It would be great to develop a new add on product to the locks to utilize the USB ports. The product would be a device that you permanently install to the bottom of the lock with a USB cable plugging directly into the USB port. The device would have a rechargeable battery pack and the front of the device would be a solar panel to keep that battery pack charged. This would allow for the lock to be a full power and not worry about replacing the battery.
This should be an option as not all devices are installed in an area that has available sun light.