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Understanding VOC

Ron

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Oct 4, 2019
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I was reviewing my system, that I haven't hooked up yet. My four 158 watt panels. each put out VOC 42.75=171v. my FM60 manual says to stay below 150.
VOC is unloaded? So if my batteries are hooked up? Wouldnt that make for a load? I also have a diversion load controller hooked up to the batteries for my wind power. So wouldn't any excess power just be diverted and keep the volts in a safe range? If the panels are putting out over the 150 v and the batteries are full the controller would shut off and the VOC would be more than the controller could handle? I guess I got hung up on the fact that VM is 31.67=126.68 for the system. I do get that the diversion controller needs to be hooked up before the panels or the wind power is started. Does someone here understand this better than I do or at least can add to my thinking.
 
Yes, Voc = the voltage the panels rise up to with no load. The trick is that at some time of the day, even with a battery connected, the system will be lightly or perhaps not at all loaded, ie the battery is fully charged, your DC loads are turned off etc. When that happens the panels will rise up to near or even Voc and then things break. This can even happen early in the morning before your charger decides there is enough power from the panels to enable its output. Early in the morning may also get the added bonus of low temperature voltage rise in the panels making the issue more likely to happen.

If your diverter is connected to your PV side it could potentially be used to clamp the array voltage so that it never goes into unsafe territory for your charger but I'm not aware of a wind controller that can do that. You'd really need to do a careful read of the capabilities of your wind controller.
 
In addition to what gnubie said, don't forget that the Voc rating is at 77F (25C). If it gets cold where you are, the voltage will go up. I would never put 4 of those panels in series.
 
I was reviewing my system, that I haven't hooked up yet. My four 158 watt panels. each put out VOC 42.75=171v. my FM60 manual says to stay below 150.

And that is only at ambient 25C. If you assume -15C is possible, and if temperature coefficient of panels is -0.4%/degree C, Voc on a cold day could be 16% higher.

First glance, it looks to me like panels wires 2S2P would be good. Voc stays below 100V. Two strings of panels in parallel doesn't require fuses.
What battery voltage? You have 632W (STC) of panels. At 12V that's 50A. FM60 should be good for 60A.

Vmp of 31V, two in series is 62V should work with an MPPT charger for 12V or 24V batteries, but not likely 48V batteries.
 
My newbie thinking was that any excess power would go through my wind diversion controller and help power my water heater elements. This is a 48v system. Maybe three panels for VOC 128.25? 95.01V I do have a separate 24v system for a greenhouse, I could use one panel there.
 
The open VOC is very critical with you solar controller working correctly. When I installed mine, even though it was not at maximum PV in the day, my unit was not happy going beyond listed on my controller. My unit is rated at max of 150 VOC. I was running 112 VOC with installed panels and added a panel to 156 VOC. The controller went ballistic, lights blinking rapidly with over VOC status. Took the panel back off, setup was ok. So in this case, just a little bit over the VOC of 150 messed things up a bit.
 
Maybe three panels for VOC 128.25?

Probably.
If temperature coefficient of Voc is -0.4%/degree C and record coldest temperature is -15C (40 below ambient), Voc would reach 149V.
Check PV panel label/data sheet and weather records for your location.
 
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