DC PV design rule question: Voc less than 8*Vfloat

DIY
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1

Hi all,

As we’re all aware, Victron systems have this design rule for DC-connected solar that limits the open-circuit string voltage to 8 times the battery float voltage. This is understandable from a technical perspective, but as an installer it already caused me quite some difficulties.

I had several potential customers now that had existing solar panel strings with Voc’s of around 380-430 volts. The inverters were old and not able to communicate to Victron, so naturally we wanted to connect the string to an MPPT. Unfortunately, the voltage just touched the theoretical limit in cold-weather conditions.

At the same time, some of the systems that I’m actively monitoring also have DC PV and I notice the voltage never even comes close to the Voc voltage. Not in cold-weather conditions, not in early morning or evening. It always stays around the Vmpp, which is 30-35% lower than Voc for most panels I encounter in the field.

This is leading me to at least question if the 8*Vfloat is fully justified, or perhaps overly cautious. What are the situations where one would actually see such a voltage and run into trouble?
I hope someone can shed some light on this so I can keep confidently rejecting the devil on my shoulder whispering that ‘it will be fine to connect this string’ :slight_smile:

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2

It is based on a hardware limitation, internal transformer? iirc.
It isn’t a random number that was produced by the great engineering dartboard in the sky.
This has been discussed in other posts on this same topic.

3

I’m not questioning the transformer limit; I’m questioning when one would see Voc in practice. But indeed I see some other topics have similar discussions.

4

I get you.
MPPT voltage limits tend to be absolute and unforgiving.
While it may be seldom that you would see it, it isn’t something you would want to put to chance.

5

Thanks, fully agreed.
If Victron developed an MPPT with a wider range / conversion capability, it would enable so many use-cases..

6

If you look at the internal capability of solar for the HS, that runs up to 600Voc, so they do technically have something in the range already.