I am wondering if I could implement something akin to Lenovo’s old powerbridge (https://share.sketchpad.app/23/80d-6154-ba78a1.png) wherein there are 2 batteries connected to the laptop (one at a time). When the first drains sufficiently, a switch is made to the second battery. When a charger is connected, the second battery is charged to a certain threshold and then a switch is made back to battery 1, which is then charged to a certain threshold.
I am at this point too ignorant to implement such a thing, but I figured that someone on here might not be and could either tell me I’m a fool or point me in a direction. 
Failing that, perhaps we could get an update on the external USB battery case that was teased a couple of times? I’d love to just buy one of those and call it a day (assuming that hooking up a battery the normal way and also via usb might achieve the same result as my proposed design).
(I’m really trying not to burn my house down by building a battery with 18650s and crossing my fingers.)