Victron MPPT RS, Float Voltage, VOC...does the 8*Float Limit require a Temp Correction?

[AlaskanNoob]

Continuing my habit of learning something new after I make expensive decisions on what I think I know...

I have seven arrays of 7 x LG440N2T-E6 bifacial panels in series, with each array connected into a tracker of two MPPT RS 450/200 units.

When I sized the arrays, then designed ground mounts & had them engineered & sent to a metal fabricator, I had sized the arrays based solely on this calculation:

VOC * 7 * 1.23 (my temp correction) = 423V = less than 450V = good to go

Now I see in the manual that my PV string cannot be more than 8 x the minimum battery voltage when at float.



My batteries are Pylontech US5000. The float value I see in the Compatibility Guide is 51V. Does this mean my arrays cannot exceed a VOC of 408V?

The VOC of my panels is, I think, 48.9V. 48.9 x 7 = 342.3V.

342.3 is less than 408V so I think I am good here and can use strings of 7 panels?

Or did I do my string VOC calculation incorrectly by not including a temp correction? Because if I use my temp correction, then my string VOC is 421V which is more than 408V, so I may have an issue. Many thanks for any help!

 

[MisterSandals]

<deleted off base post />

Fun fact: -40C = -40F

 

[AlaskanNoob]

Using temp coefficient = .26%

delta c x 423 Voc x .0026 = 27V change
delta c = 27 / (423 x .0026) = 25C change

So you will exceed 450V at around 0C. You NEED to limit to 6S if you are in the Alaska i have seen ... where even water freezes.

Fun fact: -40C = -40F

The 423V number has already been corrected for temperature. It appears you are correcting it for temperature twice?

What I am wondering is the MPPT RS manual talking about increasing voltage and how VOC of the array can't be more than 8 * float value. It doesn't specify that VOC must be temperature corrected for that limit, so I am wondering if that is a given that it just assumes as calculating VOC? Does that section of the MPPT RS manual assume that I will correct VOC for the string with a temperature correction?

I used historically low temperatures when I did my temp correction, which increased the VOC of the string from 342.3V to 423V.

If, in addition to this calculation of string size, the MPPT RS specific limitation dealing with float ALSO applies adding a temperature correction to the VOC and the manual assumes you will do that, then if I experience historically low temperatures (on a sunny day) then my string voltage would go up to 423V which would be more than the MPPT RS limitation of 408V which would occur when the batteries went into float charge. In this unlikely scenario, it appears the charge controllers would simply go offline and show an over voltage? They can handle significantly more over voltage than that I think. So I'm not sure it's a huge deal, and that is assuming that part of the manual expects me to put the temperature correction in there.

If I set the float voltage for 52V, then in this unlikely event of a daytime, sunny, historically low temperature event then 423V would be just a bit over 416V. If I set float voltage to 53V then there would be no issue, again assuming the manual expects me to apply temp correction for that calculation. Of course, I have no idea what raising float to 53V would do to my batteries overall. I think it would keep them too highly charged from what I'm reading, and the compatibility guide says to set float at 51V. Although the batteries will control the charge controllers and get what they want despite what I put in settings.

 

[MisterSandals]

The 423V number has already been corrected for temperature. It appears you are correcting it for temperature twice?

Doh! Sorry, i missed that. Carry on.

how VOC of the array can't be more than 8 * float value.

Never heard of this constraint before. Interesting, wish i had some info to share, sorry.

 

[AlaskanNoob]

Doh! Sorry, i missed that. Carry on.

Never heard of this constraint before. Interesting, wish i had some info to share, sorry.

No issues, I think it will be fine. I used this chart and used a temp correction of 1.23 which is just north of -40F. Coldest I've seen is -7F in the five years we've been here and we're on the Coast and I don't think it has ever been -40F here historically. I think the lowest was -20F. So I think it's all good, I'm just mostly curious. I would guess the temp correction is supposed to be applied because voltage is voltage, but the MPPT RS can handle a bit of over voltage without being damaged so probably not a factor.

PV array temperature correction table (NEC 2017)

There are calculators like this one made by @upnorthandpersonal which help you calculate PV array voltage and power for low temperatures based on the specific specifications of your panels. These are great tools and will give more precise...

diysolarforum.com

 

[AlaskanNoob]

Interesting. I just realized there is a temp correction to VOC in the panel specs themselves which results in a much better number than the temp correction chart above.
-32C is the coldest recorded temp where I'm at. So that's 57 degrees from the 25C standard most panels use (even though the specs use a 20C standard). Going with the more conservative 57 degrees, that means the temp correction to VOC is 57 * .26 = 14.82 (or a temp correction of 1.15 to be conservative).

So temp corrected VOC is 48.9 * 7 *1.15 = 394V

I'm assuming since it's the specs from the panel manufacturer, it's more accurate than the chart? I definitely like the number better!