I am trying to build a solar powered phone charger using small size solar panels. I have two solar panel connected in series, each of them are 4 V and 50 mAmps. When I connect the solar panels to the input of the following boost converter, it shows its charging but the battery level actually goes down, instead of going up.

http://www.amazon.com/Module-Battery-Converter-Charge-Samsung/dp/B00C93Z8JY/ref=sr_1_9?ie=UTF8&qid=1451591616&sr=8-9&keywords=dc+dc+step+up+converter I am not sure if using a boost converter a good idea since in cloudy days the voltage goes down to approximately 4-5 V. The current for the solar panels are 50 mAmps. So is the boost converter actually lowering the current? Thank you

  • Sorry I meant connecting the solar panels in parallel giving an output of 4V and 100 mA
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    \$\begingroup\$ 2x solar panels each producing 4V. When connected in series they produce 8V. Phones need 5V. Why are you using a boost? Have you fried a phone yet (I hope you're not too attached to it)? \$\endgroup\$ Commented Dec 31, 2015 at 20:24
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    \$\begingroup\$ Instead of adding a correction, fix the original! It's a waste of time to read that you're connecting the panels in series, then find out they're actually in parallel at the end. \$\endgroup\$ Commented Dec 31, 2015 at 21:19

2 Answers 2


I'm confused -- if you have two 4V solar panels in series, your open-circuit solar cell voltage is going to be 8V, and you want to have a buck converter to get 5V for charging a phone. Hooking up 8V to that Amazon module will likely damage it.

If you have two 4V solar panels in parallel, now a boost converter would make sense to get 5V, and that module could potentially work for you.

Regardless, think about your problem in terms of power. A solar panel with 4V open-circuit voltage, and let's say your 50mA number is short-circuit current, we can back-of-the-envelope as a 200mW device. If you put these both in parallel, you have about 400mW (theoretical) you can deliver to your device, assuming perfect power transfer (which of course we cannot do).

Now, assuming that you are not doing any peak-power tracking and are applying the output of the cells straight into a boost converter to get 5V, assuming perfect everything (which again, is not true), your best case delivery to a phone is 5V @ 80mA...which is not very much. I doubt that's enough to even trickle charge a modern smartphone. If we assume that your energy conversion process is 80% efficient, that's even less transfer -- only 64mA delivered ((0.8 * 400mW) / 5V).

In the scenario where you have placed them in series, you would have the same amount of power available, but you would use a buck converter instead of a boost converter.


  1. Don't hook up the panels in series to a boost converter.
  2. You don't have enough power in either scheme to do what you want and charge a phone sufficiently (best case, you charge a phone that is off very, very slowly).
  • \$\begingroup\$ Thank you for the answer. Now it makes sense to me. Many boost converters doesn't mention source current requirement in the specification. For example, the following product says if you input 1.8 V, the output current would be 500 mA. So I thought I can use one 4 V, 50 mA solar panel as an input.adafruit.com/products/1903 \$\endgroup\$
    – Tanvir
    Commented Dec 31, 2015 at 21:14
  • \$\begingroup\$ Ah yes -- when you see current numbers for power supplies, that is generally a maximum number, not a minimum. So for instance, that product when supplied with a 1.8V source that is capable of sourcing sufficient current, can in fact output nearly 500mA. But if the 1.8V source itself can only source 50mA or so, you're not going to get 500mA out of it. \$\endgroup\$ Commented Dec 31, 2015 at 21:19

No, a boost converter doesn't "lower the current" coming out of a solar panel.

Your description is vague, but most likely what's happening is that the boost converter is loading the solar panel to the point where the voltage collapses. Or, it puts out less than what the batteries want, so ends up drawing little power from the panel. That would leave the voltage high but the current low.

You have to look carefully at both the input and output of the boost converter. The output must match the end-of-charge voltage of the battery. A proper battery charger also controls the current, particularly at low state of charge, but your solar panel sounds so wimpy compared to the battery that you don't have to worry about that.

Find the voltage at which the panel delivers maximum power in full illumination. Put a big cap across the panel, and have the boost converter stop drawing power when the voltage gets below that point or maybe a little lower.

When the battery doesn't need much current and you've got more than enough power coming out of the panel, the panel voltage will be above the set point and the boost converter will run, limited by the battery voltage. When the battery wants more power than the panel is making, the boost converter will drain the panel voltage until it gets to the threshold you picked earlier, and then shut down. The panel will then dump the little power it is making onto the cap, which will cause it's voltage to go up. This causes the boost converter to come on again, charging the battery a little bit until the cap voltage goes too low. There should be a little hysteresis between the boost on and boost off input voltage thresholds. Basically, when there is too little solar power, the boost converter will run in bursts to just use the power that is available.

  • \$\begingroup\$ Thanks for the answer. But I am not sure how I can control the boost converter to stop drawing power below the mppt voltage. I wanted to find a boost converter that is already in market. Do you have any suggestion about what boost converter I should use for the solar panel I have and what capacitor I should use? \$\endgroup\$
    – Tanvir
    Commented Dec 31, 2015 at 21:50
  • \$\begingroup\$ @tanvir Some boost converters like the TPS61221 have an "Enable" pin that can be tied to the ground to disable the chip or put it to shutdown mode. It requires you to check your device though. \$\endgroup\$
    – user59864
    Commented Dec 31, 2015 at 21:59

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