The super capacitor charging very slow

Thread Starter

LAOADAM

Joined Nov 21, 2018
371
Hi,
I am charging my super capacitor of 2.7V - 500F with a 300W Boost Converter Step Down to 2.7V, it's charging very slow at 0.02 - 0.04 A, why? Do I have to use a current adjustable power supply?
Thanks
Adam
 

Papabravo

Joined Feb 24, 2006
15,464
Boost converters don't normally have a lower output voltage than the input voltage. My guess would be there is unaccounted for resistance. Charging a capacitor always involves an R and a C. That resistance is designed to keep it from discharging an enormous amount of energy in a short period of time. That energy is:

\( \frac{1}{2}CV^2\;=\;1823\;\text {Joules} \)

That is the same amount of energy in a 500 Kg. block of steel traveling 2.7 meters/second slamming into a brick wall, or a bit less than half a gram of TNT.
 
Last edited:

Thread Starter

LAOADAM

Joined Nov 21, 2018
371
Boost converters don't normally have a lower output voltage than the input voltage. My guess would be there is unaccounted for resistance. Charging a capacitor always involves an R and a C. That internal resistance is designed to keep it from discharging an enormous amount of energy in a short period of time. That energy is:

\( \frac{1}{2}CV^2\;=\;1823\;\text {Joules} \)

That is the same amount of energy in a 500 Kg. block of steel traveling 2.7 meters/second slamming into a brick wall.
Thanks.
Sorry I made a wrong name of it, it is BUCK step down.
I guess I need make adjustment on the CURRENT pot, which I didn't do.
 

Papabravo

Joined Feb 24, 2006
15,464
Thanks.
Sorry I made a wrong name of it, it is BUCK step down.
I guess I need make adjustment on the CURRENT pot, which I didn't do.
Be careful about connecting the power source directly to the capacitor. Without some device to limit the current an accidental discharge could be painful or perhaps fatal. Don't try to charge it too rapidly either. You want to be deliberate in the way you go about this. Folks will sometime use a light bulb for this purpose.
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
371
Be careful about connecting the power source directly to the capacitor. Without some device to limit the current an accidental discharge could be painful or perhaps fatal. Don't try to charge it too rapidly either. You want to be deliberate in the way you go about this. Folks will sometime use a light bulb for this purpose.
Thank you.
I am first time to deal with this, what device should I add between?
Best
 

DickCappels

Joined Aug 21, 2008
7,155
I use a current limited switching supply to charge my 100F capacitors. No, I would not want to slap one across my bench supply -why stress out either the bench supply of the cap?

1617021035148.png

1617021067735.png
This is the voltage across a 100F ultracap in millivolts vs time in seconds -about 700 ma as set by R2.
 

MisterBill2

Joined Jan 23, 2018
8,214
That 500 Farad capacitor is requiring a whole lot of charge. Consider the time to charge a 500 Microfarad capacitor, (500 mfd=500x 10to the -6power farad. So your new capacitor is about 10,000 times larger. Another factor is that the effective series resistance (ESR) may not be small on your new capacitor.
One other thing is that a maximum voltage of 2.7 volts there is not much of a shock hazard at all. Short circuiting the capacitor is not advised, but that is quite different.
 

Papabravo

Joined Feb 24, 2006
15,464
That 500 Farad capacitor is requiring a whole lot of charge. Consider the time to charge a 500 Microfarad capacitor, (500 mfd=500x 10to the -6power farad. So your new capacitor is about 10,000 times larger. Another factor is that the effective series resistance (ESR) may not be small on your new capacitor.
One other thing is that a maximum voltage of 2.7 volts there is not much of a shock hazard at all. Short circuiting the capacitor is not advised, but that is quite different.
It's not the shock, it is the amount of energy released in a short period of time. It is what happens to the package when that energy is released.
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
371
I use a current limited switching supply to charge my 100F capacitors. No, I would not want to slap one across my bench supply -why stress out either the bench supply of the cap?

View attachment 233915

View attachment 233916
This is the voltage across a 100F ultracap in millivolts vs time in seconds -about 700 ma as set by R2.
Thanks.
I saw this:
can charge at 3 A with a Bench.
I'll try you circuit.
 
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