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.

Usually on the battery case or tool spec if not the individual cells.

Let me repeat this... The capacity printed on the cell is its nominal
capacity. It is NOT the capacity of that particular cell. Individual cells
can sometimes differ 20% or more from this value -- usually on the low side.

It's important to match the capacity of nicad and NiMH cells, because they
have a very abrupt cutoff. This means that one cell can drop way low, while
the other cells are at a voltage high enough to continue powering the
device. If you let the device run long enough after this point, the weak
cell will drop to zero volts, then reverse, possibly causing it to leak or
explode.