If you use 9 volt batteries, because of the way the terminals are setup, you can link them together. Say you have 2 on the bottom side by side, the third will fit on the top and connect to a positive from one battery and the negative from another. They do this on the video with a cheap EBB pistol. If you did that then it wouldn't fit inside any stock, but you could tape it all together and wire it up. Seems like a lot of work instead of just buying a cheap 1500 mah battery that is always charged to have as a backup.