# The super capacitor charging very slow

Joined Nov 21, 2018
634
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

#### Papabravo

Joined Feb 24, 2006
16,829
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.

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#### dl324

Joined Mar 30, 2015
13,141
it's charging very slow at 0.02 - 0.04 A, why?
Do you have a meter in series with the cap to monitor current?

Joined Nov 21, 2018
634
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
16,829
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.

Joined Nov 21, 2018
634
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

#### BobTPH

Joined Jun 5, 2013
3,684
A constant current source.

Bob

#### Alec_t

Joined Sep 17, 2013
12,210
A possible reason for the slow charging is that an uncharged capacitor is effectively a short circuit across the buck converter output. That would cause any decent converter to shut down, or go into current-limit mode, to protect itself.

#### Sensacell

Joined Jun 19, 2012
2,893
A supercap "looks" like a dead short to the regulator, as previously stated, it's probably not starting correctly.

#### Papabravo

Joined Feb 24, 2006
16,829
Thank you.
I am first time to deal with this, what device should I add between?
Best
Any device to limit the current. An appropriately sized resistor is an obvious choice, but a garden variety light bulb is also an alternative.

#### Yaakov

Joined Jan 27, 2019
3,519
Any device to limit the current. An appropriately sized resistor is an obvious choice, but a garden variety light bulb is also an alternative.
Abd the light bulb has the added side effects of showing the state of charge.

#### DickCappels

Joined Aug 21, 2008
7,742
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?

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
9,304
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
16,829
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.

#### Papabravo

Joined Feb 24, 2006
16,829
1 million times larger.
We seem to be having "orders of magnitude" problems. What could be the reason?

Joined Nov 21, 2018
634
Do you have a meter in series with the cap to monitor current?
Thanks.
I used a multimeter to do it. and shown 0.04A TO 0.18A.

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#### MisterBill2

Joined Jan 23, 2018
9,304
We seem to be having "orders of magnitude" problems. What could be the reason?
Probably the error was due to not being fully awake yet.

Joined Nov 21, 2018
634
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.