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bombadill avatar image
bombadill asked

Curious for this - One string for two inverters

screenshot-2024-01-18-at-144119.pngHello,

I have a curious situation in my old system that I am replacing and try to understand why this is possible:

I have two Effekta AX-K 5000V Inverters, setup in parallel mode, so they talk to each other, connected in lead-acid battery 48V.

For panels, I have 14 250W panels, so total 3500W-ish. I am replacing the effektas with Victron system, but obviously there is problems here with the old setup.

I cannot access the panels on the roof to see how they've wired them up, and the company does not provide any information, so I am solely reduced to thinking the options they have chosen.

The main question mark is how they decided to connect the string to the inverters. They pipe up the ONE string that comes from the roof to a automatic fuse, so + - from the main string, comes to the fuse, which, in the other end, they have separated (the fuse output) to two -- and ++ strings. These strings then go to the Effekta Inverters, which they then control and charge/invert.

So my question is, since each of the inverterrs max. VoC is 150 and the one string that comes from the roof, is obviously more, what is the reason or even the thinking here? I would imagine that separating the wires in the fuse does actually nothing for the inverter to be able to "see" the total VoC change to half. Is there something I don't see here?

Also, this would mean that I would need a big-ass MPPT to control the string that comes to the roof currently. I have Victron 150/85, but in this case I would need two - but I still would need two separate strings coming from the roof for the both of them, right? Not the one that they've manually split in the fuse.


Thanks for the info...screenshot-2024-01-18-at-145623.png... Italians...

MPPT Controllersmultiple inverters
screenshot-2024-01-18-at-144119.png (51.7 KiB)
screenshot-2024-01-18-at-145623.png (1.3 MiB)
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Alexandra avatar image Alexandra ♦ commented ·
@bombadill

Are you planning on keeping a 24v system?

If you switch to 48v then the 150/85 will be plenty.

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Alex Pescaru avatar image
Alex Pescaru answered ·

Let me understand better:

1. From roof come only 2 wires?

2. The old inverters allow max. 150V PV input?

3. Do you know more about the panels? At least their type in order to know their Voc?

4. Do you have the possibility to measure the open voltage on the wires that are coming from the roof?

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bombadill avatar image bombadill commented ·
I did indeed measure 115.2 from the wires.

Also remembered wrong, current max. Voc for inverters is 105 voc. So they made some interesting choices in the past.



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Alex Pescaru avatar image
Alex Pescaru answered ·

In this case, for sure, because 14 pieces can't be divided otherwise, they can be connected like this:

(1) 7 strings in parallel composed each string of 2 panels is series, about 76Voc, without corrections.

This way will be below 145V input of the old inverter(s), which in fact is 115V.

Otherwise, the only other configuration that will be:

(2) 2 strings in parallel composed each string of 7 panels in series, about 266Voc, without corrections.

But configuration (2) is too much for the old inverters because of the max Voc, and also for that Victron 150/85, whose max input Voc is 150V.

Measure that voltage from the roof and the mystery will be solved... :-)))

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bombadill avatar image bombadill commented ·

1. Yes - from the roof only 2 wires.

2. Well, they state in the tin on the side that max VoC is 150. Though I double checked this from the material, and it seems that for that specific model the value woud be 105. But I'm pretty sure it read on the side of the inverter that it's 150.

-If I look on the values that they produce from the PV per inverter, they do show me different values. so another might be pulling 500W and another 300W (just an example).

3. Panels are Schuco 250W, MPE 250 PS 60 EA, with VoC 37.48V. (best to my knowledge)

4. I might have if I can get a multimeter to inserted to the stubs that go to the Breaker.


Specs for the AX-K inverter (5000 model)screenshot-2024-01-19-at-82444.png

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bombadill avatar image bombadill commented ·

Thanks. Yes, I did more digging and found your suggestions of the 7 strings in parallel to be more likely scenario.

I will measure when I will get there next time (it is my holiday home). Lot's of legacy stuff and the old owner was not really taking care of the energy system or the batteries. Let alone even know that you should never drop the battery voltages of the lead acid batteries under certain thresholds... so here we are.

But. Luckily, I already have all the materials at the house. Just doing everything better this time.

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Alex Pescaru avatar image Alex Pescaru bombadill commented ·
Indeed.

And the 150/85 will for sure handle the 7 strings in parallel, because I don't believe the Isc of the panels is bigger than 10A and the 150/85 can handle up to 70A input.

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Related Resources

MPPT Product Page

MPPT Error codes

MPPT 150/60 up to 250/70 Manual

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