[jandrese]

Unfortunately it is not that simple. All of the cells in a pack need to be balanced the same so it’s not as easy as just swapping in a replacement for a dead cell. Even if you replace all of the cells at once they need to be balanced.

The biggest time sink when building DIY battery packs is testing and charging every cell to the exact same voltage.

[criddell]

You should definitely replace all of the cells at once, even if you are just using drugstore batteries. If you don't they are not going to last very long.

Manufacturers are not being as careful as you are. They bundle batteries from the same manufacturing batch with no testing and rely on battery monitor and balancing ICs.

https://www.ti.com/battery-management/monitors-balancers/ove...