Storing Volts

30 September 2011

By John Donovan, Low-Power Design, for Drive for Innovation

While electric vehicles have been around since the late 19th century, they only became practical with the development of energy storage systems that sport a lot better horsepower-to-weight ratio than bulky lead acid batteries.

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 lithium-ion—140-300 Wh/liter for NiMH vs. 250-620 Wh/liter for lithium-ion—that’s 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 500-1,000 times, with lithium-ion batteries being good for 400-1,200 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 you Dad had lithium-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 lithium-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, a senior materials scientist for the Argonne National Laboratory, "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.

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 our 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 Environmental Protection Agency claims that rechargeable batteries are not an environmental hazard if they’re not dumped in landfills; European governments aren’t quite so sanguine, since lithium-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 own their cars for a very long time.

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