In theory, it's a wonderful concept: once their tenure in an electric car is over, lithium-ion battery packs with as much as 70 percent of their energy capacity remaining can be sold and reused.

In practice, however—especially at scale—the idea turns out to have some practical challenges that must be overcome.

At the simplest level, a battery pack from a wrecked or dismantled electric car could be paired with photovoltaic solar panels to let a home or building not only generate solar energy but also store it for later use.

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That may one day become a practical, low-level example of reuse for such packs, especially once enough of them are available for a secondary market to be sustainable.

But as a recent blog post from consulting firm Navigant highlights, some battery packs are turning out to retain their energy capacity much longer than anticipated.

One example is the 2012 Chevrolet Volt driven more than 300,000 miles by auto millwright Erick Belmer on a cross-state Ohio commute to his job at a GM plant.

2012 Chevrolet Volt crosses 300,000 miles, March 2016 [photo: owner Erick Belmer]

After that distance, Belmer says it has exhibited no apparent degradation in either its battery capacity or its electric range.

Few of the 1 million or so plug-in electric vehicles now operating globally have yet been wrecked or dismantled.

But the total energy capacity of available second-life packs could reach 11 million kilowatt-hours within 20 years, Navigant says.

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To make stationary energy storage reuse practical for a stream of used batteries, standard protocols must be developed for screening, testing, and grading the condition and energy capacity of each individual battery pack.

Even if batteries can be assessed and rated in a standard way, however, utilities and other large-scale energy storage users face what Navigant calls a "vexing" question.

Specifically, at what price and in what condition would a heterogeneous stream of used packs make more economic sense for long-term use against a large-scale buy of brand-new lithium-ion cells or packs?

Mercedes-Benz energy storage system

With the price of lithium-ion cells anticipated to continue falling at roughly 7 percent a year, how much discount would be required to make a used pack with a shorter projected remaining life cheaper than a brand-new one with the latest cells and hence a longer life?

Some utility executives, however, have suggested in conversations with Green Car Reports that sourcing hundreds or thousands of individual used packs wouldn't be worth the uncertainty and effort required for a giant company like theirs.

It might require a third party to assemble thousands of such packs, and resell them with a warranty on their future capacity and performance.

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Navigant suggests that automakers may be uniquely positioned to do just that, capitalizing on their deep knowledge of use and durability from tests on the batteries they will have fitted to an ever-expanding volume of plug-in electric cars.

But that would still require the makers to set up a new business to assess, purchase, store, and sell arrays of used battery packs to energy-storage customers.

Other observers have suggested that local or regional solar-panel installers may soon start to offer the addition of used battery pack as an extra-cost option for new arrays, possibly offset by the need for fewer panels if energy can be stored on-site as well as fed back into the grid under "reverse metering."

Photovoltaic solar power field at Volkswagen plant in Chattanooga, Tennessee

Navigant recent published a research brief, Alternative Revenue Models for Advanced Batteries, that looks in detail at the secondary market for used battery packs.

That report is priced at $1,700 for one to five users, and $2,550 for an unlimited enterprise license.

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