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

Just as they have for the last several years, portable devices will continue to drive growth in the electronics industry. Far from just handsets, the mobile internet—also encompassing “the internet of things”—represents a huge expansion of the semiconductor application space to include a wide range of wireless home entertainment, automotive safety and autonomous industrial, military and medical devices. The mobile internet promises to be 10-100x larger in unit volume than the desktop internet ever was.

Amerasekera distinguishes between two types of portable electronics: performance “hub” devices such as computers, multi-media devices, wireless hubs and PDAs which have 1W to 5W needs today; and distributed, largely autonomous systems with micro and nano watt needs. A typical autonomous system—for example, wireless strain gauges in bridges and aircraft wings—has a life expectancy of up to 10 years. Assuming such a device is powered by today’s typical 100 mAh cell phone battery, the average power available from the battery is less than 1 µW. That isn’t possible with today’s technologies.

The problem is that battery technology has been scaling at about 2x every 10 years compared to semiconductor technology, which scales 2x every 18 months. The gap between what portable electronic devices demand and what batteries can deliver will continue to grow. Don’t expect much improvement from the battery camp any time soon. “The energy density of lithium-ion batteries is so high that they’re really like small hand grenades,” said Amerasekera. There isn’t much left on the atomic scale that has a higher energy density and isn’t radioactive.

How Do You Manage?

Lacking more capable batteries, silicon performance advances require power management. A lot of very effective techniques have been developed over the last several years. At 65 nm leakage power was reduced 300x vs. what it had been at 90 nm through a combination of SDRAM retention, logic power gating, channel length reduction, logic retention, process/temperature AVS and dynamic voltage and frequency scaling (DVFS). At 45 nm new techniques were devised—including adaptive body bias (ABB) and Retention ‘Til Access (RTA)—that resulted in 1000x reduction in active power. Still, Amerasekera—like Jan Rabaey in his keynote yesterday—is concerned that we’ve run out of tricks at the component level that will scale.

According to Amerasekera, future advances in ultra-low-power electronics will come at the system level. He gives the example of running an FFT in software, which requires 28 uW. Running it in hardware requires only 1.6 uW, an 18x improvement. Dropping the core voltage yields a further 1.8x power savings, for a total improvement of 28x. The SoC running the FFT now draws < 1uA.

3D chip techniques have finally evolved to the point where they can help optimize bandwidth, power and area. Currently 3D means package on package (POP) or stacked die. FinFET technology now enables more dense dies, and and die-to-die interconnects—vias connecting disparate digital, analog and RF layers—are becoming…viable.

Renewable energy—wind, solar, hydro and heating systems—has tremendous potential, though they’re all faced with economic as well as technical challenges. Energy harvesting also has a lot of potential, but efficiencies of such systems are quite low, as is the amount of energy they can deliver. Still, there’s a place for them going forward.

Divide and Conquer

Basically, the mobile internet will need a variety of energy sources:

• Batteries for general functionality
• Storage caps for high current functions
• Energy scavenging for extended battery life
• Wireless power sources for connection to the grid

The mobile internet will also require intelligent energy management and control, including

• Highly efficient on-chip power processing
• Control of energy sources and delivery
• Management of power demand and access
• Unreliable energy sources (aka wind, solar, etc.)

The challenge for the next decade will be coming up with another 2-3 orders of magnitude of power reduction to meet the demands of an increasingly wireless world.

Engineers always enjoy working on interesting problems, and this one should stay interesting for years to come.