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String Voltage Near Voc?

Cmy

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Joined
Nov 18, 2022
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139
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Installed two west side 12 panel strings, Aptos 440 DNA120 BF10, Voc 41.51v and Vmp 34.71v, and one east side 10 panel string (shorter roof). Time to mount the Growatt 11.4.
Ran string wires inside, checked voltage, and polarity. Noon local in north Texas, 50F outside - the right 12s measured 483v and the left 12s 484v. The 10s on the east side measured 405v. All three strings 10-15v below Voc while I was expecting close to Vmp. Am I getting a bifacial boost even though they are mounted close to the roof?

Coldest temp adds 16% to Voc, ~ 578v on the 12s. Still below a Growatt 11.4's 600v max but getting close and over the MPPT's max 550v. Are 12 440s too many? Could always turn off the 12s if we have a rare 0F period that adds a 20%+ increase.
 

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Just because its 50°F doesn't mean the panels are at that temp.

Sun illumination heating can raise the dark colored panel temp quite a bit.

For PV charge controller max Voc consideration, you have to assume no warming by the sun.

Worse case is typically early morning with just a little amount of sunlight. No significant sun heating of panels yet. Inactive PV charge controller with no load on array yet, so array at Voc for given ambient temperature. Panel will go to Voc as soon as illumination generated current exceeds panel leakage current.

At this initial Voc condition, little to no excess PV power is available to run charge controller until sun illumination get stronger. SCC may dwell for a few minutes with Voc voltage before attempting to fire up solar charge controller. Other SCC's attempt to fire up only to collapse Voc when load is applied. They wait a few minutes and try to startup again. Eventually the sun gets strong enough to support additional loading on array.

Other consideration for Voc maximum voltage is when PV illumination power exceeds needs for inverter loading or battery charging causing PV charge controller to reduce PV delivered power by lightening load on array which causes array to move towards Voc voltage. Voc equals zero current output from array.
 
Last edited:
Nothing connected yet except PV wires into the building. Panel install came early before I had disconnects and inverter mounted. Was checking my wire continuity after similar voltages on the panel string ends yesterday. No batteries planned, just the Growatt grid tied.

Still learning, didn't realize Voc was before a load and Vmp with the load.
 

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