An 1800 watt inverter will draw over 150 amps from your battery at full load. Depending on your battery bank, that may be too much current drain. If you draw too much current from the battery the voltage will drop and the inverter will shut down.
If you have a large battery bank, it could be that the wiring to the inverter is too small. Depending on the length of the wire you should probably use #2 AWG.
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