From grid-tie to microgrid / hybrid backup

OK, that’s that then. I was hoping to easily comply with CoCT’s ridiculous 15A grid-charge rules for batteries using that setting, but it’s detrimental to AC-PV charging too. I really don’t get how they can think up that rule.

But with ESS it isn’t really an issue unless “Keep Batteries Charged” is on, which will just require changing that setting at the same time. Although it still wouldn’t comply, since you could just not do that.

Not dumb, but also not a problem, unless you keep your batteries full all the time.

When there is PV, the grid-tied PV inverter pushes it back. If the MultiPlus can see it, either on its AC-out or with the help of the ET grid meter, it “pulls the power down” into the battery and charges. Any excess goes onto the grid. This works for any brand of AC PV.

The Modbus control of the Fronius from the Cerbo (and one or two others) allows it to not go onto the grid as well, i.e. limiting export. This also works with the Fronius’s optional Smart Meter without the Cerbo, and I assume with any other brand that has a grid meter to limit power. The Victron kit then isn’t in the loop and the PV grid meter sees the battery charging as a normal house loud. You would then have your ET & AC PV grid meters back to back, possibly tweaking the grid import threshold a bit. (Mine is currently like that, but not for that reason.)

In either of the ESS “Optimized” modes this just works, since you tell ESS there’s AC PV somewhere, either the input or output. If you use “Keep Batteries Charged”, then there’s nowhere for the power to go, so you lose out. But the same would be true if you charged with DC MPPTs.

So what you are saying is that there is still a need for a maximum charge power setting.

There is a slight difference in where things are implemented with these two settings. The newer one that DVCC uses (the current limit) is implemented in the Multi. The GX device merely relays the information from the battery (or from you) to the Multi. The limit applies regardless of where the energy comes from, because the point of this feature is protecting the battery.

The older power limit setting, affects how the ESS control loop sets the “setpoint” on the Multi, in other words the work is done on the GX device. The setpoint is continually calculated so that after the loads is covered, not too much extra is imported for battery charging.

Because the calculated setpoint acts on the INPUT of the Multi, which is inactive during an outage, that setting does not affect PV-inverters on the output, as the documentation says. It is not really a feature, it is documented more as a quirk one might say.

Of course there is a lot of overlap between the two types, and as it always goes with these things, it causes confusion. So one went away…

Yes please. If I could name it, it would be “Grid import charge current limit” – i.e. only take this much current (or power) from the grid to charge the batteries (but only when you need to!). It is about the position of the ET, not AC-in or AC-out, so it can only be implemented from the Cerbo, presumably tied to ESS.

It is not:

  • a grid limit,
  • an AC-in limit or even
  • a charger limit,

it is specifically “grid current going to the battery”.

Meaning that the following should be excluded from the calc:

  • AC PV on AC-out gets a free pass through the charger
  • AC PV on AC-in gets a free pass through the charger
  • any loads on AC-in or AC-out gets a free pass

Current should still be clamped to the Multi’s charge limit or the DVCC charge limit of course.

(Hopefully if I make it sound complex enough it’s fun to build!)

In terms of the DVCC feature goal, your explanation makes more sense than the manual. If you want that feature, it should work all the time, like it does now.