The case is printed in pieces to make the assembly easier. To start we have the cell holder which is responsible for holding the two 18650 cells and the hard ware that contacts the cells. It also has holes for screws that allow for the use of a lid. I bought 18650 cell contacts off amazon and I messed around with a few ideas as to how to connect them to the case and landed on heating the contacts up while on top of 3d printed squares. The metal will heat up and melt the plastic and allow for them to stick into the plastic squares. These squares will then be glued into the case and from there we can solder the contacts in parallel and run the wires to the charge controller.
The charge controller used in this project is one I designed but realistically any charge controller that monitors the battery voltage as its being charged and discharged should work. This is an important component to use as you don't want to over discharge or over charge 18650 cells. I am using a micro USB breakout port to supply 5 volts DC to the charge controller to charge the batteries but since the batteries are removeable you could charge them externally using a sled charger.
The cell holder is secured on top of the battery cover door using JB plastic weld. I'm sure its possible to print these two pieces as one but I thought it would be a lot easier this way. The battery door I am using isn't mine. I did however modify it to suit my needs. I found an STL file on thingiverse by a creator who actually made their own 18650 battery pack. They put in a lot of effort and I am really glad to be using it as its way out of my skill range to design. The reason I am using theirs is due to the quality and the fact that they incorporated perfectly aligned holes that match up to the Xbox One controllers battery tabs. If you place rounded head screws in these holes you can use them to make electrical contact with the controllers battery tabs.
Here is a link to the STL file of which I take no credit for. Xbox One Controller, 18650 Battery Holder, MK II by SirRawThunderMan - Thingiverse
Since a fully charged 18650 cell reaches 4.2 volts some of you may be wondering if that's a problem for the Xbox One controller since it was designed to use two 1.5 volt double A batteries for a total input of 3 volts. I had this thought and decided to power a controller using my bench power supply set at 4.2 volts for several hours. During this time the controller functioned normally and no additional heat was recorded so I concluded that it isn't that much of a concern. As with all projects, do them at your own risk.
Thanks for making it this far! :) Hopefully you enjoyed this project as much as I did!