Also, if you put two more NiCd batteries in series which are not at the same charge level, the higher voltage battery will discharge into the lower voltage one. Then, as time goes on, both batteries might end up going flat.
My solution has been to keep batteries in groups and to always keep those groups together. Charge and discharge/use the groups together. Having two or three groups of batteries also helps them to last longer. While one is charging, one is being used and the other one is "resting" in reserve.