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I have an off grid solar system which I reciently upgraded from 12v to 24v. I have two 12v solar panels which I put into series to boost the voltage to 24v (I will refer to this as a single 24v panel as to not confuse) and then I bought a 24v solar panel and put it in parallel with panel.

Now everything should be 24v and I should be getting lots of amps. However as with all panels they are not exactly 24v they are usually a little higher. Because of this the panels do not make the exact same voltage. One puts out around 30v while the others make around 32v.

Now I understand that when devices are put into parallel the voltages accross the board drops to the lowest voltage however I thought that would look some thing like this:

If I have two panels like so:
10amps @ 30v = 300w
10amps @ 32v = 320w
= 620w

Put them in parallel and the voltage drops slightly
10amps @ 30v = 300w
10amps @ 30v = 300w
= 600w

So yea a little drop but not really a concern. However it seems that this too is not the case. It looks more like one of the panels is not providing any power and instead is drawing a little power from the array. Something like this:
10amps @ 32v = 320w
-1amps @ 32v = -32w
288w

These numbers are generally estimates but I can definately see a drop in amps when I connect up the second panel.

My questions is why is this? How close do the voltages need to be for the amps to stack up and work together? Also if anyone has any information on the theory of why this is that should be great.

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  • \$\begingroup\$ Connecting them together via blocking diodes will at least eliminate the reverse current. However if one panel's MPP voltage is higher than the other's open circuit voltage, the other will still contribute no power. Giving each its own controller might be the answer. \$\endgroup\$ – Brian Drummond Mar 24 at 12:26
  • \$\begingroup\$ Hi Brian, do you know how close the voltages need to be join forces? \$\endgroup\$ – Nectar-Bomb Mar 24 at 20:49
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A solar panel is, fundamentally, a photodiode*. If you forward-bias that diode, current will flow through it.

To elaborate a bit: You may have used photodiodes for circuitry that has to respond to light. When used for light sensors, they're generally reverse-biased and operated in the photoconductive mode, but they can also be used in the photovoltaic mode, where they are allowed to develop a voltage as a consequence of light striking them. If you scale this up to a point where it can deliver significant current, what you get is a solar panel.

*yes, there are differences in how they're made and what they're optimized for, but they are basically the same idea.

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