The way I would like my EG4 18KPV to work is that the grid would not input any power until the battery charge falls to 20 or 25% (maybe in the middle of the night) and then it would turn on and charge the batteries.
What parameter(s) is it that would control this and what should I set it to?
The Ruixu pamphlet says "The Low Voltage cut-off" is 44.8v
Ensure that Battery Backup Mode is disabled, verify that you have sufficient solar and battery capacity to support your loads, and set the On-Grid Cut-Off SOC (%) to 20% or 25%.