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In my design I have super capacitor (1.5 F) of maximum rated voltage of 5.5 V. I am charging it to only 3 V. I am using boost converter to convert it to 3.3 V so that I can utilize the super capacitor at lower extent and at the same time it is not charged to higher side (near 5 V) so that stress is less and super capacitor life is more. boost converter can convert from 1-3.2 V to 3.3 V.

Is my designing is correct? Is using super capacitor down up to 1-1.2 V correct idea.

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    \$\begingroup\$ What you are doing seems fine, but you can go higher than 3V. Feel free to use 95% of allowed voltage, moat energy is there. \$\endgroup\$
    – user76844
    Aug 17, 2018 at 7:11
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    \$\begingroup\$ You will get better run time if you use a buck-boost converter and charge your capacitor up to 5 V, but what you're doing is fine. \$\endgroup\$
    – Colin
    Aug 17, 2018 at 8:11
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    \$\begingroup\$ You're being over-cautious. Going to 4.2v would give you twice the stored energy of 3v, and you're still well clear of that 5.5v limit. 5.2v would give you three times the stored energy of 3v, but perhaps that's pushing your comfort zone too much. \$\endgroup\$
    – Neil_UK
    Aug 17, 2018 at 8:41
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    \$\begingroup\$ I am sorry, but your calculations seem weird. If you can save 60% of your supercap, additional 50 cents on IC is nothing. How much does your supercap cost? \$\endgroup\$
    – user76844
    Aug 17, 2018 at 8:56
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    \$\begingroup\$ Hard to judge without specifics. Where i worked with supercaps few years ago, they went to around 90-95% of maximum voltage and everyone were happy. Anyway, using only 36% of available energy storage seems very strange. Triple de-rating is normal for military or aerospace, not for commercial/industrial electronics. \$\endgroup\$
    – user76844
    Aug 17, 2018 at 16:02

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To maximise capacitance over the lifetime of the device, you are correct to limit the voltage. Also critical to lifetime is limiting the operating temperature.

From the experimental data below, you can see that just a small change in voltage resulted in a huge loss of lifetime (in this case, 20% capacitance loss).

source: https://www.garmanage.com/atelier/index.cgi?path=public&B&Energy_storage/Aging/Derating enter image description here

This "lifetime" of course is less relevant if a 20% loss in capacitance is not a problem for your design. Still, ESR will increase. It is application dependent.

Find how small you can make the supercapcitor before you start having problems and derate with that and your expected lifetime in mind.

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    \$\begingroup\$ What’s the voltage rating of the capacitor in your example? \$\endgroup\$
    – winny
    Aug 17, 2018 at 16:57

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