Exactly that's what I'm looking for. I've seen such things for sale, but I want something that delivers more than 50% efficiency...
My use would be like this:
As an example those small round 5000mAh powerbanks. I have about 2 dozens of cells that fit into it. But those cells alone can't charge my stuff. So I could deplete the powerbank and swap cells.
Similarly when I'm on longer trip with my 2600 mAh.
Some my wonder why I just don't buy several powerbanks.
Well, as said I have plenty of cells, but depending on what I'm doing I use them for something different.
When traveling (flying) I would use them to keep my laptop charged.
When not traveling I put them in my power hungry flashlights.
Besides of that I like multi-functionality.