By the mid-90’s automakers had pretty much given up on being able to go very far on batteries alone, which led Toyota to introduce the Prius—the first commercial hybrid—in Japan in 1997. In EV mode the Prius is powered by a sealed 38-module 6.5 Ah/274V NiMH battery pack weighing 53.3 kg. That works out to 1.78 kWh total capacity. According to the EPA’s formula, one gallon of gasoline is equivalent to 33.7 kWh—almost 20x what the Prius’ battery alone can deliver. So it’s hardly surprising that the Prius relies primarily on its internal combustion engine for propulsion.

The Chevrolet Volt features a much larger battery with a considerably higher energy density than the Prius. The Volt uses a 16 kWh (197 kg) manganese spinel lithium-polymer prismatic battery pack, which alone can power the Volt for 35 miles (56 km). The Volt’s lithium-ion battery is 2.5x larger in terms of energy density than the Prius’ NiMH battery (.0812 vs. .0319 kWh/kg). Considering that the energy density of NiMH is under 2x that of NiMH—140-300 Wh/liter for NiMH vs. 250-620 Wh/liter for lithium ion—that’s well on the high side of what you would expect.

In addition to having a greater energy density than NiMH—in terms of both weight and volume—lithium-ion batteries also display a much lower self-discharge rate; a greater maximum number of charge/discharge cycles (i.e., they last longer); a more linear discharge rate, which enables more accurate prediction of remaining capacity; and they perform better at low temperatures.

As far as durability goes, both battery types are about the same: NiMH batteries can be discharged and recharged 500-1000 times, with Li-ion batteries being good for 400-1200 cycles. Since replacing an EV battery pack can be a very expensive proposition—currently about $8,000 for the Volt—manufacturers typically guarantee them for an extended period. GM guarantees the Volt’s battery bank for 100,000 miles or eight years.

Not Your Dad’s Li-Ion Battery

OK, assuming your Dad had Li-ion batteries, the ones in the Volt are better. The Volt’s battery design is based on technology developed at Argonne National Laboratory. The Lab used x-ray absorption spectroscopy to study new cathode compositions. They came up with a manganese-rich cathode that resulted in a dramatic increase in the battery’s energy storage capacity while at the same time making it less likely to overheat, and therefore safer and easier to maintain. To complete the trifecta, the new cathode material is also cheaper to manufacture.

Even if there isn’t much beyond Li-ion in terms of energy density—unless you’re comfortable with a thorium-based energy source—there’s still room for improvement. According to Khalil Amine, an Argonne senior materials scientist, “Based on our data, the next generation of batteries will last twice as long as current models.” Chances are your car would give out long before your battery does.

Recycling

When your Volt battery bank finally sends you an End of Life notice, what can you do with it? For one thing you could keep it and use it to help recharge your new Volt battery. Or you might rig it to an inverter bank as a backup source of electricity during power outages or at least peak billing times.

If GM gives you a credit for turning in your old battery on a new one, what can they do with it? The EPA claims that rechargeable batteries are not an environmental hazard if they’re not dumped in landfills; European governments aren’t quite so sanguine, since Li-ion isn’t exactly something you’d like to wind up in your water supply. Both the cathode and anode material can be recycled, which is what most jurisdictions require.

In the end the Volt’s energy storage system turns out to be as high-tech as the rest of the car. Considering how much more reliable electric motors are than internal combustion engines, Volt owners could wind up owning their cars for a very long time.

[This article is part of a series on the Chevy Volt for the UBM/Avnet series Drive for Innovation.]

You Want Gas with That?

There are two basic types of hybrid drivetrains: series and parallel. Series hybrids have a gas engine that turns a generator that charges a battery bank that powers an electric motor that powers the car; the engine is not connected to the drivetrain. The Chevy Volt—which GM refers to as “an extended range electric vehicle (EREV)”— is essentially a series hybrid, though with a twist that we’ll describe in a moment.

In parallel hybrids both the electric motor and the gas engine are connected to the transmission through clutches that enable one or the other to power the vehicle.

Then of course there’s the series/parallel hybrid. In this configuration the two power sources are joined in a planetary gear set that enables either the motor or the engine to power the vehicle, or to share the burden as needs be. Despite being primarily an electric vehicle, the volt actually falls into this category. When you need rapid acceleration, the engine works in parallel with the motor until you let up on the accelerator. Also, the gas engine takes over from the motor when you exceed 70 mph. That’s an appropriate place for the motor – which has its greatest torque at low rpm – to hand control over to the engine, which generates maximum torque at high rpm. Besides, at 80 mph you’ve ceased being an ecopurist and are just in a hurry.

Both the Volt and the Prius are essentially series/parallel hybrids. The main difference is that on the open road the Volt relies more on electrical power and the Prius more on its engine. The Volt as a result has a considerably larger battery bank: 16 kWh for the Volt vs. 5.2 kWh for the Prius. Not surprisingly the Prius has a larger gas engine: a 1.8 liter/98 hp engine vs. the Volt’s 1.4 liter/80 hp engine. OTOH the Volt’s 111 kW (149 hp) electric motor can generate 273 lb-ft of torque, considerably more than the Prius’ 80 hp, 153 lb-ft motor. You might think of the Prius as a gas/electric hybrid and the Volt as an electric/gas hybrid.

Looking at the table, the Volt has about the same power as my Mazda 3, though it gets >3x better gas mileage—and infinitely more for trips under 35 miles, where it’s purely electric. It’s also a lot quieter and more fun to drive.

The Train is Leaving the Station

In late 2010 GM formally introduced the Voltec powertrain on which the Volt is based, though its roots go back to 2007. The basic design combines a small gas engine and a large electric motor that drives the vehicle, though they can work smoothly in tandem when it makes sense to do so. The large lithium-ion battery bank is designed to be recharged at home overnight—in 10 hours from a 110 VAC source or 4 hours from 220 VAC.

The table shows the basic specifications for the 2011 Chevy Volt. GM has announced plans to use the Voltec powertrain in other cars, SUVs, and even trucks, bring down the cost by using the platform across a much larger base of vehicles. Expect the Volt specs to scale for SUVs and trucks. Even Porsche is getting into the act, toying with the idea of an electric 911 (though not the Turbo GT2).

Maybe drivers won’t miss the roar of a big engine so much while they’re quietly zipping past yet another filling station advertising gas for $4/gallon.

Note: This article was first posted at http://www.driveforinnovation.com/get-on-the-drivetrain. Please check out the site if you’re at all into electric vehicles and follow Brian Fuller as he pilots the Chevy Volt across America–well, parts of it anyway. –JD

Debugging Your Car

The Prius brake problems are the case in point. Drivers have complained that the brakes on the 2010 Prius momentarily stopped working at low speeds, especially on slippery surfaces. Toyota said Thursday a software glitch is to blame for braking problems in the 2010 model. They’ll offer a software update.

The Prius and other hybrids increasingly rely on complex electronics to combine the regenerative braking with brake pads, so that battery recharging absorbs as much energy as possible. Getting regenerative braking, ABS brakes and conventional brakes to work together turns out to be a non-trivial problem.

This is the kind of software bug that you don’t fix retroactively with an emailed service pack. I can see a popup window on my LCD dashboard via a wireless link: “Updates are available for your car. Click here to download and install 2010 Prius SP3. Reboot may be required after installation.” Click download and look for a coffee shop where you can hang out while your car roboots. Let’s hope all the CAN and LIN bus drivers still work.

Meanwhile Back in Detroit…

Meanwhile, Ford Motor said Thursday that there was a problem with the brakes of the hybrid version of the Ford Fusion. Ford said customers could receive a free software update, but it did not begin a formal recall.

Ford said the problem in its Fusion was caused by the car’s unnecessarily switching between its conventional brakes and the regenerative brakes, which absorb energy while braking to charge the battery. The Prius also uses regenerative braking. According to the L.A. Times,

Ford said a software glitch on Ford Fusion and Mercury Milan hybrids built on or before Oct. 17 could cause drivers to perceive a loss of braking as the car shifts unnecessarily from regenerative braking into the conventional mode.

While Consumer Reports was test driving a Ford Fusion, brake failure almost caused an accident:

Consumer Reports said that with the regenerative braking disengaged, as happened in the incident near their track, the brake pedal needed to be pushed more than an inch farther down to engage the conventional brakes.

System-Level Design

As cars become increasingly computerized, these sorts of problems will become more common. There are a number of stringent standards that hardware manufacturers must meet to qualify for inclusion in automotive systems, but there is no similar “automotive qual” for software of which I’m aware (correct me if I’m wrong). There certainly doesn’t appear to be in any case.

This is one area where the immaturity of system-level design tools is a lot more than a nuisance—it’s outright dangerous. No extremely complex system is going to be completely bug free, but it’s imperative that none are released before all the “A-level” bugs are identified and eliminated.

The Prius and Ford Fusion problems are a red flag for the automotive industry. They need to get on top of the software and systems or there will be big trouble farther down the road.