In the past I have taken My NS down to 50% at much higher than 4.5 amps of the 20 Hr rate. Voltage rebounded to 12.32v while still under a 0.2 amp load.
NOrthstar PDF sheet indicates 50% OCV is 12.11v
I bet your battery performs better after the recharge.
Do watch the temp rise when feeding it the vector. The IR gun is accurate enough. I was surprised at the temp rise on mine@ 40 amps initial, but mine has some reflectix on it as it is exposed to engine/exhaust heat.
I will be removing the reflectix on the battery and make a simpler shield to deflect engine heat and that radiated from exhaust and Cat.