often one cell in a NiCad array will get a reverse charge.
when that happens, that cell won't charge normally, and the other
cells won't completely discharge.
you can check this out by measuring the voltage across each cell. A
good way to rejuvenate the array is to completely discharge each cell
individually, by shorting out the cell with a 4.7 ohm resistor (or a
just a piece of wire, if it's nearly dead anyway.)
Then the array should charge up normally.
chet.