Something wrong ?

Ruben

Solar Enthusiast
Pip 5048gk mppsolar inverter

2 x 10 panels in series than paralel to the inverter

panels are 250w 8.5 amps
Open circuit they all measure around 33v

if i measure 1 string (10 panels in series) on the roof i get around 320v, which is good
The i go measure at the breaker before going to the inverter , i get like 220v
I hit the breaker which disconnects the panels from the inverter, measure again and i get 320v

is this normal or am i having a big voltage drop???
Wires coming from the panels to inverter is
8awg

thnx in advance
 

hankcurt

Solar Addict
Couple of questions:

When you measured the voltage on the roof, was it with the solar wiring connectors disconnected?
Do you have the model number of your panels, so I can look at the datasheet?

The panels should have two voltages on the datasheet. One is Voc, which is the open circuit voltage. The other is Vmp, which is the voltage at the maximum power point. The Vmp will always be lower than the Voc.
 

Ruben

Solar Enthusiast
When i measure on the roof , its disconnected
you can say open circuit of the 10 panels in series
As soon i connect the panels and measure again at the breaker i get the lower voltage
 

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Ruben

Solar Enthusiast
When i measure on the roof , its disconnected
you can say open circuit of the 10 panels in series
 

hankcurt

Solar Addict
I would say there seems to be a problem somewhere because even your open circuit voltage is too low.

The Voc is 37.85 volts, and assuming its warm (45°C) on your roof, the temperature coefficient of -0.35%/°C means your panel Voc should still be about 35.2 volts. So at that temperature rise, a string of 10 should have an open circuit voltage of 352 volts. You are getting 320 volts. The difference of 32 volts is very close to what I would expect if one of your panels was shorted.

For each of your panels to be at 32 volts due to high temperature, thus creating a string voltage of 320 volts, the roof top panels would have to be at 69°C (156°F), which is hot even for a roof top installation.

My first suggestion would be to test each of your panels to see if they each create about 35 volts when they are disconnected. It might save some time to isolate each string of panels and measure each string voltage separately to see one string is generating a lower voltage than the other, and if you find that result, then check the panels in the low string.

The large voltage drop when you are trying to draw current from the system could also be caused by a bad panel or a shaded panel. The maximum power point voltage is 30.5 volts, so you should be seeing about 300 volts since you are using a MPPT charge controller. If none of these problems is the cause, then I would check for a loose connection at your wire termination points.

The 8 awg wire is more than large enough to avoid excessive voltage drop given that the total current from your rooftop is less than 18 amps. Even with a 100 foot run of 8awg wire, at 18 amps the voltage drop would only be 2.26 volts, so you would see 298 volts. After correcting for the temperature related power loss of -0.41%/°C, with the roof at 45°C (113°F) and the maximum power point current of 8.27 amps, you should still see around 275 volts at your charge controller.

These calculation are based on the datasheet I found for your panels here.
 
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Ruben

Solar Enthusiast
Btw
The panels are used and each measure open circuit around 31-32v
So 10x 32 v = 320v open circuit on the roof
But how come i get 220ish with the panels connected at the breaker that goes to the inverter?
if i disconnect the panels at the breaker from the inverter i get the 320v but as i connect the panels again via the breaker it gives me the
220ish v again , shouldnt i get the same voltage going to the inverter ?
 

hankcurt

Solar Addict
The panels have two voltages listed on them.

One is an open circuit voltage, Voc. This is the voltage that the panel will create when the circuit is open and there is no where for the electrons to go. This is the maximum voltage that the panel can create.

But since the panels are creating a voltage by light hitting atoms in the silicon semiconductor, which causes electrons to jump across a barrier, once you give the electrons somewhere to go, the voltage will drop down until the rate of more electrons jumping across the barrier is the same as the rate of electrons leaving the panel.

The mppt charge controller tries to match the rate of electron flow from the panel with the ability of the panel to make more electrons jump across the semiconductor barrier. This creates the most current flow at the best voltage, which gives the maximum power from the panel. So the voltage that results from this process is called the maximum power point voltages, Vmp.

So If your panels have some degradation and are showing a lower Voc because of it, your Vmp would also be lower.
 

hankcurt

Solar Addict
This pdf lesson from Teach Engineering does a pretty good job of describing how MPPT and solar panels work together.

It has this graph in it of the voltage / current curve for the panel they are using in the classroom.
1608750005789.png

Notice that if you are all the way to the right, the voltage goes up to 6.4 volts, but there is no current flow, so there is no usable power. Once you start letting current flow, the voltage drops a bit, and the power output (represented by the pink line) starts rising. But if you try to take more than 0.1 amp of current from this panel, the voltage starts rapidly dropping and the power output drops.
 
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