I have been spoiled for choice when it comes to batteries on older ThinkPads. At present, Framework laptops have only one internal battery that is rumored to be middling in terms of capacity relative to power consumption and not swappable without tools.
Assuming no other third parties release a battery pack for the Framework 16 expansion bay in the next 5 years and the expansion bay power delivery system supports charging input, I may consider building my own based on the guide: https://www.instructables.com/Battery-Powered-Raspberry-Pi-in-Repurposed-Laptop/
Mostly just a note to myself so I don't just forget about the work of a very dedicated Raspberry Pi owner to make SMBus charging work for him. I say this because I had long thought that making my own battery pack that reports battery percentage to the OS would be unrealistic.