It's a common complaint: we'd like to be greener, but the cost is exorbitant. When it comes to plug-in hybrids and all-electric cars, it's an argument that has some justification – largely because of the cost of the powerful lithium-ion batteries they run on. So how can we reduce the cost of batteries without curtailing their performance?
Let's rule out just waiting: battery costs are expected to fall, but not fast enough to bring electric cars into the mainstream. But there may be a way for batteries to offset their cost by earning some money for their owners.
Some electricity utility companies and car makers are investigating the possibility of using electric cars to store power for the grid  when they are in the garage, with some form of financial incentive for the car owner. But all the additional discharging and charging would shorten the life of the battery – and if you've just paid out $40,000 on an electric car, that may not appeal.
Life after death
There may be a more palatable version of this so-called vehicle-to-grid idea. Used lithium-ion batteries that no longer hold charge well enough to power a car could provide electricity storage for the grid, says Sankar Das Gupta at battery maker Electrovaya , based in Mississauga, Ontario, Canada.
Currently the car industry considers a battery to have reached the end of its life when repeat use means it can only be charged to 80 per cent of its original capacity. That would still be perfectly acceptable for grid-scale storage, says Das Gupta.
His company is involved in a project with partners including Eric Bibeau  at the University of Manitoba in Winnipeg, Canada, in which they will combine used lithium-ion batteries from demonstration vehicles to build a 150 kilowatt-hour prototype storage system and monitor how it performs when connected to the grid.
Saving for later
With hopes for future energy supply pinned on intermittent sources of electricity generation such as wind and solar power, it has become more important to be able to store electricity . These sources cannot be turned on or off to match demand, so only storage can ensure that over-capacity is not wasted, and that the power stays on when the wind stops blowing and the sun sets. Utilities and researchers are already investigating a range of ways to solve this dilemma, including flywheels and compressed air .
So using second-hand lithium-ion car batteries would kill two birds with one stone, says Brett Williams  at the Transportation Sustainability Research Center at the University of California, Berkeley: helping car owners recoup some of the expense of their vehicle's battery, while simultaneously benefitting producers of renewable energy.
These repurposed batteries are likely to appeal more to energy companies than leasing time on ones still housed in a car, says Williams. "The stationary device can commit to be connected and available throughout the day," he says.
Das Gupta will present his idea at the Clean Technology  conference in Anaheim, California, next week.