Now that the 14500 version is working, I updated the 18650 design with the necessary changes and ordered boards from OSH Park. I named this version UPS-18650-NS for consistency with 14500 naming -- the NS stands for "No Switch". The schematic is only slightly different than the UPS-14500-IS: deleted the input switch, changed the charger to 500mA/6.5 hours. There are a couple more capacitors at the microUSB input and the booster input, but they may not be populated. C12, the tantalum capacitor at the microUSB jack, is there to prevent large transient voltages when a wall wart is hot plugged into the UPS (I never intend to do that.)

The layout has gobs of empty space filled with metal buses. And the ground plane on the back side is more extensive. The metal helps to carry the heat away and keep the temperatures lower. The wide buses also keep power losses to a minimum. I'm really happy with this one.

And the obligatory 3D view:

I also ordered a solder paste stencil from OSH Stencils. This time the stencil is 5mil polyimide, instead of 4 mil steel, and I shrank the stencil openings on the MSOP-10 pads. Hopefully, it will prevent the solder shorts that have been a plague on that part.