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Could you confirm I could charge a 13F 5.4v supercaps with 2.8v 3w source and discharge it at 5v? Is it efficient method to change input/output voltage or better use a step up converter?

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It's not so much about being efficient as being impossible. If you have a 2.8V source, you can only charge the capacitor up to 2.8V. It will then discharge at 2.8V to start with but this voltage will gradually decrease as it gets discharged.

To get 5V out of it you will either need to use a step up converter to charge it to 5V, or feed it into a step up converter when you discharge it, or both. Using a converter on the input means you store more energy in the capacitor and hence you can get more out of it later. Using a step up converter on the output means you can maintain a steady 5V output despite the voltage gradually reducing during discharge. Doing both maximises the length of time you will be able to maintain that 5V output, if that is what you need to do.

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  • \$\begingroup\$ Thanks Finbarr, clear as a bell! :) what of the three options you mention will be more efficient considering the source is a solar panel \$\endgroup\$ – Andrea May 9 '17 at 9:25
  • \$\begingroup\$ It's not so much about efficiency as making the circuit do what you want. I would guess you want to maintain the 5V output for as long as you possibly can for something, so you're probably going to need a converter on the output. With a bit of switching, you could even use the same converter for charging and discharging if you wanted. \$\endgroup\$ – Finbarr May 9 '17 at 9:46
  • \$\begingroup\$ The step up on the output is a must. The step up on the input is highly recommended for significantly more capacity and due to the nature of solar panels and voltage dropouts due to things like clouds. \$\endgroup\$ – Misunderstood May 9 '17 at 9:46
  • \$\begingroup\$ You could do something marx generator esque, charge in parallel, discharge in series. Probably more complicated than a boost converter though. \$\endgroup\$ – Colin May 9 '17 at 10:03
  • \$\begingroup\$ The idea is charging a mobile phone. The supercapacitor could not help in avoiding stepup on the input? I was thinking also to add a battery: with a stepup converter on the output I add in turn a lifepo 3.2v 1500ma, a 3.7v 4000ma and a two nimh reaching 2.4v 4000ma. The last one after 5h of bright sun I was able to charge 30% max of a 2200ma phone. \$\endgroup\$ – Andrea May 9 '17 at 12:21

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