As it says on the tin ; all most people really need is a Single USB-C for power/Display and wired network these days. Since all the ports are PD capable it should be feasible to build expansion card form factor mini battery extenders to sit in place ; makes more sense to me than a passive space saver slot.
I guess the trick is finding a reasonable Mah cell form factor that fits into the space contraints and a suitable USB MCU to pair with it.
It’s been discussed before, but pretty much discarded because the amount of battery you could fit in would be completely negligible 
An expansion card has around 3 or 4 cubic centimeters of internal volume IIRC.
The Framework 13’s battery has a volume of 133 cubic centimeters.
So an expansion card can fit ~2-3% as much volume as the main battery. Based on the main 61 Wh battery (which is already LiHV, so pretty high energy density) that calculates to being able to fit maybe 2 Wh max of capacity.
And that’s ignoring the space taken up by any of the circuitry to manage the battery the charge the laptop. Accounting for that I would be impressed to see even 1 Wh fit into an expansion card. That’s maybe a couple extra minutes of runtime.
Hrm I have seen 7.4V 1000mah spicy pillows that you could stack and a little USB-PD controller providing 6 Cells 6000mah I wouldn’t scoff at that
I highly doubt that those are real and aren’t way too large to fit.
If even a single one of those could be squeezed into an expansion card that would be a density of 2 Wh/cm³.
Extremely high density batteries are less than 0.5 Wh/cm³.
Sure actually holding 1000mah for any length of time is unlikely. But short lived storage in tiny sizes is definitely real. Gettng USB-PD’s down to size is easy enough I have AAA batteries with USB connectors i.e and the PS4 Dualshock packs are pretty similar size to the FW card.
And if you give up ‘Just’ fitting into the expansion slot, there are plenty of thing 6000Mah packs that wouldn’t raise the base beyond the rubber bumpers (but I would be leary of them blocking vents in that case)
Yeah the secret ingredient in those is lying.
Are you mixing up ma(current) and mah(capacity) here?
One that can actually put the contents of it’s magic battery into the laptop ad any reasonable speed is still going to take up most of the expansion card but with magic batteries that isn’t really an issue XD.
I am actually kinda considering making a ridiculous battery slice for the framework once I got it for a meme (basically just a layer of 18650s below the laptop). But if you want to just use the space between the body and the feet, even if you don’t care about the vents that is not a lot of volume and would require very thin cells.
A battery in the expansion bay makes more sense, the cards just don’t have enough volume with current battery technology
Could work if you are willing to let go of one side of ports on the laptop and have a bar on the side.
Despite ‘magical’ battery inferences ; these cells do exist I have several 3.7 500mah that fit that I use for MCU projects . My 1000mah was based on parallel 3.7v cells to bring up to a usable 7.4v for feeding to a PD controller. I’m not sure If you could get more than 2 of the 500mah tho in the space; as ideally you would want 2 in Paralell and 2 in Series. Definitely if you were prepared for a bump at the bottom to bring it inline with the rubber bumper it would be possible to fit a 2000mah (total @ 3.7v) pack.
Yeah I’m not keen on Bars along the side ; there are ultra-thin PD chargers of 6000mah that possibly could be used as clip unders that would be better if you are going that route.
What are the actual dimentions of those packs?
The 902030’s I have are 9x20x30mm
That is 5.4 cubic centimeters (more than an expansion card) and only contain 1.85 Wh of energy (3% of the main battery).
Plus, you need room for a step-up convertor, USB-PD controller, and li-ion charge (recharge) controller. Then there are conversion losses. In the end, even using every Expansion Card slot would still net you so little that it just doesn’t feel worth it.
Whit that little capacity a single cell 5v only power-bank controller would be more than enough so no need for 2 in series, that’ll just gain you the extra privilege of needing a balancer too and those don’t use 0 space. And even with your magic cells there are 22W pd chip-sets that use single cells and work quite well but there the electronics would fill a significant part of the volume of the card.
With a single 500mah cell which might physically actually kind fit into the card you already be pulling 2 to 4C doing a measly 5W (which to be fair is more than enough to drain the thing really quickly).
Yup - those are pretty much what I was working with assumptions wise.
So back to looking at a ‘clip under’ solution.
There are 6mm width 5000mah (3.7v with stepup to 5v - so lies) banks around. I’m not sure what cells they use and they don’t seem to be available with PD controller so are ‘dumb’ ; but two of these with the cells ripped out and a custom case that fit into the Expansion ports to contain a PD controller and regulators; possibly wouldn’t raise the laptop above the rubber bumpers. Which seems to be a better idea.