EVS 20 was a cornucopia of concepts for new generation transportation power, not the least of which was the myriad of battery sessions covering multiple chemistries for many applications.
In this installment, battery sessions are summarized, by paper, to provide the reader with the state of technical thinking and capabilities of battery power in transportation. Batteries and ultracaps for transportation are very important to the quest for cleaner air, greater efficiency and energy independence. In the case of the pure EV, the battery is the only power source, but in hybrids, whether internal combustion powered or fuel cell powered, the battery works in a synergistic mode to absorb braking energy, add performance and provide for the most efficient operating region of the primary power source. In this meeting we see an emergence of serious applications of ultracaps in hybrids.
The following thumbnail summaries of presented papers are organized by chemistries. Further details on presentations referred to here are available from The Electric Drive Transportation Assn. at www.electricdrive.org or from the organizations which provided the papers.
Double Layer Capacitors (Ultracaps or Supercaps)
“Important Issues of a Capacitor Storage System,” Okamura Laboratory, Inc. and Power Systems Co.
Ultra capacitors for hybrid transportation, developed by Okamura, have been supplied by Power Systems to Honda for the FCX and to Nissan for their diesel hybrid truck. At the time the capacitors were delivered, the energy density for the FCX was 5.2 Wh/kg and 6.3 Wh/kg for the Nissan truck. Today, ultracaps are being produced with 10 Wh/kg energy density and 5.8 kW/kg of power density.
Current concerns or considerations for the choice of ultracaps in hybrid vehicles include serial charge control, electrolyte safety and design trade-offs of internal resistance and energy density.
Ultracaps require many cells in series to develop the useful bus Voltages for vehicles. Ideally, the charging of multiple serially connected cells would only require a single bus Voltage charger, but on reality, differences in cell characteristics beyond the control of manufacturers can cause cell Voltage imbalance and lead to degraded cell performance or failure. To use bypassing circuits, which handle the heavy charging currents, would add unacceptable costs. Applying switching converters to individual cells would again add bulk, cost and reduce reliability.
The problem of electrolyte safety has led manufacturers away from the more dangerous acetonitrile (AN) which has a flash temperature of 2 0C. It also generates cyanide gas when burning, and thereby is a poor choice for use in an ultracap which would be placed in public transportation. The comment was made that although AN has these safety deficits, it is safer than Lithium-ion batteries, which have failed to produce the catastrophes envisioned in the last decade of the twentieth century, although many safety recalls have been required.
The safer, propylene carbonate (PC) has about one half the capacity of AN and three times the internal resistance of AN. PC also has reduced performance at low temperatures. For a temperature of 20 0C, the internal resistance of an ultracap increases 12 times in cooling to -35 0C. In a pack of ultracap cells, which were cooled, the Voltage would increase inversely proportional to the capacitance because charge would be conserved. Such an increase would not be a desirable feature of the pack.
The present solution of using the PC electrolyte with higher internal resistance, it is important to notice that the mathematics of both charge and discharge provide for the internal resistance as a limiting factor of the power, but the power is also dependent on the charge or discharge time in the relationship RC/t. Cells charged for the proper amount of time, based upon the design with an appropriate current source, can achieve good performance. In summary, to achieve desired performance, accept the internal resistance and increase the energy density of the cell for the application. Increasing energy density twofold will provide the same results as decreasing the internal resistance by half.
Long term results of using the ultracaps by Honda and Nissan will be anxiously awaited by others considering ultracaps, especially as their capacity continues to increase.
“Application Studies of Electric Double- Layer Capacitor System For Fuel Cell Vehicle,” Honda
Honda’s first commercialized fuel cell powered vehicle, the FCX, is a hybrid which uses an 80 cell, 216 Volt, ultra capacitor instead of a battery as Toyota does on its Prius. This approach is not new with Honda. They had configured the Insight hybrid with an ultracap until the augmentation power from the ultracap was deemed insufficient, requiring a return to battery power.
The 28 kW (max), Ultracap chosen for the FCX has a specific power density of over 1500 W/kg and a specific energy density of 3.9 Wh/kg. The Ultracap has 30 % more output power and twice the power density of Nickel-metal hydride batteries commonly used in hybrid vehicles.
The addition of an ultracap was required principally to provide peak power beyond the performance capability of the fuel cell. By comparison to the Honda Civic IC/battery hybrid, the FCX requires 3 times more assist power. The FCX ultracap produces twice as much power for the equivalent weight of battery used in the Insight.
Although the ultracap output Voltage linearly tapers, it can be placed in parallel with the fuel cell, eliminating the cost and complexity of a Voltage regulator. Regenerative braking is also employed to both replace charge in the ultracap and recuperate energy to improve overall efficiency.
Another benefit of the ultracap is its relatively low internal resistance. It has about 7 to 8 percent greater energy efficiency than Nickel-metal hydride batteries during charge and discharge.
Overall system complexity is reduced with the ultracap when considering its lack of memory effect and Voltage proportional to terminal Voltage. Because of the Ultracap’s long cycle life, it might be considered to last the life of the vehicle (when fuel cell endurance builds to a level commonly provided by today’s IC engines.)
To achieve the greater power performance, Honda focused on improving the fill density and collector structure so that direct connections could be made between collector plates and the case. No information was offered regarding inner cell chemistry.
“Distributed Modular Lithium-ion with Intelligent Device For Hybrids,” Industrial Technology Research Institute (ITRI), (Taiwan)
To implement Taiwan’s year 2000 mandate to produce 2-wheel zero emission vehicles, the ITRI has consolidated this program with lean-burn IC engine technology to pursue hybrid configurations for metropolitan light-duty vehicles.
A parallel hybrid with a 375 cc IC engine and a 24 cell series-connected 8.4 kW Lithium-ion battery is controlled via an intelligent CAN bus. Balancing of all cells is performed to promote performance, 2000 cycle life, and safety. Lithium-ion has been chosen because of low internal resistance, reasonable power, durability, safety and acceptable cost.
From a system performance profile specification, the engine and battery requirements were set to include 20 Ah capacity, 1000/kg power density and 65 Wh/kg energy density. The power output is to be 8.4 Kw above a 50 percent state of charge. Taiwan is apparently a warmer climate so temperature performance has only been evaluated down to + 5 OC.
The battery management system consists of one control unit and four sensing modules which can be expanded for additional pack control. With the CAN bus, the electric motor , integrated starter, generator and vehicle control is rapidly interconnected with simple wiring and good noise immunity.
The battery system is being tested and integrated with the power train. Although battery cost is still a problem, the acceptance in the hybrid community should increase volume and reduce costs.
Lithium-ion for Light Hybrid, Industrial Technology Research Institute (Taiwan)
‘Home brew’ batteries and vehicles are the goal within Taiwan. At the center of their work is a planned hybrid two seat auto with a lean burn engine and Lithium-ion batteries. The vehicle is to have a range in excess of 200 km and carry batteries which drop in cost from $700-$800 to $300-400 per kWh per kg.
Using experience from Ultralife, Taiwan, an 88.8 Volt three pack consisting of a total of 24 cells has been constructed to begin the design. Characterization and capacity testing with cycling at 60-80% state of charge is being performed. Cells have passed safety testing, consisting of nail penetration, overcharge and external short circuits. Final battery pricing of $109/kWh, including controller, are anticipated in high volume. Based on the past ability of Taiwan’s to compete in world markets with high tech products, there is the possibility of seeing another global hybrid manufacturer emerge. (Ed. note: the next question may be whether they would be sold at Wal Mart?)
“Impedance Studies of High-Power Lithium-ion Batteries for HEV Applications,” Samsung SDI Co., Ltd.
The Korean influence on the choice of future HEV batteries continues to support Lithium-ion. The work by Samsung in preliminary, with prismatic 3.6 Ah cells using Lithium cobalt oxide in the cathode, artificial graphite for the anode, a 20 micrometer thick polyolefin separator and an organic carbonate electrolyte with LiPF6 salt.
In the development process, the performance relative to the need for improvements was analyzed with cycling tests using pulses to determine performance. Cells at 50% state of charge were discharged at a 5C rate for 18 seconds, then rested for 32 seconds before recharge back to the 50% SOC. During testing, an ac Ohmmeter was used to provide data for determining the major sources of impedance.
Testing was carried out for 180,000 cycles, to determine various components of the change in cell impedance. Results showed that the major contributors were the current collectors and the cell polarizations. Because this testing was done to find these areas which require improvement, it is assumed the next part of the development program will be to improve these impedance components.
“Lithium-ion for Idling Stop Vehicle,” Toyota
Today’s popular hybrid vehicles are almost exclusively powered by Nickel-metal hydride batteries. But, Toyota is offering a special concept ‘Idle stop’ control which uses Lithium-ion technology. This is one step below the hybrid drive in that the battery is only use to restart the engine and provide accessory power while the vehicle engine is off. Looking at the Los Angeles freeway during rush hour, it is easy to see how the Idle Stop approach can drastically reduce emissions as all those vehicles sit parked six-wide, across the I-10. According to Toyota which has demonstrated the system in a vehicle designated the “Vitz,” This model has the lowest fuel consumption of vehicles except for those under 660 ccs and hybrids.
The system provides power when the driver applies the brake and the vehicle comes to a stop. The engine shuts off and the heater, air conditioner, radio and lights are powered by the 14.4 Volt, 12 Ah, 4-cell Lithium-ion pack. When the driver releases the brake, the Lithium-ion battery restarts the engine which is the only prime mover in the vehicle, separating it from the ‘hybrid vehicle’ classification. The Lithium-ion battery is recharged from the alternator while the engine is running and can also absorb regenerative braking energy. When the battery charge is low or the temperature too high, the system does not stop the engine.
“Advances in State of Charge Estimation for Lithium-ion polymer Battery Packs,” Compact Power & University of Colorado
This presentation is an excellent example of evolutionaryprogress in the development of a model to predict the state of charge of a Lithium-ion polymer battery in a HEV application. The presenters began the work with a model based on extended Kalman filtering and reported results in 2001. Improvements and limitations were continually evaluated with better results, but often did not extend from cell level to battery management in the pack. Complexity also extended the execution time to limit usefulness.
With the latest level of development, the model error has been reduced and execution time has been cut by a factor of 50. The current version considers the effects of open circuit Voltage, cell relaxation, cell internal resistance and hysteresis.
“Lithium-ion Polymer for Transportation Applications,” LG Chemical
As the rechargeable Lithium-ion chemistry unfolded in the last decade, the design and manufacturing excellence of the Japanese insured the success through safety and performance. After the market was established, the Chinese began to become a major supplier with cost being a large incentive in their acceptance.
Korean influence in Lithium-ion has been added to the list of suppliers and the thoroughness of their presentation at EVS supports their inclusion in the membership of serious suppliers. LG Chemical has provided large size Lithium-ion polymer batteries for transportation applications and is expanding the offering with new sizes and improved performance. Details of the performance was extensive with only the calendar life and low temperature data left out. In the case of calendar life, there may be difficulty in providing accurate information at the outset of such a new program.
LG emphasizes its focus on safety by choosing a manganese spinel with graphite chemistry. The spinel has long been accepted as having superior thermal safety over cobalt electrodes. Additionally, the Mn based cathode material is abundant, with relatively low material cost. In the new 7.5 and 5 Ah cells, a blended carbon provides improved power density both in charge and discharge. At discharge rates to 30 C, the cell delivers over 85% of its 1 C rated capacity.
Detailed performance at 1 and 2 C rates shows initial peak power densities over 5000 W/kg at State of charge between 80 and 20%. Cycle life in the PNGV life cycle pattern produced 160,000 pulse cycles with only a 20% reduction in initial power.
The combination of inherent safety and reasonable costs of the spinel and polymer materials with the high power performance can make this supplier a force in power for HEVs, electric bikes, scooters and wheelchairs.
“Integration of a Lithium-Metal Polymer Battery Pack into an Electric Vehicle,” Avestor
This is one of the chemistries which has been under development for many years but continues to be pursued as a high energy density, potentially low cost battery. Since electric and hybrid vehicles are still experimental or of low volume, Avestor has directed the immediate applications to telecom backup where the tolerance to low and high temperatures allows it to be offered with a 10 year warrantee. The telecom experience will allow reliability and cost performance data to determine the acceptability to EV and HEV applications, which will hopefully emerge in the next decade.
“Improving Battery Thermal Management Using Design for Six Sigma,” Advanced Engineering Solutions & the National Renewable Energy Laboratory
The level of understanding needed to implement such design is formidable, but the overview of the approach can be made understandable to those not statistically well versed. First, one must admit to the complexity of battery utilization which includes battery performance and life. To put a single battery in a test device, measure its power and energy delivery, and then cycle it under appropriate load conditions are only the first steps in determining the battery’s capabilities. If we need only one cell at standard temperature, pressure and accelerations, one additional ‘g’ test might be in order. But, when the battery will be designed into tens of thousands of hybrid vehicles encountering a variety of operational and environmental conditions, much more understanding is needed to assure performance and life.
Central to the performance/life issue is the thermal management. While either charging or discharging, the current flow in batteries creates heat. High rates of charge or discharge, often desirable in an HEV battery, leads to excessive cell temperatures. Internally generated heat interacts with the heat of the ambient surroundings because other parts of the vehicle are producing heat, and the air surrounding the vehicle may either be adding or subtracting heat from the battery. If these conditions do not make for enough complexity, added variances occur due to manufacturing tolerances of battery parts, and assembly dimensions which restrict airflow.
To make the process accurate, the Panasonic prismatic Nickel-metal hydride cells in a Toyota Prius were chosen because of the extensive testing which has been accomplished on this vehicle at the National Renewable Energy Laboratory (NREL). Finite element modeling defined the geometry of the cell, which included the physical characteristics such as thermal conductivity, heat capacity, internal resistance and module placement gap.
A parametric deterministic model used three variables of thickness, resistance and flow rates with assigned normal distribution probability densities representing variations as quantities in production increase.
To quantify the quality of the design, a level of six sigma was selected corresponding to 3.4 defects per million. While this may seem excessive in auto transportation, it must be remembered that Toyota and Honda have established their overall quality performance with major attention to subsystem reliability.
Using information from this model, transient response can be determined for applications such as the 40 kW Freedom car profile. Performance to establish the time to reach steady state temperatures can be determined, and choices of input parameters and design constraints are factors which contribute to the quality level achieved.
The degree of sophistication in this approach may lead the way to better understanding of battery performance and ultimately could improve quality for optimal design and manufacturability in stationary, portable and transportation applications.
“U.S. D.O.E. Collaborative R & D on Electric and Hybrid Electric Vehicle Energy and Storage Technologies: Current Status & Future Directions,” U.S.D.O.E.
For the person wanting to see the big picture of battery development now and in the future, this presentation is both detailed and yet complete in less than 12 pages which include 1 1/2 pages of credits and references. To make the subject identification simpler, it should be noted that this presentation does not mention double layer capacitors specifically.
The DOE supports R & D for innovative auto technologies through the FreedomCAR Partnership. This includes hybrid electrics, battery electrics, 42 Volt systems and fuel cell vehicles. The goals are to establish and reaffirm performance and cost targets, develop hardware, and accelerate development of applied and long-term advanced battery technology. Over the last decade, development of Lithium-iron sulfur, Nickel-metal hydride and Lithium-ion chemistries has progressed..
Today, their work focuses on technical barriers associated with cost, performance, life or abuse of battery systems. Targets have been established for hybrids, EVs , 42 Volt systems and fuel cell vehicles. Costs, however determined, are big barriers to the implementation of battery power. Separator costs are a big part of the continuing work.(Ed. note: Units of measure are not consistent in that the long term production price of a HEV battery is a maximum of $800, whereas the selling price of a 42 Volt system battery is listed at $360. Based on past experience, better measures are needed. Motorists have been the brunt of vicious markups in selling price as the product passes through the chain of price buildup and distribution. Ultimatley, the final retail price becomes whatever the market will bear.)
Today, specific programs include liquid-cooled Nickel-metal hydride packs, Lithium-ion battery life and development of Lithium-sulfur battery systems. Benchmark testing of emerging technologies is accomplished at independent test facilities, although results are held in confidence between the DOE and developers.
Responsibilities for advance Lithium-ion battery research is segmented within various Laboratories. Argonne works with battery system development and electrochemical diagnostics. Brookhaven performs x-ray diagnostics, Idaho is involved in battery testing and electrolyte development, Lawrence performsspectroscopy and microscopy diagnostics and Sandia is in charge of abuse, accelerated life testing and statistical analysis. The presentation describes programs within each of these Laboratories in greater detail.
With the well structured approach of the DOE battery program, additional strength is added through cooperation with other international agencies such as the International Energy Agency, and Japan’s Lithium Battery Energy Storage Research Assn.
“Energy Management System for Combined Storage System,” University of Bremen
Initially the vehicle power system seems most unusual. It is composed of an electric van with three independent storage systems - Zinc-air batteries, Nickel-metal hydride batteries and an ultra capacitor. These power sources are all connected to a 230-360 Volt bus which powers a nominal 40kW, (65 kW peak,) drive motor.
The large energy storage is in the 105 kWh Zinc-air battery which provides the highest energy density of 125 Wh/kg but only a low power density of 26 W/kg. This is the ‘long range’ power supplier.
The 3 kWh Nickel metal-hydride battery is the power boost for hill climbing and meets larger profile assist requirements, providing 300 W/kg.
Finally, the 150 Wh ultracap provides additional short bursts of power for acceleration and passing.
The energy management system recharges the ultracap first from regenerative braking and then adds other regenerative energy to the Nickel-metal hydride battery. When insufficient regenerative energy is available, the Nickel-metal hydride battery is brought up to a 70% state of charge by energy from the Zinc-air battery.
The vehicle provides not only an experimental test bed for identifying the best control schemes, but also a real world road vehicle to determine if the performance is suitable for driving conditions. Unfortunately, the Zinc-air battery requires removal for regeneration of the zinc anodes which is not conveniently available.
Rather than define this vehicle configuration as an end design, the concept is being extended to a minibus which will take advantage of higher performance batteries and ultracap. Beyond that, the concept of management of combined systems provides a base for other combinations of power including IC engines, fuel cells and photovoltaics.
“Overview and Status of Japan’s Advanced Energy Vehicle Project”, Japan Automobile Research Institute.
This program was initiated in 1997 and involves Honda, Toyota, Isuzu, Mitsubishi, Nissan and Hino in contracts with the New Energy and Industrial Technology Development Organization. The goal has been to develop technologies and vehicles which have increased efficiencies.
Now seven years into the program, experimental vehicles are in the final building stages. Already, Nissan’s Diesel bus has been completed and tested to demonstrate that fuel efficiency of 2.1 times that of the base vehicle has been achieved.
Regenerative efficiencies of baseline series and parallel HEVs were under 40 %, requiring doubled targets of 80% regenerative efficiency for the experimental vehicles. In this report, the performance of capacitor storage was highlighted. From an earlier design, a second capacitor was developed which boosted charge and discharge efficiency by 5 to 10 percent over the 80 to 40 percent state of charge region. With this performance compared to the lesser efficiency of a Nickel-metal hydride hybrid, it was determined that fuel economy can be improved by 19% if capacitors are used in place of the Nickel-metal hydride batteries. This may be one reason we see the emphasis by Honda to use the ultracap in the FCX vehicle and may see further considerations of double layer capacitors in other hybrids.
“The Future of Traction Batteries and the EV, “ PSA Pugeot Citroen
This presentation is an excellent status report and forward looking evaluation of the electric vehicle. PSA is able to provide such a view because it is the world’s leading manufacturer of volume produced electric vehicles since 1995. It accounts for 65 % of the electric vehicles currently on the road in Europe. Testing and prototype development continues within the company.
Of the available battery chemistries, Nickel-cadmium is considered obsolete because of its environmental problems upon disposal and because the Nickel-metal hydride battery has brought higher energy density and acceptable power density. Loss of capacity at temperatures of -20 0C is a problem. Concerns still exist in the generation of hydrogen as a safety factor.
Lithium-ion and Lithium-ion polymer have made solid improvements although low temperature operation is a problem, especially with the solid polymer version which must be maintained at temperatures of 60-80 0C. Despite lab testing, there are safety concerns for these batteries in vehicles.
Because pure electric vehicles would be best operated from 100 % state or of charge to close to zero, the cycle life of Lead-acid under such conditions would only provide 180 cycles. Nickel-metal hydride is good for over 1000 cycles and Lithium-ion for 500-900 cycles.
Recent road testing of Lithium-ion powered sedans has provided better understanding of performance such as operation in cold climate conditions. Average driving ranges have been 165 km.
The performance capabilities of the electric vehicles, with many choices of battery chemistries, can provide suitable performance, but the shadow over all of them is the cost of the batteries. The technical advantages of Lithium-ion (& polymer) continue to build, but the costs which have dropped from 330 Euros/kWh to 150 Euros/kWh remain the penalizing barrier to EV acceptance.
As technical and cost barriers are resolved, the principal applications of EVs appears to be for urban vehicles and fleet applications.
“Development of Prismatic Nickel-metal hydride for HEV,” Panasonic
The casual observer focuses on the dramatic commercial hybrids produced by Toyota and Honda, but a third player in this exclusive club is the battery supplier Panasonic which has supplied battery packs to over 200,000 HEVs. As Toyota and Honda vehicles have collected hundreds of thousands of hours of road experience, Panasonic has shared in the experience with its batteries. The direction Panasonic takes in new developments will be reflected by the road experience and the direction of new designs by their countrymen.
Prior to 2003, Panasonic has developed a prismatic Nickel-metal hydride battery which provided a 25% increase in specific power over their cylindrical design. In the 2003 Panasonic paper, discussion continued on an even higher specific power prismatic cell with an increase from 1000 W/kg to 1300 W/kg. This increase was accomplished by reducing the resistance of internal current pathways and changes in material of the positive electrode. New intercell current pathways provide lower internal resistance, and solid solution additives (to the positive active materials) show higher terminal Voltage from very low to maximum currents. Data presented shows approximately 30% increase in specific power in the compete range of state of charge and maximum ambient temperatures.
Concurrent with the above changes, the new cells utilize an improved separator which should lead to longer cycle life. To date with only 8500 normal driving cycles reported, there is no reduction in cycle life, but the testing is continuing to hopefully show the improvement for the new separators.
‘Hybrid Bus In A Large City,” Saft
Busses and trams are the focus of the design of the model NHP battery which must recognize the large number of cycles imposed by a hybrid system powering a public transportation vehicle. By sizing the battery properly, depth of discharge can be limited to 20%. Under these conditions, prismatic Nickel-metal hydride batteries are being produced which have an anticipated life of six years. Under operational conditions, thermal management becomes a major concern. After considering the advantages of air and liquid cooling, the liquid cooling has been implemented with circulation chambers integral to the cell design. This produces a compact battery which allows uniform heat removal. Cycle testing of 20% depth of discharge from 60% peak power has shown no significant fading at 43,000 cycles. Relating the cost of the battery to the passenger, Saft estimates that the premium of the battery hybrid would only be $0.15 per passenger per ride. (Ed note: This may be a small price to pay for a healthier air quality for all the people in congested cities.)
“Zebra battery” , MES-DEA Sa
The name of the “Zebra” battery may be appropriate for this chemistry since the actual chemistry is a bit complicated. A cell is built from nickel and iron powders, sodium chloride, beta-alumina (boehmite) a ceramic electrolyte and a stainless steel case.
The reaction is:
2NaCl + Ni < = > NiCl2 + 2Na
When charged, a negative electrode of sodium forms from the sodium chloride making construction in the uncharged state safe. The sodium reacts with nickel chloride to produce current. Cell Voltage is a healthy 2.58 Volts, leading to a lesser number of individual cells in a high Voltage pack. It may be that the presence of sodium in the charged state is the major safety concern for the battery, but no safety information was presented.
The desirable features for the Zebra battery includes its high energy density of 120 Wh/kg, its economical and easily available materials (nickel, iron, sodium chloride) and its friendly recyclability. Calendar life is claimed to be more than 10 years, and 1000 to 2500 cycle life is anticipated. It has high energy density of 1230 Wh/kg. Compared to other nickel based chemistries, the Zebra requires only 1.53 kg of nickel to produce one kWh of energy while Nickel-cadmium and Nickel-metal hydride require between 3.5 and 6.8 kg of nickel per kW/h.
As of the latest report, November 2003, at EVS 20, the ZEBRA battery is said to have achieved:
• More than 4 million miles in road vehicles
• More than 60,000 miles in one vehicle with no maintenance
• More than 5 years life in a car
• More than 12 years demonstrated calendar life
• Performance independent of ambient demonstrated in arctic and desert conditions
• Proven safety
More information on the battery can be obtained at Beta Research and Development Ltd. at their website at www.betard.co.uk
“Lead-acid Battery for HEV Applications,” European Advanced Lead-acid Battery Consortium
This is an update of the saga reported in detail in BD’s May 2003 issue. While Nickel-metal hydride has captured the HEV production offerings of Toyota and Honda, efforts as mentioned above show aggressive consideration of Lithium-ion and Lithium-ion polymer as the ‘other source’ in future hybrids whether they be with IC or fuel cell engines.
Where does that leave Lead-acid? The perception is that the low-cost battery has neither the energy density nor the cycle/calendar life to compete in this modern HEV world. Pundits can expound on the subject until the public is as tired of hearing about it as they are tired of reports of the political elections. Talk is cheap; it will take road experience, production capability and cost to the consumer to decide if Lead-acid can be a player.
That is exactly the path being pursued by the European Advanced Lead Acid Battery Consortium in their Rholab program(me). This three year program is dedicated to determinet the road capabilities of a currently manufactured Lead-acid battery in a well characterized vehicle, the Honda Insight. Hawker, pure lead, starved electrolyte cells with improved double ended terminals to reduce resistance are bench tested, and then installed in the Insight to determine performance capability. This venerable battery, which has been around since the dawn of the VRLA era, has nothing to prove in its basic performance. However it needs to perform on the road with vibration and high cycling in partial state of charge conditions which require high charge and discharge rates to accommodate acceleration and regenerative braking.
At this point in the program( Nov 2003), the cells have been assembled into packs, after having good results from the bench testing. Reduced Voltage drops in 1000 second pulse discharge have shown the viability of the twin tab design. A decline in the pack Voltage was observed after 65 Rholab cycles which has led the team to consider modifying the negative active material with added carbon and a second approach of conditioning the cells by bringing them up to full state of charge after every 10 Rholab cycles. Such a pattern can be easily implemented in an automatic battery management system, and if successful in the long run, could be a transparent ‘fix’ for the Voltage dropoff problem.
Before installing the Lead-acid battery, the Insight has been bench marked with the Nickel-metal hydride battery pack. This adds performance data to the well investigated Honda and allows the Rholab data acquisition package to be validated prior to use with the Lead-acid power.
Work is in process to convert the Insight over to the Hawker battery. A 50,000 mile test comparable to the Rholab bench cycle test will be performed. One might say that such a driving distance would be less than that experienced by the Insight or Prius with Nickel-metal hydrides, but then there is the need to consider the comparable initial and replacement costs. A motorist with saved dollars in his/her wallet/purse probably would accept a Lead-acid battery replacement at 50,000 miles if it was scheduled (not traumatic) maintenance. Other factors such as calendar life and suitability to temperature extremes not only need to yet be presented to the automotive public for Lead-acid but also for Nickel-metal hydride, Lithium-ion, Ultracapacitors and any other candidates. Driving 100,000 miles in two years is not the same as driving 100,000 miles in 10 years.
The viability of hybrids in reusing braking energy and tuning of the engine for optimum performance has been established. It is real world road performance such as Rholab’s work which will help to bring this vision of battery suitability into focus.
“Vehicle Integrated Monitoring with SOC/SOH Intelligence,” Midtroncs
Lead-acid batteries have often been left out of consideration in hybrid applications, but the real world may still be a domain for the venerable heavyweight if sophisticated battery condition is needed in applications such as idle-stop or start-stop. Such an application needs to know if the capability of the battery is sufficient to restart the engine each time it is shut off. While such information may be easy to get for a new, cool fully charged battery, the repeated engine on-off regimen of an ordinary driving cycle, with variable demands for engine-off power, requires managing the load priorities so that sufficient cold cranking Amps will be available as the engine is brought back to life.
The intelligent Battery Monitoring System (iBMS) uses microprocessor control to measure battery conductance. The system also considers Voltage, charge/discharge current and temperature to arrive at battery state of charge and the state of health (in percent) which should constantly decline with use and age. No longer is the power capability idealized for a battery. The vehicle intelligence must know if the remaining power/energy will restart the engine, and if so how long can it provide power to drive components with prevailing temperatures, yet still fulfill the restart mission.
A new term was mentioned, ‘State Of Life’, which is computed from the measurements, but left undefined. Perhaps BD can find additional information on this in future issues.
In 2005 the inGEN system will be offered in a European passenger vehicle and other systems are used in military vehicles and heavy-duty trucks.
Perhaps with appropriate intelligence tailored to sophisticated and hybrid applications lead is not dead after all.