I have a 20W 5V solar panel (real output more like 10W in bright sun). Would like to harvest as much solar power as possible to a power bank to power a bluetooth speaker (< 5W consumption). Bright clear skies are a luxury where I am. The panel voltage fluctuates depending on the clouds etc, - damaged the circuit of a powerbank. Need something to act as a buffer.

I am considering a circuit using super capacitors (2 in series 2.7V 500F) as energy collectors / buffer -- let the voltage build up to ~ 4.5V. Switch on the circuit at 4.5V and a boost/buck circuit mops up the power to charge a power bank. The switch is on till the voltage drops to ~2V. When the voltage drops to ~2V the switch turns off and stays off till the voltage rises up to ~4.5V again.

Are such configurable switching circuits available where one can set the upper and lower voltages? Or Is it easy to make one?

Or should I try a different approach altogether?

Looking forward to your suggestions.


simulate this circuit – Schematic created using CircuitLab

PS - Apologies, I am a newbie in the world of circuits.

  • 2
    \$\begingroup\$ Skip the supercaps. They will do you no good. Get yourself a MPPT buck/boost controller. \$\endgroup\$
    – winny
    Commented Aug 28, 2017 at 19:49
  • \$\begingroup\$ I would have loved to go the MPPT way. But, this is a foldable panel with a 5V USB output like this one. \$\endgroup\$
    – JetStream
    Commented Aug 29, 2017 at 23:52
  • \$\begingroup\$ So? Your supercaps will still not do you any good. \$\endgroup\$
    – winny
    Commented Aug 30, 2017 at 5:58
  • 1
    \$\begingroup\$ you need an MPPT charger- consider st.com/resource/en/datasheet/spv1040.pdf if you put supercaps across the panel- the panel output voltage will collapse as the panel will see an effective short circuit across itself. \$\endgroup\$
    – Pawan Bahl
    Commented Feb 10, 2019 at 17:23

2 Answers 2


Maximum Power Point Tracking (MPPT) will extract the maximum amount of power available from your solar panel. For this small amount of power output you probably want the best efficiency you can get. Using a supercap won't give you 5 volt regulation if that's what you're trying to achieve.

  • \$\begingroup\$ The MPPT converter can then be used to drive a battery/supercap charger to provide the buffer in addition to the output load. \$\endgroup\$ Commented Dec 25, 2019 at 18:11

Your circuit has nothing to balance the voltages of the super capacitors. One might charge to 3V or more then be destroyed. Your schematic wrongly shows a Zener diode instead of a Schottky diode.


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