# supercap charging-discharging time

#### ilginsarican

Joined Jul 13, 2017
137
Hello,
I have 10 pcs 200F supercap in series.
I want to calculate charging and discharcing time but these formulas does not work:

Charge time formula: τ=RC for charge time=5τ
I limited the charge current to 1.2A, so I assumed R=27(supply voltage)/1.2=22,5Ohm.
But it takes 10 minutes to charge to 27V.
I want to know that whether I'm calculating something wrong.

#### mvas

Joined Jun 19, 2017
539
What did you calculate for the Charge Time = 5t ?

5t will charge to 99.3% x 27v = 26.8 v

#### BobTPH

Joined Jun 5, 2013
6,080
Does your series connection include balancing circuitry? If not, that is a problem.

If chaging at a constant current, the formula is:

dV/dt = I / C

Which means if you divide the current by the capacitance, it tells you how many volts it will rise per second.

Bob

#### ilginsarican

Joined Jul 13, 2017
137
What did you calculate for the Charge Time = 5t ?

5t will charge to 99.3% x 27v = 26.8 v
I calculated 5X
What did you calculate for the Charge Time = 5t ?

5t will charge to 99.3% x 27v = 26.8 v
I calculated 5XRXC=(5x22,5Ohmx20F)/60=37,5 minutes.

#### ilginsarican

Joined Jul 13, 2017
137
Does your series connection include balancing circuitry? If not, that is a problem.

If chaging at a constant current, the formula is:

dV/dt = I / C

Which means if you divide the current by the capacitance, it tells you how many volts it will rise per second.

Bob
there is balancing circuitry for each capacitor and also current limiter circuit top of capacitors.

#### BobTPH

Joined Jun 5, 2013
6,080
That is because you are charging at a constant current. That is not the same as charging through a fixed resistor. Use the correct formula that I posted.

dV / dt = I / C = 1.2 / 20 = 0.06 V per second.

So for 27V, it takes 27 / 0.06 = 450 seconds = 7.5 min.

It probably takes a little longer because your power supply can no longer provide 1.2A as the voltage approaches 27.

Bob

#### ilginsarican

Joined Jul 13, 2017
137
Can
That is because you are charging at a constant current. That is not the same as charging through a fixed resistor. Use the correct formula that I posted.

dV / dt = I / C = 1.2 / 20 = 0.06 V per second.

So for 27V, it takes 27 / 0.06 = 450 seconds = 7.5 min.

It probably takes a little longer because your power supply can no longer provide 1.2A as the voltage approaches 27.

Bob
Can I use same formula for discharcig?

#### ericgibbs

Joined Jan 29, 2010
16,803
hi il,
This is what LTSpice shows for the Charge/Discharge time.
E

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#### BobTPH

Joined Jun 5, 2013
6,080
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Can I use same formula for discharcig?
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