MPSolar 2424LV-MSD on a power trip... troublshooting.

Panel issue

I have had a call out to an electrician for better part of a month now and while they have told me they will try to get me scheduled as soon as possible I am beginning to think that they aren't willing to come out as far as I am for what seems to be a very small job.

Looking to this group to tell me if there isn't something very simple that I am not doing correctly.

I have a main panel with a split to a subpanel with a manual disconnect throw switch that used to be for a generator to power essentials. My plan is to turn the gen panel off at the main panel and manual throw switch, causing everything on the subpanel to be effectively completely "off grid" and power everything on the subpanel through batteries. My goal is to get past our local PGE shutdowns with battery backup. Currently I don’t have solar panels yet.. wanting to get this working before investing in the solar panels… but planning on 1000watts down the line for longer outages. I see this as a very large UPS for some essentials.

MPSOLAR 2424LV-MSD installed
2- 12volt 200AMP gel batteries
I have both a 100amp breaker and on/off switch between the batteries and the 2424 device.

I am plugging into power off the main panel to charge the batteries on its own breaker. (if it makes a difference, this is a GFI protected circuit)
My problem is when I go to plug in the outgoing into the subpanel. As soon as I do, it trips the GFI to the circuit on the main panel.

PDP so far.
--If I don’t have it plugged into grid power but battery power into sub panel I can power all the essentials off the batteries just fine so I don’t see me as overloading the 2424LV
--If only charging batteries, no problem.. so I don’t see me as overloading from that plug

--If I have Main panel sub panel breakers turned off, the manual throw switch turned off shore power and the 2424 not plugged in to grid, it powers my essentials just fine

the real tripper is…
Main panel to subpanel is off
Manual throw in subpanel is off
ALL breakers inside the subpanel turned off
And even then, it manages to throw the circuit to the grid power when I plug into the subpanel.

What am I doing wrong?

continuing to research and found this from TicTac...

Earthing considerations i.e. you must have a path to earth separate from your neutral return to ensure RCD/GFIs continue to work.

From this I am assuming it is the plug I am using to GRID power, being it is GFI. Two possible solutions, not sure which is best. (1) change out the plug to non-GFI.. (2) figure out how to "path to earth separate from your neutral return".

Recommendations?

I'd vote for #1.

Not sure how your panels are setup but usually the main panel is ground and neutral bonded and secondary panels are not as they get power from the main. So if you are now turning a secondary panel into a main you may need to look at that.

Thanks.... I was able to find a non-GFI plug running my garage doors and will use it.
now all running fine.

I thought about my answer above last night. yes.. I have come up with a workaround using the non-GFI plug. is that good enough? should I work to the sub-panel "ground and neutral bonded" as Monty talks about? to me, workarounds are a way of life.. as long as there are no safety concerns and they work long term and aren't just a short term answer.