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I don’t ever recall it happening at night, one strange time it almost always happened (again only during the day) was when the fridge would shut OFF (not coming on) the inverter would shut off for 5-10 seconds then come back on, don’t know if that would give you a hint about something or not…
Sounds like when the load leaves the inverter, it allows the battery voltage to rise above the inverter input threshold voltage before the controller can correct it down to the bulk voltage level. The inverter sees the voltage correction of the controller and resumes to normal..
When the load is on the battery from the inverter, the controller is letting all of the current from the panels pass through unrestricted to assist the battery to supply the inverter. When the load is removed, the voltage can spike for a few nanoseconds until the controller responds and brings it back down to the adjusted bulk level. The inverter sees this spike and does what it’s designed to do.. Shut down if the input voltage goes too high..
Worn out batteries will allow this to happen, being the internal resistance of the battery is higher (less resistance) and it would be easier for the panels to push the voltage up. If the battery provided a better load to the controller, the voltage spike wouldn’t be so extreme..
I’d venture to say that your batteries are weak and you have the bulk voltage setting on the controller too near the input voltage threshold of the inverter..