What is the lower limit of NiCd cell voltage?

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I have some old NiCd cells, and I have a bench power supply.

From what I read about NiCd batteries, I understood that a psu alone can’t charge them fully because it can’t measure the delta V.

I saw some circuits for that, but I couldn’t understand how do they work. Can you explain?

I also have old fast 1 hour NiCd charger with voltages 7.2-14.4. (Does it mean I can connect some cell combinations in between these ranges? Like 7 cells for 8.4V).

Another question: after I fully charged a battery of 6 cells, their voltage was 8.5V. I wanted to know how good they are, so I connected them to a resistor and monitored the current and voltage, and started a stopwatch to measure the time. The resistor draws about 1A, and I thought if the battery will supply 1A for an hour, it has 1Ah capacity. But the current is slowly dropping and I don’t know how to calculate that way, and I don’t know at what voltage to stop discharging. I still monitoring the volt/amps of the battery by the time I’m writing this question.

The battery supplied 1A for 25 minutes, and then the current slowly fell down, and now, the voltage is 6.13 and the current is steady at 0.77A for a while.