Not that easy. They have a battery management controller which contains cell charge data and a fuse which bricks the pack if it’s not handled properly.
Have a good look around on the internet.
Have a good look around on the internet.
It's a lot of work, but the validation was all automated via the charger. The pack I ended up with had healthy cells all within 10% of each other in terms of capacity.
You can get a cheap analyzing slot charger ($30-40) and do it yourself. That won't be quite as accurate as a big expensive industrial battery tester, but it will be accurate enough to make safe battery packs for consumer electronics.