Batteries/Automotive/Hybrid/Lead for Hybrid page 050928
Without making the case for hybrid vehicles, their expanding presence in both busses and automobiles suggests that some level of benefit and demand does exist for them. For purposes of simplicity, this presentation will only focus on the hybrid which does not have plug-in capability, obtaining all of its battery energy from a combination of IC engine charging and braking regeneration. It also does not seaparate the degree of battery assist. Hybrids achieve greater fuel economy by recovering braking energy and allowing the IC engine to operate in a more highly efficient domain to improve economy and reduce emissions.
Battery power in transportation has fallen into a perceived well of futility after the failure of pure electric auto power via California’s mandate. The vehicles did not offer the needed range, recharge convenience, cost or vehicle size because mandates could not produce the necessary technology.
The rules have changed with the advent of the gasoline/battery hybrid vehicle. With the IC engine operating over a more efficient range and providing on-the-road recharging to the batteries, the battery pack no longer has to be of such enormous size that it eats up the back seat and trunk. Hybrid builders see the total battery pack cost go down because gasoline is the energy source allowing less cells to be used. This opens doors for higher cost high-tech batteries such as Nickel-metal hydride, Lithium-ion, or possibly Lithium-metal polymer.
But, is Lead (-acid) dead? Not if you observe the efforts being carried on by U.S. and European Lead-acid battery consortia. They want a Lead-acid hybrid in every garage and are willing to spend the money supplied by government, lead producers and battery manufacturers to prove they can be price and performance competitive. With hybrids using less than the full charge-to-full discharge range of a battery, the possibility of Lead-acid providing acceptable performance at an economical price may reopen the door of opportunity so cruelly slammed shut after EV failures. The road to success is far more difficult than merely packing today’s cells on the shipping dock. Lead-acid will have to prove itself on the not-always-friendly roads with performance equal to IC powered autos, while eliciting smiles on drivers’ faces in the showroom, at the gas pump and with acceptably infrequent visits to the service department.
The requirements of a battery in a hybrid vehicle are not the same as that of a SLI or a deep discharge battery. The SLI battery starts at full charge, emits a pulse of energy, and reverts back to a state of rest to receive a top off charge, never seeing significant depressions in state of charge unless it is in a poorly tuned Minnesota auto during an Alberta clipper. The deep discharge battery provides a lower current, but it does so until most of the stored energy is removed, after which it is usually fully recharged. The hybrid battery, while called upon for relatively short bursts of high charge currents to augment vehicle acceleration, must also be ready to accept high levels of regenerative energy from braking and spends most of its life in the region midway between full charge and discharge without achieving either condition. This profile is both good and bad, for the charge limit below full state of charge and above full discharge adds cycle life. However the hybrid battery is called upon for thousands of these partial cycles as the vehicle accelerates and decelerates. For the battery the unanswered questions are: “ under these new conditions what is the cycle life?” “Are there other effects which will reduce life and performance?”
Can cost, including replacement, be isolated from perceived value? A lesson from the SUV market.
Assuming the hybrid provides the needed road performance the project bean counter will commandeer the spotlight adding that higher
costs will make or break the project. In a product which lasts for more than 10 years and has a history of money. making follow-on service and repair, the deeper cost question involves an unquanitfyable overlapping of sticker price, performance, product image, and the acquisition of perceived owner/driver power and status. When the time comes to replace batteries what will they cost or will they be covered by a warantee?
Returning to the auto hybrid, there is no doubt that it will give higher gas mileage and reduce harmful emissions. Although theseattributes are the cornerstone of the hybrid design, average consumers usually pay lip service to ecological choices by others, but they don’t consider those values in their personal buying habits which show SUVs to be the most popular vehicle configuration with over a 40% market share.
There is no data suggesting that SUVs provide competetive cost effective, safety promoting, emission reducing or foreign-oil-dependence-reducing transportation. The perceived image of power and size creates the U.S. market for a population which has immense discretionary and recreational wealth. To this end, it makes sense to apply hybrid technology to SUV’s, for if gas consumption can be reduced by even a small percentage, it could make a significant reduction in the total oil used and pollution generated for a given number of miles traveled. The greater selling price added to the hybrid SUVs may be little hurt by the additional cost of components since these vehicles are in a much higher price category than average sedans and mini-vans.
For hybrids, the greater question may be whether the added costs of showroom schooners will add to the customer required feelings of
power, status the image of cleaning up our economy and air. A massive public advertising and education campaign would be necessary to make any hybrids ‘trendy.’ If it could be done, the hybrid owner might show-off his/her car by parking it in the driveway rather than hiding it in the garage. This advantage could extend to the automaker which could offer the vehicles at a lower profit (or loss) to gain the image of being ecologically responsible. Although many would weep over reduced profits, the giant auto company could add ‘special’ pricing to other models to achieve its total profit and could shift public relations budgets. Government incentives based on reduced oil demand and pollutants to both car companies and consumers further improve the case for the production and sale of hybirds.
These perception factors greatly complicate the cost issue, but after removing all this smoke, if Lead-acid can do the job at lower initial cost, even though it may require more vehicle lifetime replacements, manufacturers may figure ways to meet buyers’ price expectations while satisfying perceived needs.
The consumers’ lack of understanding regarding battery replacement issues is presently being played out in the Personal Digital Assistant (PDA) market where most new devices have factory-only replaceable Lithium-ion or polymer batteries. These little two to six hundred dollar handhelds give such great perceived image of status to the owner that the ensuing problem of the compromised or worn out battery will either require device disposal or a long out-of-service return to the factory for battery replacement. At the time of purchase, this is not considered as the buyer gladly parts with the money for the gleaming device which is usually called on to do the same as, or little more than, a pocket note pad and pencil. When replacement time comes, there maybe a bit of gnashing of teeth. Similarly, if Lead-acid hybrids require battery exchange at the nominal four years as experinced by the present SLI batteries, the owners’ preconceived understanding and the economics must be acceptable or public opinion could turn away from hybrids.
We will leave the cost argument with the assumption that Lead-acid life will be shorter than competing chemistries, (BD has no data on Nickel-metal hydride or Lithium-ion calendar life.) but Lead-acid’s subsystem cost must be significantly lower, too. Drivers have become used to replacing car batteries at about the four year point, so a first target for Lead-acid hybrids may be in the same ballpark. What the
replacement cost will be and who will foot the bill still needs to be determined both at an introductory level and a high volume level. Battery producers and car companies anticipate Nickel-metal hydride or Lithium-ion hybrid battery costs should sink to under $300/ kWh when volumes reach 100,000 packs per year. Such numbers, whether achievable or not, put the target for Lead-acid at something below this figure. Even murkier is the cost of the complete battery subsystem which includes the package, charging, temperature controlling and safety elements.
One might say that government incentive pressures have a major impact on the price and success of the hybrid, but in the long run, history shows that while government funding at the R & D level stimulates new technologies, it is not good at legislating economic supply-demand forces.
EALABC to Prove Performance
Powering hybrid vehicles with commercially available Lead-acid batteries is being demonstrated by the European Advanced Lead-Acid Battery Consortium, (EALABC) Hawker Batteries Ltd., Provector Ltd, the University of Sheffield and the University of Warwick. Their program will incorporate Hawker Lead-acid battery packs in a direct replacement for the Nickel-metal hydride packs in a Honda Insight. This ‘Foresight hybrid will be a test and demonstration platform for Lead-acid in real world conditions.
Such an exchange is far from just being a quick battery swap exercise; because to be successful, the Lead-acid batteries must address a number of issues to prove the ability to compete as a hybrid power sink and source. While looking at the Lead-acid performance in the Insight, the observer must be cautioned to not overgeneralize on applicability to all hybrids, because as shown in the detailed analysis of Toyota Prius and Honda Insight by the National Renewable Energy Laboratory1, battery state of charge under the EPA’s Urban Dynamometer Driving Schedule (UDDS) varies considerably from the Insight to the Prius.
Why select the Honda Insight?
When comparing the battery in the Honda to Toyota’s Prius hybrid, the charge discharge requirements differ because of each system’s design configuration. The Toyota relies totally on battery power to begin motion and does not call on the IC engine until speeds of between 13-25 mph are achieved. The Insight battery provides motive power and simultaneously starts the engine as it begins to move. This difference will put unique demands on each battery system, so while the vehicle road driving profile may be the same for the two vehicles over a single course, each battery system will be called on for different charge/discharge performance.
The system Voltage is 144 Volts in the Insight and 273.6 Volts in the Prius. Because multiple cells have to be series connected to achieve the bus Voltage, only 72 cells will be needed provide the 144 Volts while it would take 134 series cells to feed the Prius. In a pioneer program, selecting the lower bus Voltage requirement limits the risk imposed by cell failure and conditioning. Then too, the lower nominal energy of 936 Wh also means less cells than that required for the 1778 Wh of the Prius. With less cells, the challenge becomes more manageable. It appears to be a walk-before-you-fly approach.
Why select the Hawker Cyclon Cell?
The cell itself is important both as a single cell and as a cell within a system of cells. To simultaneously invent a new cell and qualify it in a hybrid application is risking too much, so the EALABC is calling on the Hawker Cyclon sealed design which uses pure lead thin plate grids and enjoys decades of successful performance. Hybrid operation asks for high power, both during discharge and during dynamic braking. Simultaneously, hybrid start/stop operation places the requirement of great cycle life on the cell. The best operating region for such give and take operation is in a partial state of charge. Failure mechanisms2 in this state will be one of the outcomes of the Foresight Program so that even more robust batteries can be built. Early gassing which could reduce cell capacity will be reduced with advanced separator materials. Sulfation reduction of the negative plate will be pursued with improved carbon materials. Recharge resistance reduction is facilitated with the cell twin tab configuration and will be further pursued with revisitation of the material purity issue and the possibility of a synthetic expander.
How detailed must the testing be?
The complicated operation of the battery does not allow simple bench life cycle definition such as that from the PNGV Power Assist Life Cycle and the EUCAR Power assist Life Cycle, which only defines current with time for a typical cycle. From the rich base of dynamometer and road testing, specifically for the Insight, more representative bench test profiles are established and find definition in the RHOLAB ‘Real-data’ definition, making bench test results more representative of real world operation. This more realistic bench test is another of the walk-before-flying steps in a scientific approach to quailfy ing Lead-acid for hybrids.
The final test is on the open road, combining the demands of charge and discharge with the harsh realities of actual stop/go acceleration/deceleration, overlayed with temperature, shock and vibration. Not only will the batteries see calm smooth roads such as those in beautiful Thousand Oaks, California, but also the bone-shocking potholes experienced in Chicago, Illinois’ working streets. If hybrids are to be ubiquitous, they must eventually function in Minneosota’s forty below (Faherenheit) winter wonderland and Palm Springs’ 120 degree (Faherenheit) sauna. Only with a real world Lead-acid hybrid can all these conditions, already accommodated by today’s ICE autos, be experienced.
Where does 36/42 Volts fit in?
It is most likely not a curiosity that 144 Volts can be made up of four 36 Volt modules in series for the Insight. From the experience to be gained in this high Voltage application, the 36 Volt modules will gain valuable experience for the 42 Volt bus which it is anticipated will replace the 12 Volt batteries completely by the year 2010. If validation can show the Hawker 36 Volt modules suitable for the Insight, it will go a long way to open the doors for use in 42 Volt autos, whether hybrids or non-hybrids.
A critical failure mode is to have one cell fail in a series string.. A single cell failing to a high resistance state would eleiminate the module and the entire supply. To accommodate failure possibilities and off-pack conditioning, the Rholab module has 19 cells, allowing one cell to be off the circuit for conditioning. By allowing any of the cells to be off the bus, it appears that the pack could accommodate a single cell failure and still provide full 36 Volt operation. For such intelligent control, the module dedicates a separate microprocessor to each cell on the pack, complicating the hardware, but providing reliability and honing the individual cell with on-board conditioning.
Coordination, standardization and safety
The combination of usability, cost effectiveness and safety for any technology is expanded when there is industry-wide coordination and standardization. For Lead-acid in hybrid applications, the work done and being done to facilitate 42 Volt automotive systems will improve its focus in providing product. For years now, the Industry Consortium on Advanced Automotive Electrical/Electronic Systems facilitated by the Massachusetts Institute of Technology with Dr. Thomas Keim4 as its Principal Reserach Engineer has made 42 Volts a coordinated rather than a fractionated effort.
Also in the 36/42 Volt realm, is the American Society of Testing Materials (ASTM) which has formed a new subcommitee D09 to develop new test methods and standards for conducting tests in road vehicle applications. For mor details on D09, see the Questions and Answers provided by George Zguris in his ‘Ask George’ column added to our website this month.
Other ASTM and Underwriters Laboratories (UL,) and Society of Automotive Engineers (SAE,) Institute of Electrical and Electonic Engineers (IEEE) testing standards will facilitate development and safety of the higher Voltage batteries.
A word of caution regarding safety must be applied to the impression of standards regarding the electrical circuits of hybrid vehicles. While battery modules generate 36 Volts, the
total bus Voltage in the Insight and Prius are much higher. Forty-two Volts was subjectively selected as a new automotive bus standard because it is an upper limit of acceptable working Voltage where a careless maintenance/repair person should not be dangerously shocked. When two or more 36 Volt modules get tied in series, the total bus Voltage goes into the region beyond casual safety concerns for which the 42 volt battery module cannot be held liable. At this point, the higher Voltage must be addressed with standards beyond those of 42 Volt systems, and only allow limited access by trained professionals to eliminate the shock hazard which could be fatal. (Ed. note: electrical shock hazard is not easily specified. From the viewpoint of applying electricity to the skin of a normal person, the variable conductivity of the skin is usually the greatest barrier. At 42 Volts, to achieve a current of 10 mA, sometimes identified as the threshold of ‘can’t let go’ current, the body/skin resistance would have to be 4,200 ohms. Assuming internal tissue to be very low resistance and skin as a series element, each contact point would be 2,100 ohms, not unachievable with hands wet with sweat or contaminated water. If these numbers do not capture the reader’s interest, consider first that the trip limit of a ground fault interrupter is 5 milliamps of fault current and if that is not enough, UL Standard 544 limits patient contact leakage of medical devices to the AC line (and ground) to 10 microAmps. The bottom line here is that even at 42 Volts, a person should understand and take standard precautions to eliminate the possibility of electrical shock.)
Let the Games Begin
With the Insight and Prius, Nickel-metal hydride has struck a feasibility stake in the ground for hybrid power, but its success has not eliminated competetion because further experience including data on self discharge and calendar life along with lower production costs need to be determined. Lithium-ion and Lithium metal polymer need high visibility successes to make them a contender. From the work Lead-acid hybrid researchers have completed, have in process and have planned for, results may show a bright future which may be due to more than just cleaner air.
1.) Battery Usage and Thermal Performance of the Toyota Prius and Honda Insight for Various Chassis Dynamometer Test procedures. National Renewable Energy Laboratory
2) High Rate Partial State-of-Charge Operation of VRLA Batteries. BCI Session on 42 Volt Battery Systems, 2002
3.) The Development of a Reliable Highly Optimized VRLA Battery for HEV Applications. The EVAA Electric Transportation Conference, 2002
4.) Technical and Cost challenges for 42 Volt Implementation, BCI Session on 42 Volt Battery Systems, 2002.
5.) From a long lost Latin document which used the word ‘Tinquor’ in a way which could be loosly interpreted as ‘tinkerer,’ making the phrase ‘Those who mess with higher Voltages should use caution, lest they fall prey to being knocked on their posterior regions.’