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I'm building a solar system w/ battery for my camper van. Two identical 110W panels wired in parallel, going into an MPPT charge controller and battery (a Yeti 1000 by Goal Zero, 96.8Ah (10.8V) lithium).

When I measure the voltage and amperage directly from the panels, disconnected from the charge controller / battery, in peak sunlight with a multimeter, I got 20.4 volts and 4.69 amps or so for panel #1, and 20.4 volts and 4.58 amps for panel #2. When I disconnect the final cable that ultimately plugs into the charge controller / battery, and measure in the same way using the multimeter, I get 9.82 amps and 20.8 volts total.

However, when I connect it all back up, in peak sunlight, on the charge controller display, I'm seeing 85 watts or so. By disconnecting each panel, I determined that panel #1 gives around 30 watts, and panel #2 gives around 55 watts. This is on a ~40% charged battery. My main question is this -- why do the panels have similar measurements when measured in isolation from the rest of the system, but have this large difference (30 watts vs 55 watts) when it's all connected together?

I know the charge controller does some voodoo which necessarily drops the efficiency, but this is a pretty big drop. And it doesn't explain why the wattages of the two panels themselves are different. Also I'm in the middle of a heat wave, 105F+... could that somehow explain something?

Any ideas, or diagnostic tips? As a beginner, this is a great opportunity for me to learn about electricity. Thanks!

P.S.

Here's how it's all connected:

The two 110W panels have + and - MC4 cables coming out of them, which are plugged into MC4 "branch connectors" (MMF and FFM) to make them in parallel. Then the branch connectors are plugged into an MC4 extension cable (to be able to reach the battery), ~10 feet or so, with MC4 connectors at each end, which are then connected to an MC4 to Anderson PowerPole adapter, which plugs directly into the MPPT charge controller module of the Yeti 1000 battery.

Panel specs:

Peak power: 110W Solar cell efficiency: 23% Maximum power voltage: 17V Maximum power current: 6.47A Open circuit voltage: 20.06V Short circuit current: 7.12A Power allowance range: +/-3% Maximum system voltage: 500V Values at standard test conditions: Air Mass - AM1.5, Irradiance - 1000W/m2, Cell Temperature - 25°C

Battery specs:

Cell Chemistry: Li-ion NMC Peak Capacity: 1045Wh (10.8V, 96.8Ah) Single Cell Equivalent Capacity: 290.44 Ah @ 3.6V

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  • \$\begingroup\$ What is your latitude? How are these panels rated at 110 W? (I know how some panels are rated -- equator at noon on a clear day. But not all of them are necessarily rated the same way.) How did you measure the current? Did you set up a voltmeter and an ammeter at the same time on both panels right at the entry point to the converter and read their values at the exact same time that the converter itself was reading numbers to you about the power it was extracting from each one? In other words, are these all simultaneous measurements, but taken at the entry one way and read from the display? \$\endgroup\$
    – jonk
    Aug 22, 2020 at 21:07
  • \$\begingroup\$ Or were you measuring the panel voltages and currents into a charging battery? (I may be confused on that point, but I think I got things right.) Also, regarding your ambient temperature (which in no way tells you the panel operating temperature) you might look at something I quickly googled. It may also help when you are thinking over numbers. \$\endgroup\$
    – jonk
    Aug 22, 2020 at 21:10
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    \$\begingroup\$ "with a multimeter on a ~40% charged battery, I got 20.4 volts and 4.6 amps or so for each panel." - but not at the same time, right? Spec says 20.06V open circuit. Under load the voltage should be significantly lower. Did you measure voltage with the panel(s) connected and supplying current? \$\endgroup\$
    – Bruce Abbott
    Aug 22, 2020 at 21:12
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    \$\begingroup\$ By symmetry, it should not matter much which one you shade. But shading one panel is not necessarily a good way to determine the output of the other panel. Disconnecting one of the panels would be a better way. Depending on details, a shaded panel may actually steal substantial power from the other panel in the sun. \$\endgroup\$
    – user57037
    Aug 23, 2020 at 20:56
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    \$\begingroup\$ @user I think I gather that it follows the one, itself, around to where ever you put it. So that panel probably is NOT up to snuff. I'd return it, myself, and/or demand that they prove to me that the panel meets specifications. (At that kind of money, anyway.) I think you got shorted on one of them. Return both, if that's what they require. I think you've done enough testing. Your converter results are probably sufficient evidence. Those are flexible and coated panels. There are just too many ways to go wrong in manufacturing and too easy for them to not be as good as they should be. \$\endgroup\$
    – jonk
    Aug 23, 2020 at 21:56

1 Answer 1

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In this system there are too many things that we don't know how they work. My suggestion is to, if possible, independently validate things. To test your solar panels, use a power resistor that will extract near the maximum REALISTIC power from the panels. The idea is you connect a power resistor to the panel, put it in full sun, and measure the voltage on the resistor. The two panels should give similar voltage readings assuming you test them at very close to the same time when the view of the sun is not obstructed. The power delivered to the resistor is V^2 / R, where V is the measured voltage and R is the resistor value.

Resistor value: approximately Vmpp/Impp. From the lensun link, that would be 17/6.5 = 2.6 Ohms. 2.7 Ohms is a standard value. I think a 50 or 100 Watt resistor should be OK provided you only connect it for about 1 second (look for a resistor with short-term overload rating). Use this to test both panels independently. They should be close to the same. If they are, then stop worrying about the panels.

I don't have full details for the Goal Zero. Does it do MPPT? We don't know. If it doesn't, then you shouldn't necessarily expect to get anywhere near the rated power of your panels.

By the way, MPP for solar panels is given under unrealistic test conditions. Mainly, the temperature is at 25C. In general, when solar panels are held in full sun, they will be much hotter, like 40C. The maximum power will be lower at higher temperatures. And if they are not pointing absolutely perfectly straight at the sun, then there will be further reduction do to reduced projected area. So don't expect to see 220Watts.

The only way I can think of to validate the Goal Zero device is to connect it to a known good solar panel such as the type they actually recommend. And verify that it performs according to their advertised specifications.

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  • \$\begingroup\$ Thanks for the comments. I will try the resistor. I'm hoping it demonstrates that the second panel is just inferior, as that would be the simplest explanation. The Goal Zero has an MPPT add-on, which is what I've been using. \$\endgroup\$
    – user
    Aug 23, 2020 at 21:56

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