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.

 

What did you calculate for the Charge Time = 5t ?

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

 

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

 

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.

 

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.

 

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

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?
Load consumes 450mA constantly.

 

hi il,
This is what LTSpice shows for the Charge/Discharge time.
E

 

Can

Can I use same formula for discharcig?
Load consumes 450mA constantly.

Yes.

Bob