Presuming you don't toss it over the nearest cliff, what do you do with a used electric-car battery pack?
Even after battery capacity has degraded too much for use in cars, lithium-ion cells still have usable storage capacity.
Now, BMW plans to put those used batteries to good use.
MORE: Electric-Car Batteries: What Happens To Them After Coming Out Of The Car?
The German carmaker is working with Bosch and Swedish utility company Vattenfall on a project to repurpose used battery packs for grid energy storage.
BMW, Bosch, and Vattenfall Second Life Batteries project infographic
Called Second Life Batteries, the project will begin with a prototype energy-storage system in Hamburg, Germany, according to Transport Evolved (via Charged EVs).
Storage systems like this one help balance the grid by absorbing excess energy during periods of low demand, and discharging it when demand picks back up.
The batteries in these storage stations can also be charged using renewable sources such as solar and wind--compensating for some of the unpredictability of weather conditions.
The Hamburg project will use recycled battery cells from more than 100 BMW electric cars, most likely retired ActiveE models from the pilot program that preceded the launch of the i3.
Those cells will provide a storage capacity of 2 megawatt-hours, and peak power output will be 2 megawatts. The facility is to be designed and managed by Bosch, and operated by Vattenfall.
ALSO SEE: California Energy Storage: Why It's Good News For Electric Cars
The three partners expect the facility to be operational by the end of 2015.
If the project proves successful, it could provide a useful outlet for the battery packs from today's i3 electric cars and i8 plug-in hybrids as their automotive service days wind down in future years.
But BMW is hardly the first automaker to get involved with a battery-recycling project, and it surely won't be the last, either.
2012 BMW ActiveE
Used lithium-ion cells will become more common as electric-car adoption increases, but the low cost of their materials makes just breaking them down for scrap economically unattractive.
Energy storage, on the other hand, is gaining traction and creates multiple new business opportunities.
It offers utilities a way to take stress off their infrastructure, and proponents of renewable energy a way to make them more practical to integrate into a grid designed for huge fixed generation plants burning hydrocarbon fuels.
Photovoltaic solar power field at Volkswagen plant in Chattanooga, Tennessee
California will soon implement an energy-storage mandate, which will see the state's three largest investor-owned utilities buy more than 200 MW of storage capacity.
That could make use of lots of electric-car batteries, and the concept shows how the automotive and utility industries could become increasingly intertwined, as both begin to rely more heavily on battery technology.