This is a very academic and theoretical minded look at the issue that misses the fundamental point of a larger battery in vehicles that plug-in. Higher energy in a larger battery does displace more fuel directly, but what it really does is provide sufficient EV power, so that ALL ELECTRIC RANGE (AER) is actually possible.

The researchers here take the easy way out and make the gross simplification:

"for simplicity and fair comparisons we restrict attention to the range-extended PHEVs that run entirely on electrical power in the charge-depleting range and switch to operate like an HEV in the charge-sustaining range"

They simply take a Prius and simulate it as if it could drive the first 7,10,or 20 miles on electric power alone. But a small battery they end up advocating for could not actually drive the vehicle entirely on electric power alone, and has no real world AER. Hence, this is a fundamentally flawed study.

For instance, in order for a Prius to drive preferentially on electric power alone, it would need a 70kW or so battery pack, since the Prius car has approximately 70 kW of total usable power with the engine off. The C-rating for an energy-type battery pack is typically no higher than 5 or so, meaning that it can provide power at at best approximately 5 times its 1 hour rated capacity. So a 70 kW battery is going to have about 14 kWHr of energy on board. Approximately 60% is available for EV discharge within safe and durable limits, so we have a usable 8.4 kWHr of battery energy. A car like the Prius will use approximnately 250 Watt hrs per mile, so it would have an EV range of about 8.4/.25 or about 34 miles. If the vehicle had less energy, it would not have sufficient EV power, and the AER (All Electric Range)that the academics toss around becomes meaningless. The minimum real world AER for a Prius is about 34 miles. For vehicles with less energy on board the engine will be going on and off a lot and the kind of analysis that professors did misses the boat entirely.

Other things beyond just the power rating of the battery constrain usability of EV power in a plug-in hybrid. The drive motor power, the electronics, the gearing the thermals systems, etc, all have to be rated for 70 kW in order to make a Prius a vehicle that has a meaningful AER. Just the other day, the Seattle fleet of Plug-in Prius conversions demonstrated this exactly, getting no better mpg than the base Prius: http://www.autobloggreen.com/2009/02/24/disappointing-results-just-51-mpg-for-converted-phev-priuses/

A vehicle that is configured to have sufficient EV power to have a real world electric driving is known as an E-REV, like the VOLT. The benefits of E-REVs relative to PHEVs without a large battery and subsequently without AER capability (like the Plug-in Prius) was published in the GCC last year: http://www.greencarcongress.com/2008/02/gm-study-shows/comments/page/2/

One of the problems with E-REVs and PHEVs is that there is lot of hype and hope and positioning among opinion leaders, yet very few such vehicles actually exist, and very few people have actually driven one over a period of time and understands the real limitations, constraints and balances that need to be considered. The best academic work at this time is more likely to be hands-on the precious few fleets that exist.

I would have thaught that what you want is enough battery power to do all urban driving on E power at least.
This will generate huge pollution benefits for the community and some decent fuel savings for the driver.
For short runs, the car should run on e power anyway.

I think focusing on AER is going to nedlessly postpone the adoption of very useful, shorter range PHEVs as people wait for huge batteries.

It depends alot on how long the depression lasts. If it lasts as long as I expect then plug ins are fairly well dead this and next decade.

Alot of the people who would have bought a plug in now are unable to buy a yugo.

When people get used to the plug-in paradigm, they're going to want an all-electric range that matches their normal daily usage. So 20 miles might be great for drivers who live within 10 miles of work but drivers who go 25 miles each way are going to want at least 50 miles range. Charging stations at work will help immensely and will be likely be demanded by many employees. They will begrudge those times when the gasoline engine kicks in because they know that means they're that much closer to having to stop at a gas station.

All of us now are used to regular gas stops and paying whatever the oil industry decides they can squeeze from us. As a society, were so accustomed to it that it's hard to imagine alternatives. An example is the vast network of EV charging stations many envision to exactly replace the role of today's gasoline filling stations. I say there will be relatively few such stations and they will account for less than 10% of EV charging.

In 10-20 years there'll be a lot more appreciation for being able to avoid filling/charging station trips and for being more free from big oil's whims and greed. 90% of the time EVs and PHEVs will be charged at home or the workplace.

The primary goal of PHEV's is all-electric range of 60 to 70 km (37 to 43 miles) on a fully-charged battery pack. Given that most commuting is under 15 miles in range a PHEV version of the Prius or Ford Fusion will need a lot less fillups if you do most commute driving.

Mind you, I'd love to see Ford reduce the size of their HEV drivetrain so it fits in the current Ford Fiesta, though.

It might help to have car rentals with PHEV/EV for rent. If you are having your car repaired, you can get one with 50 mile range and try it out. Recharge it in your garage with 110v over night and you might find that it could make a good second car.

This is interesting. The case I try to make for 10-20 mile range on electricity alone has more to do with its effect upon land-use and development. We simply drive too much and too far. Households with a car that may go 10 miles on cheap electricity (rooftop photovoltiac) will drive less. In time, more routine trips can be made without having to drive. Walking, bicycling can be safer and convenient and mass transit more practical to arrange.

Or, we can live in a future world like Tom Cruise in "Minority Report", where in sausage-shaped hypercars magnetically glued to 'verticle' off-ramps on the outside walls of high-rise condos or leading to gigantic unsupported freeways hundreds of feet off the ground, we speak derisively about "the Sprawl" and escape from Futurecrime Authority forces in completely computer controlled cars may not be possible. Thanks a lot, Spielberg!

Engineer-Poet,

The Saft cell you refer to is not a high energy cell, and it's design is for high power- A Crate of 14.5 reinforces that. Looking a the practial details, this cell has a only a usable 3.5 Ahr. To make your low AER Prius with sufficent electric power for 70KW, you indicated that you would need a usable 5 kWhrs worth of cells. Your 5 kWhrs pack would need to have about 500 of these cells, with 500 intercell interconnects and 500 channels of cell monitoring and/or control.

By the way, this is a Lithium Cobalt chemistry and is intended for military use. By the time you were done with the pack it would be more expensive than a pack made of cells designed with a safer automotive usage chemistry, and in the right cell size.

This further supports the idea that the academic study didn't comprehend the practical nature of plug in vehicles when they argued that a low AER was a good thing for cost reasons.