This reminds me a lot of those tape cassette adapters many of us once had in our cars.
ReVolt is a system for powering battery-operated devices using USB power instead.
"One end is battery-shaped and goes into the battery compartment of the device you want to power, while the other end has a power connector," Luke Brennan, the inventor and a former Ericsson engineer, writes. "The two are connected by a thin ribbon cable, robust enough to deliver power to your device, yet flat enough to allow you to easily close the door on the device's battery compartment."
The ribbon cable then plugs, functionally but somewhat inelegantly, into an adapter for USB.
You can scale the batteries up with different housings to meet different battery size needs.
ReVolt is currently up on Kickstarter. At press time there was $15,584 in pledges on a $24,397 goal, with 26 days left to pledge.