804040 Lithium Polymer (LiPo) batteries are some of the most compact, lightweight batteries available on the market nowadays while still being able to hold up to 1800mAh capacity. This is almost as much as a standard grade 18650 battery, however its far slimmer and square design affords it an easier fit into specific electronics, as opposed to the 18650's exclusively cylindrical design. 

I wanted to design some extension battery packs for my VR controllers so I could charge them while I used them, for particularly long VR sessions. After considering how batteries add weight which affect balance and stability, and needing a design that could attach to a controller without obscuring necessary tracking components (or discomfort the controlling hand while in-use), I settled on this battery as my best option. I still only plan on using these to refill the controllers when they get low, as they are definitely a noticeable presence when attached, but it's largely ignorable in the displayed configuration. I use velcro to hold it in place, as the player's hand will prevent it from pulling free, and have found no movement from its position while in use. This case should print fine with PLA or PETG, though I chose the latter, as VR can be quite hands-on.

The first design had an output USB-c cable soldered into the setup inside the case, but I opted for a USB type-A port output instead. This means that cable failure can simply be replaced without the need to resolder anything, and even allows for use of magnetic connections, which are I feel essential for the longevity of your VR controller ports (if they ever pulled hard enough to disconnect the magnetic connection, just remember that could have resulted in port damage on a normal cable!)

With my setup, the type-C port can handle both input and output, but this way, I can charge the battery using the type-c as an input, and the battery can remain plugged into the controller via the USB type-A port, which charges both in tandem. Make sure your components support pass-through charging before attempting the same. 

If you use the exact components recommended with this project, the provided wiring schematic should indicate how to set these up. It's fairly straightforward, thankfully.

Obviously you don't need to use these for VR controllers yourself, that is simply a suggestion. Hopefully these prove to be useful in some way, to somebody out there.