This is a neat way of doing a distributed solar PV system.
I question his claim that only a Victron inverter can charge a battery from the AC…
@Anton and myself run our systems like this with no grid to ensure we don’t draw a few watts from the utility.
Indeed, I think there are at least three. Victron, of course, but then also the SunnyIsland can do this (as far as I know, but I don’t know if it works with non-SMA PV inverters), and I think… Selectronic (more popular in Australia) can also do it.
Edit: I seem to remember Schneider also has a “grid forming” inverter you can use with PV inverters.
Lets not forget the host of Axpert/Kodak clones that can do that as well…
They can? You sure? At least the early ones couldn’t… they’d blow up if you tried it.
I guess what I am saying is you can plug it into the battery… set the settings and use AC to charge the 48v battery. No PV needed.
I think we’re talking about different things.
What Richard is talking about, is connecting a PV-inverter to the output of the inverter (no grid supply), and then if there is more PV than the loads need, the inverter senses that energy is pushing backwards into the output, and it flips the buck/boost stage around and instead charges the battery.
The Voltronic inverters can’t do that as far as I know. They would probably shut down while complaining that the high voltage DC bus went overvoltage.
And they definitely can’t change their frequency to control the in feed power… 
What happens when the batt is full?
The loads are serviced first and then the battery. Once the battery is fully charged the inverter frequency increases which throttles the PV inverter supply. (AC regulation)
Frequency shifting. Usually based on battery voltage. As the battery voltage gets closer to the required charge voltage, the inverter starts increasing the frequency, and it reduces power. Eventually it evens out and the PV inverter makes just enough to service the loads.
It goes without saying that if the inverter cannot frequency shift… well then “what happens” is somewhat less pleasant. High voltage events and things…
The answer I was looking for as it is crucial to also note the flip side.
The Titbit: Make very sure the inverter can indeed do frequency shift and that it is always done with reliable success.
It needs to be able to do two things. It needs to frequency shift to control the PV-inverter, but it also needs to be designed and programmed to run things in reverse so it can charge the battery. Or in more technical language, that bit in the middle has to be a buck/boost design, so it can do both (buck means to convert downwards, boost means to convert upwards, of course).
Indeed! One needs to read all the manuals you can find first (and even a big outfit like Ginlong don’t have good documentation)
I also found that the local support was wanting. There are 2 types of limiting that the inverter responds to: Frequency and the CT input.
The one that’s active locks the other one out. They are both active all the time so this can be a PITA…
Frequency shift has priority over the CT. In most cases, when the CT is in use it means you are connected to the grid, and you cannot shift the grid. When the grid disconnects and you are islanded, then the CT has no reading, and frequency shifting takes over. So the way you normally set it up, only one of them is used at a time.
But if the grid was to drift up to above 51.8Hz… then yes, that will take priority over the CT, and it will start limiting.
Yes and one would expect the feed in limiting to disengage but it doesn’t.
However if you cycle power it starts up ok.
I thought this issue might be fixed with a firmware upgrade but they don’t recommend this because it’s an old inverter and there’s a risk of bricking the thing