Took apart my dead power bank to fix it and finally understood the cylindrical vs pouch cell thing

I went through the same thing with a couple of banks I rebuilt last winter using cells I'd pulled from an old laptop pack. Kept the original holders and just matched capacity on the replacements, tested each one with a cheap cell checker before wiring them in, and both units have been fine since. The round cells are forgiving that way, you can swap one without butchering the whole assembly.

The slim banks I've torn down with flat cells inside were a different story. Those cells are usually glued to the board or wrapped around each other, so replacing one means prying, bending, and hoping you don't punch through the wrapper. I killed one doing exactly that by shorting across the top with a pair of needle nose pliers, wich was a dumb move, so now I tape anything conductive near those cells. On the space question, I think it comes down to what the case was designed around. A chunky brick with room to spare makes cylinder cells the obvious pick because they're dirt cheap in quantity and consistent. A pocket-slim bank realy does need the flat stuff to hit the thickness the marketing promised. Both approaches work, they just suit different shapes, and honestly the cylinders age better since replacements are everywhere.

I acutally went the other direction with mine. When the pack in my old Anker-style brick died I rebuilt it with two flat pouch cells I'd pulled from a defunct tablet, stacked flat with thin foam between them. Got the same capacity in about half the thickness, which mattered because I wanted it to live in a jacket pocket. The trick was figuring out the polarity on the tabs with a multimeter before soldering anything, since pouch tabs aren't labeled the way a canned cell's button is.

The tradeoff showed up a year later though. One pouch started puffing slightly and there was no swapping just that cell, it was heat-shrunk and glued to its partner, so the whole pack came out and got rebuilt. With cylinders you can pull one and drop in a match, like the laptop-cell rebuild above, but pouches are all-or-nothing in my experiance.

One thing I'd tell anyone tearing into one of these: be extra careful prying around pouch cells, since a dented or punctured flat cell can go into thermal runaway right on the bench, and I've seen a swollen one hiss when a plastic spudger slipped. Cylinders shrug off a lot more abuse before they complain.

So my take is the opposite of yours on form factor. Slim bank, pouches win on space. Chunky brick you might open again someday, cylinders win on serviceability. Neither is wrong, it just depends what you value.

I ussually run with the round cells because the consistency in internal resistance makes planning easier. When I finished building a bank for a remote cabin last winter, I made sure every cell was rated under 30mOhms. You have to pay attention to that spec, especally if you plan on pulling continuous high loads. Since the metal casing is just part of the design, it's much better for overall thermal management than trying to pack flat cells so tightly that heat just builds up inside the plastic housing.

I've been looking at doing somethig similar with a couple of old flashlights I found lying around the garage, trying to repurpose them into small portable chargers. I was thinking of using a mix of diffrent brands for the cylindrical ones because they were just the first ones I coud find cheaply. It's kind of making me worried about capacity matching. Do you think I should really stick to just one type of cell, or is there a real chance I could get away with mixing them, assuming the size is the same?