VRLA battery, Peukert constants n may vary between 1.123 to 1.33 and K may vary from 138 to 300 respectively. The graph in Figure 13 is a Peukert plot at room temperature, 80°F.
Temperature Dependence of Battery Capacity
The useful Ahr capacity available from the VRLA is dependent on battery temperature and may be represented by the following equation
Ct = C77 * (1 – 0.065(77 – t))
Where t is the temperature in °F, Ct is the battery capacity at t °F and C77 is the capacity of the battery at 77°F (room temperature). For example, C3 capacity at 32°F, for an 80 Ahr VRLA battery is expressed as C3 (32°F).
C3 (32°F) = 80 * (1 – 0.0065(77 – 32)) = 56.6Ah
Similarly C0.1 at 80°F for an 80 Ahr VRLA battery is 36.3Ahr. Thus C0.1 at 120°F is 45.74Ahr.
Thus under a constant current discharge and variation of temperature the battery pack capacity changes the performance of the electric vehicle (EV). This is observed as a variation of the driving distance before an EV recharge.
As illustrated in Figure 14, the graph is the estimated VRLA battery capacity with respect to the battery pack temperature. A 80Ahr VRLA battery above room temperature, 77°F, exhibits a larger than rated battery capacity. This increase is larger at higher temperatures. A fully charged battery pack when discharged at 100°F can deliver approximately two times the rated battery pack current than a battery pack at room temperature under similar discharge conditions.
Figure 14: Variation of estimated VRLA battery capacity with temperature
Hybrid car battery life
Manufacturers claim the battery life to be of 8-10 years or 80,000 to 100,000 miles, but can even pass this limit. If used with proper maintenance, hybrid car battery life expectancy can go well beyond what the manufacturers claim. Some hybrid car owners say the average car battery life extended from around 150,000 to 200,000. As the battery is used over time, their capacity to hold the charge may deteriorate. Let us get to know more about the cost of hybrid car batteries.
Hybrid Car Battery Cost
A negative thing about hybrid car batteries is their cost which may even go up to a few thousand dollars. Companies providing extended battery warranties are a good alternative to lower the cost of hybrid car battery replacement. The cost of hybrid car batteries may range from $3000 to $6000. If you are thinking of lessening the hybrid car battery replacement cost, a good option is to go in for batteries from out of-service automobiles. In this way you can purchase quality batteries at a much discounted price.
2.3 Plug-in Hybrid Electric Vehicle
A plug-in hybrid electric vehicle (PHEV or PHV), also known as a plug-in hybrid, is a hybrid vehicle with rechargeable batteries that can be restored to full charge by connecting a plug to an external electric power source (usually simply a normal electric wall socket). A PHEV shares the characteristics of both a conventional hybrid electric vehicle, having an electric motor and an internal combustion engine; and of an all-electric vehicle, also having a plug to connect to the electrical grid. Most PHEVs on the road today are passenger cars, but there are also PHEV versions of commercial vehicles and vans, utility trucks, buses, trains, motorcycles, scooters, and military vehicles.
The cost for electricity to power plug-in hybrids for all-electric operation has been estimated at less than one quarter of the cost of gasoline. Compared to conventional vehicles, PHEVs reduce air pollution locally and dependence on petroleum. They may reduce greenhouse gas emissions that contribute to global warming, compared with conventional vehicles. PHEVs also eliminate the problem of “range anxiety” associated to all-electric vehicles, because the combustion engine works as a backup when the batteries are depleted. Plug-in hybrids use no fossil fuel during their all-electric range and produce lower greenhouse gas emissions if their batteries are charged from renewable electricity. Other benefits include improved national energy security, fewer fill-ups at the filling station, the convenience of home recharging, opportunities to provide emergency backup power in the home, and vehicle-to-grid (V2G) applications.
PHEVs are based on the same three basic power train architectures as conventional electric hybrids:
Series hybrids use an internal combustion engine (ICE) to turn a generator, which in turn supplies current to an electric motor, which then rotates the vehicle’s drive wheels. A battery or super capacitor pack, or a combination of the two, can be used to store excess charge. With an appropriate balance of components this type can operate over a substantial distance with its full range of power without engaging the ICE. As is the case for other architectures, series hybrids can operate without recharging as long as there is liquid fuel in the tank.
Parallel hybrids can simultaneously transmit power to their drive wheels from two distinct sources for example, an internal combustion engine and a battery powered electric drive. Although most parallel hybrids incorporate an electric motor between the vehicle’s engine and transmission, a parallel hybrid can also use its engine to drive one of the vehicle’s axles, while its electric motor drives the other axle and/or agenerator used for recharging the batteries. (This type is called a road-coupled hybrid). The Parallel hybrids can be programmed to use the electric motor to substitute for the ICE at lower power demands as well as to substantially increase the power available to a smaller ICE, both of which substantially increase fuel economy compared to a simple ICE vehicle.
<href=”#Power-split_or_series-parallel_hybrid” title=”Hybrid vehicle drivetrain”>Series-parallel hybrids
<href=”#Power-split_or_series-parallel_hybrid” title=”Hybrid vehicle drivetrain”>Series-parallel hybrids have the flexibility to operate in either series or parallel mode. Hybrid power trains currently used by Ford, Lexus, Nissan, and Toyota, which some refer to as “series-parallel with power-split,” can operate in both series and parallel mode at the same time.
General Motors announced plans to release a vehicle that will be able to go long distances in electric-only mode. It thus became the first U.S. company to commit to producing a so-called plug-in hybrid design one that has batteries so capacious that they can be recharged not only by the engine but also from wall current in the garage. It represents the next way station along the path to all-electric vehicles.
2.4 Safety Battery System
Although the lithium-ion cells in laptops and mobile phones pack twice as much energy per pound as the next-best kind, they haven’t found their way into hybrid cars because they’re worryingly prone to fires. A123, a Watertown, Mass. startup, believes it has solved the problem with a lithium-ion design using a special formulation for the battery’s cathode, or positive plate.
The safety problem that has stood in the way of lithium-ion batteries became notorious last year when laptops using such batteries were shown spouting flames in video clips that circulated on the Internet. Millions of lithium-ion batteries had to be recalled, even though no one was hurt. If masses of such batteries had been crammed into automobiles, however, the fires would likely have resulted in the deaths of the passengers.
The fires seem to begin when a small manufacturing defect, perhaps compounded by overcharging, causes oxygen to separate from the compound making up the cathode, a heat-releasing process known as oxidation. As the cell overheats, it can prime oxidation in neighboring cells, a process known as thermal runaway.
A123 overcomes the problem by making its cathodes out of iron phosphate, which bonds to the oxygen far more powerfully than does the cobalt dioxide found in conventional lithium-ion batteries. Its cells are thus far less subject to oxidation, and thus less prone to thermal runaway.
Disposal
Rechargeable batteries contain heavy metals such as nickel that can cause neurological or kidney damage with enough exposure. However, they are a preferable alternative to lead-acid batteries present in combustion cars, which are one of the most harmful consumer products on the market. Rechargeable batteries should be taken to a recycling facility at the end of their lives. Many car manufacturers will take batteries back. Toyota puts a phone number on each battery.
Heat Discharge
Lithium ion batteries are lighter and store more electricity than nickel metal hydride, but in laptops and small power tools they discharge heat and are a potential fire danger. In landfills they have even been known to explode from hot temperatures. However, Jeff Boyd, CEO of Miles Automotive Group, guarantees that its lithium ion batteries do not generate heat because of a change in chemical composition.
Handling
With an electric vehicle, avoid leaving connections open and exposed so that they do not come into contact with people or tools. According to the website EV Convert, use a short wrench when working with EV batteries, or a tool that can’t accidentally bridge from one terminal to the next, and possibly insulate the handle.
Fail Safe
The circuits in electric vehicles should be designed to disconnect the high voltage line from the battery component in case a failure or some kind of problem occurs. A heavy-duty relay connected to the ignition key can disconnect the voltage line when the key is off, and another relay attached to the accelerator pot in the engine compartment can also disconnect the voltage when you let go of the gas pedal.
Chapter: 03
System Requirements
3.1 Electrical Management
The major components like converters, regenerative braking system and super capacitor are discussed under electrical management of the hybrid system. The converters are combined into one unit to manage the power and recharging circuits in hybrids electric vehicles. Regenerative braking takes energy normally wasted during braking and turns it into usable energy and super capacitors are used to boost up the acceleration.
3.1.1 AC to DC Converters
A plug-in hybrid electrical vehicle (PHEV) is an electric-drive hybrid vehicle with an all electric operating range. It combines batteries and internal combustion engines in an efficient manner. The PHEV provides a fuel tank and combustion engine to be used when an extended driving range is needed. A battery charger is essential for PHEV.
The battery charger should have two main functions: one is charging the battery to a proper state of charge (SOC). This operation mode is called recharge mode. The other operation mode is called inverter mode, which means the battery energy can be inverted and flows back to the grid or for possibly supplying ac electricity locally. Therefore, the battery charger is a bidirectional ac-dc converter, recharge mode is ac to dc conversion and inverter mode is dc to ac conversion. There are two types of battery chargers: off-board and on-board. An off-board charger is separated from the PHEV and can allow for higher weight and volume at a lower cost to PHEV efficiency. In the PHEVs product development, the cost, volume and weight of the power electronics and electric machine (PEEM) system are important. The bidirectional ac-dc converter belongs to this PEEM system. On-board charger is designed to combine with the whole PHEV system, which can benefit from the system optimization consideration; this can lead to higher performance. The