ee21 avatar image
ee21 asked

Is keeping battery SOC at 100% with AC-Coupled PV a bad idea?

I have a grid-tie PV inverter, and an on-grid connected ESS setup. Right now my PV is parallel with the AC-In of my Multi, not the AC-Out, meaning it will not work in a blackout, which I would like to fix.

Typically, due to a number of reasons I won't go into detail, when operating on-grid using ESS, my battery SOC is kept at 100% during the daytime, until 2:00PM, at which point they discharge in tandem with the solar, selling back as much energy as possible.

When I say 100% SOC, that is my self-imposed limit, meaning my actual Lithium battery can charge up to 29.4V. My Multiplus is configured with an absorption voltage of 28.6 to extend the lifespan.

Battery is not connected using CANbus or other communication with the MultiPlus, it has its own internal BMS that can control the charge/discharge switch.

I am interested in knowing what will happen in this scenario: It is 12PM noon, the sun is the highest, and my PV interver is outputting its maximum. AC load is nearly 0, as nobody is home typically during the week, so all of that is going back to the grid. The grid fails, and all of that energy instead of going to the grid, then gets dumped into the batteries (this would be after I move the inverter to the AC-Out obviously). The batteries had already been at, what the MultiPlus thinks is 100% soc, and is at the maximum absorb voltage of 28.6, which, when 10Kw is dumped in, obviously starts to spike a bit.. Let's assume the PV inverter power isn't enough to make it rise above 29.4 when the BMS would step in the sever the connection, but does rise about 28.6V which is the maximum absorb voltage.. Will my MultiPlus trip in a DC over-voltage alarm? Or will it allow the energy to flow into the battery, raising the voltage above 28.6 for a short time until frequency shifting steps in?

MultiPlus Quattro Inverter ChargerESSac coupling
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2 Answers
ceriw avatar image
ceriw answered ·

You would set your multi up to frequency shift its output. You would set your pc inverter up to reduce its output on frequency shift. This way, the multi controls the output power of your pv inverter to meet the dvcc of your BMS

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ee21 avatar image ee21 commented ·

But since my BMS is not CanBUS connected (or anything else) with my Multi.. My question I guess is what setting on the Multi would then control the DC over-voltage shutdown? Or is that a separate value altogether.

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Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @ee21

Please read the manuals instead of using the 'ask an expert' function, this is all in the manuals.

If you correctly configure the ESS assistant (just fill in what is asked in the wizzard), the Multi will frequency shift to control the PV inverter.

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ee21 avatar image ee21 commented ·

@Daniël Boekel (Victron Energy Staff)

I appreciate your feedback, however I have read the manuals fairly extensively, and I don't feel my question in particular was addressed in the ESS manual. What I am asking I feel pertains more to the Power Factor 1.0 Rule documented here, which I have also read:

Specifically this section here:

2.2 Example and background

To understand the background, consider the following situation: the PV inverter is at full power, supplying a big load. The Multi is in inverter mode. Then, suddenly and at once, this load is switched off. At that moment the PV inverter will continue operating at full power until the AC frequency has been increased. Increasing this frequency will take a very short time, but during that time all power will be directed into the batteries as there is no other place for it to go. This causes the following:

  • When batteries are (nearly) full, the battery voltage will spike, possibly causing the Multi to switch off in DC over-voltage alarm.

My question is what voltage will the MultiPlus itself will shutdown at in this scenario? Will it shutdown when if the batteries exceed the absorption voltage set in VE Config, which is currently 28.6? Or will it continue to function unless the spike is so great it hits 33V (24V model). I do apologize for inconveniencing you on this matter, I only asked an expert as I felt I had to where else to find this information.

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Hi @Ceri.w

To get an answer, at least you'll have to specify what (size) Victron inverter you have, how much PV and the capacity of your battery.

But if all is well sized, a Lithium battery (not being LiFePO4) can absorb a lot of current at 'fully charged' without the voltage shooting up.

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