Just to add, it's sometimes cheaper to buy a different make of
battery that may be on offer and rob it of its cells to fix another.

This is not generally a good idea. Replacing the bad cells in one battery
with new, good cells, is likely to cause cell reversal during operation. You
should generally replace all the cells.

It's also useful to have some way of measuring each cell's capacity while
discharging it, so you can roughly match the cells' capacities.