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I have two solar panels connected in series, and I'm using them to charge a phone. Together they provide about 7 V when I connect their leads to a multimeter or a basic circuit (i.e. Resistors/etc.). To regulate the voltage/current, I connected them to a step-down buck-boost converter.

The problem is that the output voltage of the solar panels drops significantly as soon as I connect them to the converter, from 7 V to about 1.5 V. This does not happen when I use a regular source (i.e. A laboratory voltage source - a machine).

I tried adding a diode/Zener diode to the solar panel to make sure the current is going in only one way, but that didn't work. Additionally, I tried to isolate the solar panels from the converter by connecting it to a voltage follower circuit or any IC, but that didn't work either.

I'm not sure why this voltage drop is occurring or how to prevent it from happening. Any help would be greatly appreciated.

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Is your lab power supply limiting the current to match what your solar panels would provide under your test conditions (i.e. not max rated current on the panel, but what the current is when it's dropping from 7V to 1.5V)? Could be that your solar panels don't provide quite enough current and voltage will sink since load resistance does not change.

Easy way to check is to hook it up to your power supply and slowly reduce the current via the current knob.

If you have a really simple supply and can't control the current, then you're going to have to us a multimeter to check.

EDIT: OP has provided the follow quick schematic in comments: https://i.stack.imgur.com/0QV51.jpg

While in some use-cases, you might want to do this, generally it's better to have some form of regulation and predictable behaviour. Because there is a large variance in the output of a solar panel, it's generally better to store solar energy into some form of a power reserve first (battery, large capacitors, etc.) and then using that to power your load on-demand.

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  • \$\begingroup\$ You are indeed correct - My solar power isn't supplying a lot of current. I didn't think that this would be the problem, but playing around with the current knob, as you suggested, showed that it is. Then again, I'm not really sure what to do about that. How can I make the current larger with nothing external? \$\endgroup\$
    – Shahar
    Apr 26, 2015 at 6:31
  • \$\begingroup\$ Directly from a solar panel, you can't. A lot of the times, there's an intermediate stage between the panel and load, which is a boost converter into a battery (most of the time li-ion, for USB mobile device chargers). The li-ion can source quite a bit of current (2A+, sometimes 10A+ depending on the cell). This in turn goes into the load (maybe regulated before it goes in as well), and will provide consistent power. \$\endgroup\$
    – PGT
    Apr 26, 2015 at 6:57
  • \$\begingroup\$ Read the datasheet on the buck converter and see if you can convert it to a step-up and use it with a single panel. Step-downs usually have comparatively lower efficiency since some of the voltage will be lost to heat. \$\endgroup\$
    – PGT
    Apr 26, 2015 at 7:02
  • \$\begingroup\$ What's the current that your solar panels are putting out? If the panel's are putting out a low enough current that they could directly charge your phone, then you can hook the panels directly to phone to charge. The resistance of the phone will drop the panel's down to 5 volts and if the panel's current is not too high, then it will charge safely. \$\endgroup\$
    – Filek
    Apr 26, 2015 at 8:36
  • \$\begingroup\$ Assuming I have a rechargeable battery (which I'm guessing has to be 5V+), what do you mean by that? If I were to connect the solar panel in series with the battery, and then the battery to the converter, does that work? From "a boost converter into a battery" I'd assume that I've got the wrong line of thinking, though I don't see how I can connect it otherwise. \$\endgroup\$
    – Shahar
    Apr 26, 2015 at 15:12

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