Is there a relationship between voltage and State of Charge of a NiCd battery?
Yes it can be driven above its 1.5V per cell, but that will damage it. How much above depends on the current you are forcing into the battery. The charge rate for a NiCad is right between 1.2 V and 1.45 V per cell. When not under load or charge, a Ni–Cd battery will self-discharge approximately 10% per month at 20 °C, ranging up to 20% per month at higher temperatures. It is possible to perform a trickle charge at current levels just high enough to offset this discharge rate; to keep a battery fully charged. This is affected by both battery condition and storage temperature. Try this link:
https://www.batterystuff.com/kb/articles/battery-articles/what-the-heck-is-a-nicd-battery.html