Well, the box is a little crooked here, but the wiring is in. There are two solar arrays feeding this, although we don't have everything else ready so I only connected the wires for the picture.


And even on a gray day, we're seeing some voltage, about 66 volts. I also checked the short circuit current, which is about 0.6 amps for the two strings. These are actually old panels, dating back from the '90s. It'll be interesting to see what they can do on a proper sunny day.

This array is two strings of four "12 volt" modules in series. So 48 V nominal. I'm not sure what model these are exactly, since the labeling has been worn off, but I learned that supposedly they're around 60 watts. I don't have the exact specifications, but the max power voltage Vmp is probably around 18 V. Open circuit voltage (with nothing connected to the panels) is probably around 22 V. In full sun, then, you could see as high as 88 V at the output of this array (two series strings in parallel). At max power, the voltage would be around 72 V.

If Vmp is 18 V, then Imp (current at max power) is about 3.33 A (60 / 18 = 3.33). The short circuit current (Isc) is probably a little higher than that. For one series string, the Imp and Isc values are the same as a single module.

Photovoltaic panels are more or less current sources. They can only put out so much current (for a given area of panel you can only push so many electrons around), but they have leeway in their output voltage to keep that current flowing. At their maximum power point you'll get the most you can out of the panel - the wattage rating is at "full sun," which is 1000 W/m^2.

If you hook the panel up to a battery, the battery will pull the voltage down, and you lose some power. A maximum power point tracking (MPPT) charge controller can step the voltage down, to better match the panel and battery. (Grid-tie inverters have similar functionality, they're just trying to push as much power as possible into the grid.) This of course depends on how much current the battery can accept at its given state of charge.

As a side effect, you can usually wire the array for a higher voltage, and use the controller to step it down to a lower voltage battery. I'm looking at MPPT charge controllers for this, so we can charge a 24 V battery. The controller still has to be able to take the max open circuit voltage (88 V in this case), but a lot of them can.

Of course, you lose some efficiency if you have to make a big leap in voltage, but you save some energy in the wiring. I haven't worked it out in this case, but it probably wouldn't make much of a difference if the array were wired for 24 V in stead of 48 V.

Here's a picture from this past Friday (a week after the other pictures in this thread), of the short circuit current of one string of panels. This was on a sunny day, and I saw a little over 3 amps. Unclipping the meter pin from the wire drew a nice little arc. The other string was similar.

I left the positive wires disconnected and taped off, since the wire at the other end of the conduit doesn't go to anything yet.

@toroidalcore id be interested in their performance too. Cool set up btw

@voltur I wouldn't be surprised if we get most of their rated output from this. I think these are 60 watt modules, the labels are kind of worn but from what I can tell and what I found online that seems to be the case. So getting a good 400 watts into a battery would be amazing.

I'm going to try to get some good metering going with this system, so we can see what it's all doing.

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