On a NiCad charger, NiCad's might get to 1.5 - 1.55v under charge, depending on their condition. As I recall, many of mine settle at about 1.35v off of the charger for a bit. What is the cell voltage on charge, and off. Sorry, I wasn't sure if the 13 volts is active charging, or after the charger clicks off. Either way may not be full charge. The auto charger may not be charging and detecting peak adequately for NiCad's. Of course the NiCad's may not accept a full charge and need to be replaced.
When I want to see if a NiCad pack is good, I cycle it a time or two on a NiCad charger and get an actual pack capacity. And a cycle or two may help condition the pack and raise the capacity.