Why can’t I get 1600 w of charging?

[jondrew55]

I have 4 Dokio 410w panels. I want to charge my Ecoflow Delta Pro. The max voltage the Delta Pro can accept is 150V. I believe the Delta pro can accept a maximum charge current of 15 amps.

According to the panel specs, the maximum power voltage is 31V. The maximum power current is 13.09. So 31 x 13.09 =405.79. By that alone, 4 panels in series would give me 1623 watts.

I’ve asked this question in other forums, and have golden the same answer. I should not string 4 of these panels in series to the Delta Pro because the Voc = 37.08 and 37.08 x 4 =148.32. Right at the 150V limit. Keep in mind I’ll be connecting the panels with at least 100’ of 10 AGW cable, so there will be a few volts of loss. I live ln Orlando so it almost never gets below freezing. So maybe the Voc won’t get much higher?

So what is the real risk of a 4S connection? What should my Voc safety margin be? I don’t want to ignore advice just because I don’t like the answer.

 

[SunDave]

You will get the same answer here. Because it is the right answer. If you have a cool day or have a cloud edge effect, 4 in series could destroy your Delta Pro's charge circuit.

For reference I have 7 Rec Pure 420 panels in series with an open circuit voltage (VOC) of 49.1 volts. That would be 343.7 volts in theory. In reality, I found a peak of 408 volts in my solar assistant logging. I only looked for the peak for about a minute, so there are likely higher peaks.

Basically, when it is cold, or when the cloud edge peaks the voltage, things will not go well. When designing a system, you can pick panels to fit the application, but if you already have the panels and the device, it's up to you if you want to take the risk. Personally I would not. Unless the Delta pro is being very conservative with the 150 volt number and you have some proof of that.



I can't find the exact temperature data on your panel since I don't have the full spec sheet, but based on other similar size panels, you could still easily be over voltage at 70F. It wouldn't need to be cold at all to spike the voltage above the max allowed by the Delta Pro.

So the answer to the title of your post is that you can get to 1600 watts, just not with this particular panel choice.

 

[jondrew55]

You will get the same here…

So no way to get around this. Tossing a resistor in line to eat the potential overvoltage isn’t the answer I suppose? I was able to get almost 900W into the Ecoflow with a 2P2S configuration in direct mid day sun. And that’s fine for my purposes right now. The last thing I want to do is fry the Ecoflow. I have 3 different people in 3 different forums telling me the same thing, so I’ll take that as ground truth. Thanks!

 

[outsider]

Does your Pro have one MPPT input or two? The Pro 3's have two inputs. If you had two you could put two panels into one and two into the other. Or even 3 and 1.

 

[SunDave]

Yeah unless you know that they have quite a bit of cushion in their ratings, it's a gamble. Often times you can over do the amperage to a certain extent, but not the voltage. When purchasing a system from new (next time), now you know that there are so many different panels out there you can dial in exactly what works given input specs and temperatures.

I don't have enough experience with modifications to know if adding a resistor is a viable option. That would have to be answered by a more tech savvy forum member.

 

[Backyard]

The only viable way to use a resistor to lower voltage of a panel is to wire them in parallel with the panels thus drawing some current and pulling the voltage down. This will lower the total voltage to the SCC and reduce the overall wattage available. And because you're probably going to need to draw a few amps minimum, you'll likely need a 400W resistor minimum.

Putting it in series with the panels will cause a larger voltage drop the more current you draw. But also the inverse is true. The less current being drawn the less voltage drop across the resistor and thus the full voltage of the panels can still be present at the SCC under low load conditions

Conclusion: a resistor is a poor choice for reducing voltage on a solar string

 

[jondrew55]

Oh well, first lesson learned. I got almost 900 watts in a 2S2P configuration. Which is typically where I charge the delta pro anyway. So it’s not a total bust. I can see now how people burn up their equipment.

 

[wpns]

I live ln Orlando so it almost never gets below freezing. So maybe the Voc won’t get much higher?

So maybe you won't destroy your SCC? There's a reason you keep getting the same answer.

And getting peak power from your array is unlikely, or at least for very long (plus or minus cloud edge effects), as it requires a clear, cloudless day, panels at the proper angle, and moderate to low temperatures (the sun will heat your panels, reducing their output power).

If you need or want more charging during the day, put another couple of panels in parallel.