I'm new to Electronics but I love solar panels and hiking.

So, I bought 2 of these and one of these and I connected them.

I cut 2 USB Cables, connected + with + and - with - from each Cable and connected it to the DC-DC booster. The amperage is not doubled and the output Volts are lower than 5V which should be the constant output of the booster. The panels were solar oriented and the output was only 0.02 A with Bright enough sun from those 2 solar panels. The panels separately work fine, it is only when I connect them to the booster that they act weird.

And this is roughly the scheme I'm using, except solar panel specifications and MPPT: Charging Powerbank with Solar Panels better with converter or mmpt or without?

Is the problem in booster or is it me doing something wrong? Thank you!

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    \$\begingroup\$ How did you measure the current? \$\endgroup\$
    – pjc50
    Jun 29, 2018 at 13:34
  • \$\begingroup\$ I used a multimeter/USB chargerDoctor on the output of the USB controller/booster/DC-DC. \$\endgroup\$ Jun 29, 2018 at 13:43
  • \$\begingroup\$ And this is roughly the scheme I'm using, except solar panel specifications and MPPT: electronics.stackexchange.com/questions/325110/… \$\endgroup\$ Jun 29, 2018 at 14:06
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    \$\begingroup\$ So if you have two solar panels in parallel, the voltage will be reduced to the voltage of the worst panel. But lets say there is a lot of sun and you have 6V over the panel. What do you think the boost converter which boosts voltages from 2.5-5V to 5V will do when there is 6V on its input. You may want to check out the differences between Boost, Buck, Buck-boost converters. \$\endgroup\$
    – Remco Vink
    Jun 29, 2018 at 14:25
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    \$\begingroup\$ @aninteresteduser: electronics is a very complex subject indeed and terminology and fancy words are necessary. If you love hiking and want solar pannels, just buy a commercial product. But if you want to build something by yourself, you'll have to understand terminology and fancy word. Electronics is not like Lego. \$\endgroup\$
    – mguima
    Jun 29, 2018 at 15:09

1 Answer 1


Look at your panel specification:

Solar panel output: 6.8V 1700mA
USB output: 5V 1200mA
USB version: 2.1

This means that not only the panel has voltage regulator inside, but most likely some kind of support for USB battery charging specification. Connecting these in parallel is not a good idea.

So, check your panels - do you have direct access to 6.8V mentioned in spec? If you do, then that is where you have to tap in and that output you can connect in parallel to get theoretical (highly unbelievable, though) 6.8V 3.4A

If you don't have access to 6.8V then next thing to do is measure actual USB output voltage on a single panel. If it abruptly drops from around 4.5~5V down to 0 in low sun then the chip inside is too smart for your application, no kind of booster will help you here. You need different solar panels with unregulated outputs to connect them in parallel. These panels usually generate over 6V in bright sun.

Now, as mentioned in many comments, you need to understand the difference between DC-DC converters. Booster modules boost voltage, while at the same time reduce current. They are not happy if you put more than something like Voutput - 1.0V on their input.

When solar panel output can change from say, 3V to 6V, the output voltage 5V fits right in the middle of it. Which means you need "Buck-Boost" module, the one that can boost when input below and buck when input above the output.

As a side note, your reference to the unanswered question riddled with conceptual errors and links to questionable components underscores the lack of understanding. I applaud your willingness to learn this stuff. I don't believe anybody will be able to help you much until you do learn the basics.

Here are some connection options for you. In the first you need to figure out your own wiring, but you'd keep an option to use USB output of a single panel, if necessary. In the second you can use USB cables that you already have, but you might accidentally damage USB device if you plug it directly into panel.

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  • \$\begingroup\$ Great, I'm on the right track. Thank you very very much. Everytime I used the panel it never showed more than 5V so assume it is a max 5V solar panel and yes it drops and ups the Voltage with the suns brightness. Can you please explain a bit this part: They are not happy if you put more than something like Voutput - 1.0V on their input. What will happen if I remove the booster and connect them in paralel with only the Cables? and what if I remove the panels' voltage regulator and connect it directly to a buck-boost module? thank you! \$\endgroup\$ Jun 29, 2018 at 16:17
  • \$\begingroup\$ 1) Boost modules work when input is below output. e.g. 4V in, 5V out. 2) if you remove booster and connect USB outputs in parallel you would either have 5V at double current, or you would have unstable 5V, depending on internal regulator schematics. 3) if you remove internal regulator and connect panel directly to buck-boost module you will have stable 5V as long as sun is bright enough. ...or you accidentally damage you panel beyond repair while removing that regulator. \$\endgroup\$
    – Maple
    Jun 29, 2018 at 16:22
  • \$\begingroup\$ I don't know the basics and you helped me a lot and engaged me in looking deeper into these issues I'm experiencing...;)..one last question, if in low light the panel for example outputs a 3.6 V and sends it to the module, why isn't the module boosting it up to 5V? the input is below input. thanks \$\endgroup\$ Jun 29, 2018 at 16:32
  • \$\begingroup\$ Assuming your wiring is correct if panel outputs 3.6V the boost module should output 5V. If it doesn't there are several possibilities: 1) the panel does not output 3.6V you think it does. It's internal circuit might cut off the output to comply with USB spec. 2) If USB output in the panel really supports USB protocol as description suggests then it expects actual USB device to be connected, not some DC-DC. It might not supply enough power without it. 3) most DC-DC modules require minimal load present for normal operation. Your module might need that load. 3) your module might be defective. \$\endgroup\$
    – Maple
    Jun 29, 2018 at 16:47

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