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Old 09-03-2013, 12:41   #16
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Originally Posted by twistedtree View Post
Just be sure you don't go too high on the voltage. It's surprising how much hight the peak voltage can be vs nominal. With a controller rated up to 150v which I think is the outback rating, you need to keep the peak panel voltage below that. I found that meant no more than 48v nominal.

Peak voltage is the panels max rated open circuit voltage. But you need to calculate it at min ambient temp, not the rated temp on the nameplate. And you need to further jack it up for cloud edge lighting which through some magic of optics is brighter than full sun. I don't recall the adjustment factor, but it's in the 15 to 25% range. Then on top of all that I'm pretty sure there is a further NEC de-ratng of 15%. It all adds up surprisingly fast.
No, you can put 6x 12V panels in series, which will give an open array voltage of 132V for the 36-cell-strings based panels. The voltage is depending on the number of cellsin series, each of which is a silicium gate like a diode so about 0.6V. You don't add anything on top as the voltage will only go down from there, never up. Controller specs state the open panel voltage allowed, so you simply divide that by the stated open voltage of one panel and you find how many you can put in series. People that buy the 110-140W panels with 72 cells should buy 24V versions instead so that all cells are in series

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Old 09-03-2013, 15:05   #17
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Re: Change 12v Solar Config to 24v?

Yes and no to both of the preceding posts (#15 and #16). Voc is around 0.6V/cell at 25C, and exact Voc can be found on just about every panel data sheet.

Voc is dependent on temperature, and increases with lower temperature. Panels are rated at 25C, and the TCVoc (Temperature Coefficient of Open Circuit Voltage) is around -0.4%/C (Voc will drop by 0.4% for every degree C increase in temperature >= increase for a drop in temperature). If you are pushing the limits of your system then you need to calculate Voc at the lowest temperature, at 0C it makes about a 10% difference (again, exact values can be found on most datasheets). The highest operating voltage is generally seen early in the morning when the sun starts to get bright and the panel temperature is still low, it drops quickly thereafter, but if your controller or inverter limits are exceeded you can burn things out even though the total power is actually very low.

Edge effects on clouds can greatly increase irradiance values for a short time period. We have measured increases from 1000 W/m2 (generally considered the reasonable maximum irradiance value) to 1400 W/m2 with passing cloud edge effects. Voc does change slightly with irradiance (typically about 0.5V from 600 W/m2 to 1000 W/m2 for a 60-cell panel), the amount of change is generally insignificant. In addition, if you are near 1000 W/m2 it is very unlikely that your panels are operating at their lowest calculated temperature, as they quickly increase in temperature at that kind of irradiance level. Irradiance values can generally be ignored when calculating Voc from the published panel datasheet.

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