I have just found that another one of my LiPo battery packs has started to develop a belly, and I'd like to replace it with one where I can just swap out the cells rather than junking the entire unit. I haven't had much luck finding something that meets my, I think modest, needs.

Needs:

• USB 5V power (enough to charge one modern cell phone, slowly)
• replaceable cells (18650s would be ideal but I'd settle for something that took AA or AAA batteries too)

Nice to haves:

• USB-A and USB-C (though I can manage with either - I have a ton of cables for both)
• Fast charging (the more watts the better but all I really need is ~10W at most)
• Passthrough charging (can charge and keep the output hot at the same time)

Sky Pie:

• Hot swapping cells (can selectively discharge one cell or bank of cells, then switch to the other while the dead cells are being replaced)

For that last one, I think if I can find a pack that just meets the "Needs" requirements I can rig up two of them and a raspberry pi to handle the charge monitoring and source switching. I just would like something that uses commodity hardware like the 18650 to store the power, can deliver enough voltage to run the pi, and is designed to have the cells be user replaceable. Is there something like that on the market?