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Supercap charging


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Hi,
        Is it fairly safe to charge a fully discharged 2.7v supercapacitor from an alternating voltage source of more than its rated voltage? I plan to design a circuit to charge the one using yet-to-be-designed pull string type axial flux type generator.

Thanks in advance. 

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You need to see what the Charge current is for the super cap and treat it like you would charging a high current battery (i.e. a Lithium)

It's ok to have a higher charge voltage as long as you monitor the actual voltage of the super cap during this phase and the charge current rate does not exceed the rating of the super cap.

For example if the super cap reads 2.2v and your charging supply is 5V, then the difference of 2.8v needs to be dissipated through the internal resistance of the super cap and presumably the Rds "ON" of a MOSFET your turning on or off to charge with.  As long as the dissipated 2.8v over R (R in this case is the internal resistance of the super cap + the Rds ON of the MOSFET)  does not exceed the rated current for the super cap then you will be fine.  Also in the same setup scenario, when the super cap reaches 2.7v you will need to cut off the MOSFET entirely to prevent over charging the super cap

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The voltage across the capacitor should never exceed its rated value, indeed it's a good idea to leave a safety margin of at least 10%, so it it's rated to 2.7V, keep the voltage below 2.43V.

Supercapacitors are polarised and should never be subjected to reverse voltage for any period of time, typical ratings for short term voltage reversal are 10% of the rated voltage or 0.6V, whichever is lower, so if it's rated to 2.7V, the reverse voltage should never be greater than 0.27V and it's a bad idea to leave the capacitor reverse biased for long periods of time.

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Thanks for your quick replies. How do I select the bridge rectifier and buck converter? I think the emf generated by the generator would be under 50v with the current carrying capacity of a few mAmps(I plan to use bipolar stepper motor instead of axial flux generator). I think the output of the converter would be 2.43v which is fed directly to the 400F supercap. Am I thinking in the right direction or are there other things that I need to consider? Here is what I am working to accomplish:

Generator-->Rectifier-->Buck Converter-->Supercapacitor-->Boost Converter-->Load (bunch of LEDs, etc.)

Thanks.

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