Multiplus with AC-coupled PV battery size requirements

DIY
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1

Hi!

I have a MultiPlus 48/5000/70-100, BMS v2, 8x Victron lifepo batteries (2p x 4s - 200Ah 48v), Cerbo GX and small MPPT and panels. There is no grid.

I will soon have a larger array installed some distance away (I assume too far for DC cabling) and so intend to purchase a Fronius Primo to connect via AC. I understand that this will generally work well with Multiplus via PV assistant and frequency shifting. My new array will be ~3.6kW max so I would like to buy the 3.6kW Primo.

I have read the “factor 1 rule” - AC-coupling and the Factor 1.0 rule [Victron Energy]
For the primary rule, I presume I am ok as 3.6kW (Primo) <= 5KVA (multiplus), which is great.
However, I also see that there is a battery capacity requirement, which states 100Ah of capacity per 1.5kW of PV (for lifepo).
3.6/1.5*100 = 240Ah, which is sadly greater than the 200Ah that I have.

So my questions:
How strict is this rule? Is 200 close enough to 240?

I gather the requirement exists to account for latency in the AC voltage increasing and then the Multiplus frequency shifting and then the Fronius detecting the shift and reducing power. And to deal with the temporarily too high voltage, the Multiplus will increase the DC voltage and the batteries need to absorb the associated current. In particular, it seems that a large AC load being disconnected is considered the most likely cause of this.

I would be interested if anyone could provide more details about this process.
Is the behaviour the same in Bulk, Absorption and Float? How does this interact with the Multiplus max charge current (70A in my case)? And what if a lower charge current limit is configured, does this get ignored?

What happens if this battery capacity requirement is ignored and a high voltage (what kind of rise might 3.6kW translate to?) hits my 8 batteries that are in Float? Will the BMSv2 try to shutdown the Multiplus? Is that guaranteed to be effective? If not, is there a fire risk?

Any insights and advice would be greatly appreciated.

P.S. My system has been active for 2-3 years and I understand that it is not recommended to connect additional batteries and hence why I am not discussing that (the most obvious way to comply with the rules).

2

I cannot really answer your question, but as far as I understand that rule, you need to do the math with both solar arrays, old + new, as a whole.
So when your existing small mppt array is 1kWp and your new array is 3.6kWp, you need to calculate with 4.6kWp installed PV.

3

Interesting, thank you. I do have a 1kW existing array and the mppt is victron. I forgot to mention that I also have a smart shunt that measures battery current. I think the mppt and shunt talk via bluetooth and perhaps also via the GX (DVCC???) so maybe the system can tell the mppt to reduce power faster than the AC PV?
I presume that when the Multiplus is charging via generator input the mppt gets throttled as necessary.

Edit: If we assume that any load is less than 3.6kW, is it safe to say that when batteries are full the total PV will already be throttled to that load and so the max “surge” power will be 3.6kW?

4

Does anyone have any tips to get more engagement on posts?

I’d love to be able to get someone knowledgable, maybe even from Victron, to answer my questions so that I can move forward on this.