Since the weather has taken a turn for the worse I have broken out the freestanding oil fired heater.
This is 10 months old.
The unit works fine in normal operation.
The problem is that when the batteries become fully charged the digital timer clock gains time really fast so by 6am it shows 8am (Having reset the clock the night before)
I have checked and the multiplus increases the frequency of the output to regulate the fronius on the AC out.
This is fine but the appliance must use the mains frequency to determine the time.
I have tested the heater on a stable grid and it keeps time.
It’s just worth noting that it’s not just old AC wall clocks that have a problem with the frequency change.
System:
Multiplus 2 10kva
Cerbo GX
Fronius 5k Primo single phase
250/70 MPPT
Pylontech 20kwh batteries (4xUS5000)
[Tale as old as time] 
(Frequency shift affecting clocks - VictronEnergy)
Not much you can do about it. Off grid that is how the Fronius is controlled unless you move to node red. But frequency shifting still kicks in even then.
It is actually a major question we ask in system design - do you have any appliances that are affected by frequency shifting?
To solve it you would either need to switch to DC MPPTs or dedicate an inverter to device that needs a constant frequency.
Thanks, It wasn’t a question about what to do about it but just to bring to attention that some appliances still use the frequency of the supply to tell the time.
Where I am in France these are everywhere.
It is a very interesting experience.
One that is not usually considered.
It is a good reminder. Thanks for sharing.
So your clock gains 2 hours overnight? When the Fronius is shut down and not being regulated by Hz?
Sounds a lot like the difference between a Multiplus being set for 60Hz and not your clock’s 50Hz. Have you looked?
Exactly my thoughts, but John beat me to it… For sure at night there is no freq shift…
Not exactly two hours difference but yes the multiplus changes the frequency when the batteries are charged regardless of if the Fronius is producing or not. I.e. night.
Frequency goes from 50 - 52 hertz according to VRM but I haven’t put an oscilloscope on it.
It takes about two hours to charge the batteries 2200 to 0000
It is strange that it is shifting at night, when production doesn’t need to be controlled. There is another factor at play here.
What is the charge source during this time?
Multiplus AC in activated by cerbo relay using general flag and ignore AC.
No ESS installed.