question

phtr avatar image
phtr asked

Max VOC with MPPT RS 450/200 and Pylontech

Hey there,

we are in the process of connecting a PV system from 2003, which has been running feed in-only for nearly 20 years now, to a Victron system for self-consumption. As of the current string configuration and VOC, we will use an MPPT RS 450/200. It will be connected to a Pylontech US2000C battery bank.

According to the data sheet of the MPPT RS, the max VOC per tracker must not exceed the eightfold of the battery float voltage, which brings us for Pylontech to:

51 V x 8 = 408 maxVOC

Now I calculated the VOC per string using the individual flash values of the manufacturer and one string is fine, two others exceed the 408 VOC limit as they have 19 instead of 18 modules:

  • String 1: 406,80 VOC
  • String 2: 429,35 VOC
  • String 3: 429,22 VOC

As I expect some degradation of these 20 year old modules, which may also affect VOC, I'm not extremely concerned and would test the system for occurring over-charge protection errors. And in case of doubt, one module per string would have to be removed.

But what I'm really interested in: Is this eightfold battery float voltage rule even relevant for GX device managed systems, as the battery charge cycle in this case is managed by the Pylontech BMS values?

Many thanks!

Pylontechmppt rs
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3 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hello @phtr,

First - good work on doing your homework prior to the installation - I love to see it!

To directly answer your question, yes this rule definitely still applies to managed batteries.

Now - I have almost an identical system running at my place, using Pylontech US2000s with MPPT RS. It is worth to note that while the specified float voltage is 51V, the reality of these batteries is that when they are 100% full (the time to normally be at float), they will hold voltage at 53V with 0.4A.

In my Array, this corresponds to an open circuit peak voltage of about 412V real world.

In your case, at 53V could be 424V limit (53x8) - this is extremely close to your OCV panel rating - and given losses from age and wiring, I would reasonably expect it to work as is.

All the advice here also given my @Alexandra is very prudent - and I agree with her - I thought I could add some additional info from the world world case to help with your design decision.

It is most important to also know that if the lithium batteries are new, and especially if they are NOT individually completely recharged to 100% and cell balanced prior to installation - then there might a period when they need to balance and the target voltage may need to be overrode to even lower values. If you have a cell imbalance, and peak solar conditions, then you may have issues.

cleanshot-2022-11-07-at-135845.jpg



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Alexandra avatar image
Alexandra answered ·

@phtr

Just a thought, even with a can managed battery, the battery cannot control the panel voltage value.

I would imagine since it was explicitly stated it is a serious consideration.

As you say age and temperature affect the panel output, so also operating temperature would have to be considered here, how often will they be working at higher voltages?

And that triggering the overcharge protection often may make a warranty claim (if there was one) a little shaky, since it points to a design/set up issue.

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phtr avatar image
phtr answered ·

Dear @Alexandra, many thanks for your considerations, they are very welcome!

You're absolutely right, temperature is an important factor. The solar modules are oriented to the south with a roof pitch of 30 degrees, so that in the cold winter months here in Central Europe the lower sun intensity may eventually equal out the lower temperatures.


Thank you @Guy Stewart (Victron Community Manager), I'll try my best!

Thanks for this very interesting real world example, it gives me good hopes! As the Victron system will be grid-tied with excess feed-in, I expect VOC situations (sun is shining, but no current is taken) to happen hardly ever. Assuming that the grid remains as stable in the future as we are used to in Central Europe.


I think, I'm confident enough to give it a try!


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