Also check this out: https://www.thingiverse.com/thing:3110308
I had some cheap 18650 powerbanks that came to me as gifts. I use them from time to time to power some gadgets. The 18650 batteries it holds are crap as it can only output about 800mah. So I decided to harvest its electronics (134N3P module) and put into action a creepy 26650 5000mah lithium battery I had around. This design is a box to hold them together. The electronics are almost standard in lots of those cheapo powerbanks, consisting in a small board with a normal USB and a microUSB plugs, they are easy to find, if you are in the business you know what I mean. Put in the electronics, make contact with the battery, close the box with the sliding lid and job done, 5x times the original charge capacity. Recommended print in ABS, as the cheapo electronics could get warm and soften the PLA.
TODO: 32650? 18350? 16340? Double? Customizable? Desktop model with Current/Volts LCD? Wings?... (Hmmm no, no wings. Let's hope it doesn't 'fly away' ;-)
UPDATE (17-OCT-2016): Added pics. New fixer part added to fix in place the electronics. Also added a box model with ears to help with warping. This part tends to warp a bit when printed in ABS. Just remove the ears with a knife or something when finished.
UPDATE (11-AUG-2016): Introducing V3, similar design but included a more convenient sliding lid. Older version removed. Older pics removed.
UPDATE (11-AUG-2016): Minor changes to enhance board fitting, added a cap to the lid to press fit the board over the USB connector. V2 is now the only supported model. Removed older versions. Updated STL's and sources. Anyway I keep some pics of the older models as they are similar.
UPDATE (10-AUG-2016): V2 added. 26650 label etching, Lid with just one removal hole, added breathing holes to the box & lid also to check LEDS, fillet box edges, new V2 pic added. Updated FreeCAD sources, older model sources removed 'cause it is easy to revert changes to V1. V1 and V2 parts are interchangeable.