Using 18650s to extend cell phone battery life -- easy hack?
 

The idea is to extend or greatly extend battery life, in order to have more time off the grid (on long tours for example). I realize that there are other approaches (solar and dynohubs, for example, which do not appeal to me at moment -- please let me know of other possibilities there might be).
Most cell and smart phones seem to run on 3.7 volt lithiums. Mine are 3.7 volts and 900mAh.
I already have charging cables with USB on one end and micro USB on the other end, which charge the phones off a laptop USB or other USB.
So one approach would be to find a 1x18650 case or holder fitted with USB (or simply with wires that could be fitted with USB).
Does anyone know where such cases can be found?
(Or any other ways of doing this, preferably with 18650s, but other ideas are welcomed too.)

Did you previously post here as 'Merton'?

With all the lithium ion external packs available, this sounds a little extreme.. I have used the apc mobile power pack for extra capability on long rides

http://www.ebay.com/itm/APC-Battery-...item20be79030d

Duracell also sells one that is a little smaller but same concept..http://www.ebay.com/itm/Duracell-Rec...item2568d7c825

or go the cheap route and use AA cells: http://www.ebay.com/itm/380311445157...84.m1438.l2649

the mintyboost uses AA batteries, I'm not sure what the maximum input voltage really is