There are two easy ways for a critic of electric vehicles to deflate the eco-pride of a Tesla or Nissan Leaf driver: Pointing out that lots of electricity is produced by burning coal, so it's hardly clean, and asking what happens to the batteries that can't be recycled. They only last a few years, and their disposal comes with a significant environmental cost—one that may offset all the good EVs can do.

The dirty energy question can be answered with solar panels, but the battery issue is trickier. The best bet is reusing them, which comes with the potential to spawn entirely new industries specializing in old EV batteries.

To prove it, General Motors is now using five sets of batteries from aging Chevrolet Volts to help power a new data center at its Milford Proving Ground.

It's certainly in part a public relations stunt (look how we're solving this problem our product generates!), but one with a practical application. Over time, a battery begins to lose its ability to quickly discharge power, something that's important for a car to, say, go uphill. Other purposes, like powering a building, don't require the same level of discharge speed. So, when a battery is no longer good for taking you from 0-60, it can still keep the lights on just fine.

John F. Martin/General Motors

In 2013, there were some 70,000 electric cars on the road in the US, according to the Energy Information Administration. Sometime in the next few years, their batteries won't be good for powering them anymore, though as much as 80 percent of the storage capacity remains. At least that gives us time to figure out what to do with the things.

"Most used EV batteries will end up being used in a variety of ways related to stationary energy storage," says Chelsea Sexton, an electric vehicle advocate. Battery storage is key to making renewable energy work on a massive scale. The grid is set up to deliver electricity immediately upon its generation, so utilities produce exactly as much as is needed at any given moment. That doesn't jibe with solar or wind power, which can't just be turned on when necessary. You need a way to hold on to the energy they produce, and deploy it later, on demand. You need batteries.

Use solar or wind power to charge giant stacks of batteries when it's sunny or breezy, then drain them down as needed when it's dark and calm. Tesla's recently introduced home battery scheme addresses the same idea, only using new batteries instead of used ones from cars.

GM's idea isn't viable on a large scale quite yet. "There just aren't that many used batteries," Sexton says. The average life span of an EV battery is estimated to be eight to ten years. The first Volts will turn five this year. So while lots of ideas are being kicked around for what to do with all these batteries, we'll only see small deployments and demonstrations for the next few years, she says.

In the GM proving ground, the batteries will be combined with solar and wind power to generate 100 Mwh of energy annually. It's a small installation, but a great proof of concept for the used batteries that should begin flooding the market over the next few decades. The batteries will store enough juice to power the building for four hours in the event of an outage.