question

Just lower the amp rate of the inverter(max amps of incoming amps). I'm not sure if you have a cerbo or other display. I have cerbo and have power assist turn on. And it is easily adjusted from touch screen.

. If you set it at 30 amps it will try and charge as well supply boat. If you lower it to 15 amp max(Just using 15 amp as a example). then the inverter will use batteries to top up your requirements.

And it will go back to charging when demands are low. If the line voltage is dropping to 105vac then definitely turn power assist on.

You have to watch and make sure your batteries will still be getting charged.

I plug my 30amp RV into a 15 amp plug on house. So I lower the max amps to whatever the house breaker is.

Hope this helps

If the voltage drop you are experiencing is do to YOUR loads, then power assist can help by drawing less from shore power and taking battery power to make up the deficit. That is assuming you have enough energy in the battery to cover the period of high current draw.

If however the voltage drop is due to others at the marina then you can reduce your shore power draw to 0 and still have too low a voltage. Power assist can't raise the voltage at the inverter output because the input and output are tied together when shore power is in use. Power assist can only inject current into the stiff voltage provided by the shore power.

If the shore power voltage drops below the value set as a minimum input voltage, the inverter will reject it and begin inverting, providing all AC load power from the batteries. At this point, the AC output voltage will be what you have set it to be (120 volts is the default).

Another solution is to install a transformer with an adjustable tap for the input so you can boost the incoming shore power up to a more useable voltage.

Thanks, I am using a Cerbo GX and have lots of house battery available. At idle the voltage is 115 but when I turn on any type of load it drop to 105v. I will turn on Power Assist although it sounds like it won't get my voltage back up to 120? I noticed I'm also at firmware v430 and v502 is available, is that a 'recommended' upgrade? Thx.

Isn't the power assist voltage limited to the incoming voltage?
As the inverter will need to synchronize!

With shore power active, the output voltage will equal the input voltage. So you will not get 120 volts with 115 volts of shore power. But power assist will probably prevent future sag assuming you can find a balance between low input voltage and an input current limit that will allow the batteries to recharge after the heavy load is removed.

It seems you need to address the 115 volt shore power issue. You might look into automatic voltage regulators. These are essentially autotransformers that change taps to produce the desired voltage at its output with varying input voltage. One such device:

https://www.boatandrvaccessories.com/products/surge-guard-10176-rv-voltage-regulator-30-amp

Another option is to install a separate battery charger and run the Multi/Quattro in invert only mode when input voltage is below minimum. You can probably do this automatically by setting the minimum AC input voltage to whatever is acceptable to you. The inverter will then reject the incoming AC below that voltage and switch to inverting mode.