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Plug-ins Progress

From being a concept known mainly only by a close few even as recently as several years ago, plug-in hybrid electric vehicles (PHEV) are now being seen by an increasing number of transportation technologists and policy-makers as a near-term solution for reducing petroleum consumption and emissions of greenhouse gases.

The just-released Climate Change Technology Program Strategic Plan from the Department of Energy, for example, highlights plug-ins as one of the promising near-term solutions in the transportation sector. (Earlier post.)

The third day of this week’s California Air Resources Board ZEV Symposium was dedicated to PHEVs and the batteries that support them and all-electric vehicles (EVs).

This year marked the first year when I came to this conference and assumed that everyone already had fairly detailed technical knowledge of plug-in hybrid vehicles. It’s also the first year where I won’t spend the first third of a talk explaining what plug-ins are, and the next third why.

—Dr. Mark Duvall, technology development manager for electric transportation & specialty vehicles, EPRI

There is a pressing need to do something new in the transportation industry specifically related to addressing our petroleum consumption...I feel that the PHEV is one of those potential options to help us address those issues...ideally one of the near-term solutions.

—Tony Markel, senior engineer, NREL

The dramatic improvements in battery technology over the past few years are a major enabler and catalyst for this intensifying focus on PHEVs. Improvements in basic battery chemistry and manufacturing are not sufficient to unleash a flood of PHEVs on the market, however.

There are many ways to design a plug-in—and these have a direct effect on the size and type of battery applied in the vehicles. One of the challenges in the industry now is to determine the expectations of the different stakeholders, and then balance those objectives in such as a way as to give battery manufacturers a target to which they can work.

Charge-depleting vs. charge-sustaining modes. A plug-in battery operates in two modes: charge-depleting and charge-sustaining. Charge sustaining is the mode of operation for batteries in conventional hybrids. The state of charge may fluctuate, but on average it is maintained at a certain level while driving as it supports the engine. Discharge patterns consist of many, relatively shallow discharges.

In charge-depleting mode, the battery draws down its charge to power the vehicle. If the PHEV is operating in all-electric mode, the depletion will be fairly constant, except for bursts of regenerative charging from braking. If the PHEV is operating in blended mode, the state of charge may fluctuate but will, in general, decline until it reaches the point at which it switches over to charge-sustaining mode. Here the discharge patterns combine deep discharges with many more shallow discharges.

All-electric operation vs. blended mode operation. One of the fundamental questions designers must address is the operating strategy for the PHEV and its battery. The two basic options are to run the vehicle as an all-electric vehicle from startup, and then kick over to the engine when the battery reaches its minimum state of charge (SOC) threshold. (At which point the battery switches to charge-sustaining mode.) The other option is to run in blended mode, a charge-depleting strategy in which the engine supplements the battery up to the minimum SOC.

Cycle life vs. state of charge swing. Click to enlarge. Source: NREL

The State of Charge (SOC) Window. Regardless of whether the PHEV is designed for all-electric or blended mode, an important parameter for the battery is the SOC Window. The SOC window defines the swing: the upper and lower parameters for the state of charge of the battery, which, in turn can help define the required total kWh capacity of the battery pack for a given application.

Battery cycle life is a strong function of SOC swing. Battery SOC swing is a strong function of driving habits.

—Tony Markel

In the absence of large test fleets of PHEVs on the road, simulation has become a tool for helping engineers figure out the likely technical dynamics of the PHEV market. NREL has used real world driving data from St. Louis in its modelling, as described by Markel at the ZEV Symposium.

Among the early findings and insights:

  • Consumers are likely to experience higher fuel economy than the rated value in daily driving, although a lower all-electric range than rated (due to driving patterns). In other words, as Markel said, “People would be very pleased with these vehicles.”

  • A point related to the above: vehicles designed with all-electric range likely to operate in a blended mode to meet driver demands.

  • Fuel savings relative to conventional vehicles are almost entirely distance-dependent. Savings relative to conventional HEVs are distance dependent up to PHEV distance then constant.

  • PHEVs that saved the most fuel relative to an HEV travel about 25 miles with speeds under 60 mph and light accelerations. PHEVs that saved the least fuel relative to the HEV in the 20-30 mile range had periods of 60+ mph highway driving and the accelerations were significantly more aggressive. (See chart below.)

  • Reduction of petroleum consumption is not tied to an all-electric range.

  • PHEV benefit is strongly related to distance and aggressiveness of real-world usage.

Real world PHEV simulations of acceleration versus velocity. Click to enlarge. Source: NREL

The decision of whether to support an all-electric or a blended strategy has a direct impact on the battery and motor combination—an all-electric strategy requires a more powerful battery and motor.

The blended strategy, by allowing the engine to switch on to provide additional power, can use a less powerful battery and motor combination. NREL’s simulation work so far finds that both strategies save about the same amount of petroleum.

The blended operation scenario potentially is less expensive, and may be a little more efficient too. We’ll have to see.

—Tony Markel

Still, it all comes down to the capabilities of the battery. A special DOE meting on PHEVs in May of this year concluded, not surprisingly, that the success of the technology will be directly related to how good the batteries are.

It is our personal experience that the batteries are very good. That advanced batteries continue to provide remarkable performance improvements across the board. We are cautiously optimistic about the future of batteries.

What we have here is a technology with tremendous potential to improve the sustainability of the transportation sector.

What we know is that there are very promising current durability test data. But simple data. Deep cycling data. What we don’s have as much knowledge on is how those cells will perform specifically under a PHEV duty cycle. We have a large body of knowledge of battery capabilities...but that knowledge is very focused on battery capabilities for pure electric vehicles. There is a need for specific test data on PHEV requirements—and we need to evaluate the latest and most advanced chemistries.

—Mark Duvall

In terms of chemistries, lithium-ion increasingly appears to be the current chemistry of choice for PHEVs. Beneath that umbrella, however, there are a large number of options in terms of materials, electrolytes and specific chemistries, all of which must be tailored to meet the particular application requirements.

Baseline characteristics of batteries
Chemistry Type Wh/kg W/kg $/kWh
Source: Dr. Andrew Burke, UC Davis ITS
Lead-acid Energy 35 200 $150
Power 25 315 $200
NiMH Energy 75 200 $500
Power 45 800 $600
Sodium Metal Chloride
Energy 100 200 $300
Li-Ion Energy 120 600 $600
Power 75 1,300 $700
Ultracapacitors Carbon/Carbon 4.5 1,500 $10/Wh

Very broadly, one main distinction is between energy batteries and power batteries. An energy battery, simply, is designed to optimize the energy density; a power battery is designed to optimize the power density.

Tien Duong, vehicles technologies team lead at the Department of Energy’s FreedomCAR and Vehicle Technologies Program Office, noted during the ZEV Symposium that conventional lithium-ion batteries for power-assist HEVs “are about ready for commercialization.

Duong said that the DOE’s main focus now is to support the development of novel materials for cathodes, anodes and electrolytes that promises greatly increased power and energy.

For PHEV batteries, I would not rule out NiMH, but I would say that lithium-ion batteries are technically feasible. There are no batteries specifically built for this application that I know of. I would think that there are strong synergies with the development of PHEV, HEV and EV [batteries]. One thing we don’t know is the impact of real world operations during charge-depleting mode.

—Tien Duong

The new materials—as represented by A123Systems, but also by companies such as Altair Nanotechnologies, who was also present at the ZEV Symposium—offer designers more power or more energy depending upon the requirements.

A123Systems has a high volume battery product currently designed for power tools. (Hybrids are currently quite a small market for lithium-ion battery manufacturers, who have a much larger opportunity with laptops, cell phones and power tools. Power tools will outnumber hybrids by two or three times as measured by MWh, according to some analysis.)

A123Systems’s current product—the power tool cell—is somewhere in the middle of the range between power and energy. In other words, the A123Systems production cells put together by a PHEV conversion company such as Hybrids Plus in its Boulder PHEV demonstration aren’t really designed for PHEV operation. (Earlier post.)

Power is a function of the diffusion of the ions in the battery, notes Ric Fulop, one of the co-founders of A123Systems. The nature of the electrode materials and the electrolytes can all make a difference in terms of tilting the battery toward the power or the energy ends of the spectrum (or the area within the Ragone plot).

While that customization is possible from the battery manufacturers’ points of view, what they need are targets to hit: the specifications from the automakers.

And one of the best ways to achieve that—aside from the simulation work underway—is to get more trials out on the road that then feed back into shared project knowledge. Similar, for example, to what automakers and energy companies are doing for hydrogen in Europe. (Earlier post.)

So while the conversions of existing hybrids to plug-in operation might not represent the optimal in terms of battery technology or systems design, they do represent an extremely valuable potential source of real world data that needs to be fed back to battery makers and to automakers.




Good article Mike: It would be nice to see an up to date summary such as this on a variety of topics kept on a side panel.

P.S. All vehicles in my area are subject to emissions testing before they can be insured. I should have fun at the testing center when I bring in my EV scooter. One of the tests requires that they attach a hose to the tailpipe.

Adrian Akau

I think that ultracapacitor should be functioned in as part of the study. The high demands in power could be compensated, thus placing less strain on the batteries. I think we have to look at ultracapacitor improvement with just as much care as we do with battery development.

Amitai Palmon

I would leave the all-electric/blended mode decision for the car. Integrating with a navigation system and knowing the desired destination at startup time could help optimize mode switch based on distance, urban surroundings and traffic density.


I agree with Amitai. Most people use cars in a predictable way (to commute to work and shop). Using GPS you could say "it is 8 am Tuesday and the car is going 20 miles to work" The system would have learnt the route and would be able to turn the ICE on perhaps once for a long fast stretch. The would work even better with a diesel hybrid where you do not want to start and stop it very often - ideally once or not at all / journey.
The system could also know that the car will or will not be recharged at the end of the journey (if there is a racharging tsation at work).

In general, the combination of GPS and AI should yeild much improved battery management, optiising economy and battery size and life.
You could probably get by with a battery 1/2 the size if you had a smart battery management system.
- JM


What would the decision to use Li-On over Lithium be based on. Lithium, although possessing more power and energy per kg is more expensive per kw/hr. Does the decreased weight make up for the cost difference because the vehicle will, therefore, get better mileage per kw/hr. If so, perhaps there could be a table showing dollars per mile.

If Lithium is not cheaper per mile, considering life cycle costs, what would drive the decision to use lithium?

Rafael Seidl

t -

lithium batteries are far more susceptible to thermal runaway than those based on lithium-ion or lithium-ion-polymer chemistries. And as we have seen with the recent spate of laptop recalls, even Li-ion cells need to be manufactured very carefully to keep the fire hazard at an acceptable level. If the required tolerances are not met or the charge/discharge protection circuits fail, you end up with a small amount of metallic lithium in the cell which is liable to react with the electrolyte.

A123, Altair Nano and others have switched to special nanostructured spinel materials that greatly reduce the risk of metallic lithium formation. That is why they are stable even under extremely abusive conditions, such as the penetration test. Since cars do crash, it is essential that traction battery packs pose no greater fire hazard than fuel tanks do.

Harvey D.

Very good article. How can we convince our leaders to spend less money on oil wars and hydrogen but much more on batteries and super-caps R&D.

Batteries + Super Caps may be a worthwhile approach for PHEVs until such time as a more efficient cross-technology quick charge ESDs come around.

Many more A123, Altair, ESStor etc are required to accellerate the process and market lower cost ESDs. The key to practical PHEVs is better/cheaper on-board ESDs. The rest of the technology required is here already or could be developped quickly.


Rafael. Sorry. I mean Li-on compared to NIMH. What drives the decision to use Li-on over NIMH considering the higher costs per kw. Again, weight seems to be the main advantage, but does that make up for the cost difference?

Rafael Seidl

t -

nickel prices are actually high, as are those of some of the rare earth materials. By contrast, lithium oxides are reasonably abundant. Per Wh of capacity, Li-ion batteries would be lighter, use up less space *and* cost less. Safety concerns - and the overhead associated with addressing them - are the primary reason they have not yet displaced NiMH in automotive applications.


I like the idea of telling the car where I am going. I punch in "store" and I can get there and back home on electric. I punch in "work" and the engine comes on before I hit the freeway onramp.

allen Z

_Sodium Metal Chloride (SMC) looks like it may have a future as an excess electrical energy absorber. NaCl is cheap and common is seawater (OTEC or waste heat process), and may be just the ticket for power smoothing in solar concentrator electrical generation. Electrolyte thermal demands may be met by utilization of waste heat from power source (solar thermal concentrator), and adequate insulation.
_During the day, the excess power charge the batteries. At night, the flow reverses, and the juice goes to the customers. California (Nevada and the 4 Corner states too) may have all that they need to go largely off carbon when it comes to electricity. Then again, the 5,000 to 10,000 sq miles of land necessary might be hard to get, if the Dept of Interior and Dept of Defense are not involved.
_With scale of economy, the batteries may come down in price. On top of that, as demand increases, R&D investments for cheaper, better, longer lasting units will increase. This probably results in better SMC batteries.
_As to gas fired plants, switch large ones over to combined gas + steam turbine systems. The 60% thermal efficiency would be 50% above the current 40% average for large plants.


I would rather not make ANY conclusions and predictions about Li-chemistry batteries. As being most promising from the point of view of specific energy and specific power output, both cathode, electrolyte, anode, and all in-between chemistries are highly variable and subject to fast progress nowadays. The example is recently unveiled by Alatair Nano Li battery with Titanate spinel cathode. This battery is ABSOLUTELY safe, allows fast charge, is extremely powerful, allows deep discharge, have very good low/high temperature performance, lasts forever, BUT currently have inferior energy density. All these features are virtually unheard for any Li battery just couple of weeks ago. R&D to improve all mentioned parameters and especially energy density is underway.


We who spend time studying automotive technologies should not disregard how battery-oriented technology will affect travel patterns. If a technological advantage influences a change in how much or how far the average person drives, supporting an automotive technology that does not offer this benefit is disingenuous; much like Edison arguing that DC is better than Tezla's AC.

We drive too much, too far, for too many purposes, at too high cost and impact. Better we consider the technology that allows us to drive less, rather than the technology with the most bells and whistles and computerized apparatus.

Urban/suburban personal travel consists of 4 modes: walking, bicycling, mass transit and automobiles, not necessarily in that order. The automobile presents a severe impediment to the functionality of the other modes of travel, each of which are far more energy efficient than any automobile, and far more important to incorporate into existing and future development patterns.

Also important to consider is the value of battery power in the event of electric utility grid failure, the compatibility batteries have with rooftop photovoltiac systems, the advantage battery weight has in improving vehicle stability and handling.

Which automotive technology is most viable is incomplete without these more complex considerations.

Roger Pham

Cobasys, a company owned by Chevron, is now holding the patents on NiMh battery technology, and guess what? Cobasys has not allowed the manufacturing of NiMh battery in sizes large enough to be used in a PHEV, of course for obvious reason of protecting the oil company's petroleum interest. So, everyone, including Toyota, are looking into the use of Li-ion battery for the next PHEV. The issue is less of technology and more of turf war and financial interest.

Technically, NiMh battery is quite suitable for PHEV usage, being quite durable, more so than Li-ion and somewhat less expensive than Li-ion, but most importantly, potentially far safer.
However, A123, the developer of a nanotech Lithium battery technology, has claimed that their battery is much more durable and safer than current Li-ion battery technology.

Roger Pham

Allen Z,
Your idea of SMC battery is quite fascinating. Do you have the cost figure of SMC battery ($/kwh capacity and $/kw of power)?

Right now, NREL is counting on the use of thermal storage for storing of solar heat for night time usage. However, this thermal storage has significantly increased the cost of solar electricity over the version without thermal storage. If you consider that desert sun is quite reliable, and peak usage is more likely AC use in the summer heat, then little storage would be needed if solar electricity is used to supplement fossil fuel or nuclear power plant. Of course some solar energy storage would be necessary, since peak solar is over by 5-6 o'clock summertime, and yet, peak AC usage does not decline until probably midnight.

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