Charging a supercapacitor

[SoftwareSamurai]

Absolutely nothing connected to the cap after I charged it. I didn't even have the meter connected to it while it sat there doing nothing.
I'll repeat my test this weekend and see what happens. This time I'll try to get a measurement at least every 8 hours consistently.

[duracell]

Dear members, I have a question about charging the 2.7 V super capacitor.

I read that the maximal charging voltage for super capacitor should be 2.7 V.

However, sometimes I adjust my power supply to 3.5 V and charge them. Because then the current it's higher and it charges faster.
I connect a voltmeter directly to the capacitor and monitor the voltage rising from e.g. 1.5 V to 2.7 V
When the voltage reaches 2.7, i disconnect it from the power supply.

QUESTION 1: Why is power supply showing 3.5 V and the voltmeter connected to the power supply AND capacitor only 1.5 V when charging starts.
QUESTION 2: Is it wrong way to charge super caps, because I use 3.5 V on the power supply and not 2.7 V ?
QUESTION 3: If it is NOT wrong, can I even increase the voltage even more in order to charge them faster ? Use 5 V, and then disconnect the cap when it is charged to 2.7 V.

Thank you very much for your help

[Psi]

Q1: Am I correct that I want a constant-current circuit to charge a supercap?
Q2: Am I correct that the charging voltage should start out at whatever the supercap's current voltage is, and then be dynamically raised high enough to maintain the constant-current feed as long as the voltage across the supercap never goes above the supercap's rated value?
Q3: Am I correct that when using a supercap as an energy source, I should always add a little resistor in series (or perhaps a current-limiter) to prevent the load from pulling too many mA?

Q1: Constant current with a max voltage limit yes.

Q2: You can't do anything except start charging at the caps current voltage.
If you push massive current into the cap you may notice you're charging at 2V but when you switch it off the cap is 1.5V.
What you're seeing here is losses in the wire between PSU and cap. The cap was never at 2V, it was 1.5V with 0.5V loses in wiring.
So you need your charging system to be measuring voltage for the cut-off right at the terminals of the supercap.

Q3: Depends, it's true that a supercap will not hesitate to smoke your wiring in an instant if you short it out. So i recommend a fast blow
fuse with a good margin above normal current.
You only really need to add some extra series resistance on a supercap when using it for a circuit that expects a battery.
For example, in a car where the alternator expects to see a battery it will often stall the car at idle trying to charge much too fast because the supercap voltage-vs-capacity relationship is quite different to a lead acid battery.

[Psi]

Dear members, I have a question about charging the 2.7 V super capacitor.

I read that the maximal charging voltage for super capacitor should be 2.7 V.

However, sometimes I adjust my power supply to 3.5 V and charge them. Because then the current it's higher and it charges faster.
I connect a voltmeter directly to the capacitor and monitor the voltage rising from e.g. 1.5 V to 2.7 V
When the voltage reaches 2.7, i disconnect it from the power supply.

QUESTION 1: Why is power supply showing 3.5 V and the voltmeter connected to the power supply AND capacitor only 1.5 V when charging starts.
QUESTION 2: Is it wrong way to charge super caps, because I use 3.5 V on the power supply and not 2.7 V ?
QUESTION 3: If it is NOT wrong, can I even increase the voltage even more in order to charge them faster ? Use 5 V, and then disconnect the cap when it is charged to 2.7 V.

Thank you very much for your help

All that matters is the voltage right at the terminals of your supercap.
What you are seeing is voltage being lost in the wire to the cap.
The worry about setting the psu to 3.5v is that you might forget to keep checking the voltage at the cap and you might accidentally charge the cap up to 3.5V and ruin it.

The solution is to use much thicker wire between PSU and cap to reduce the losses in the wires.

Or, even better, use a power supply with a 'remote sense' line that measures the voltage at the supercap.
A powersupply like this will just keep increasing output voltage until the current limit is reached or the voltage right an the cap (sense line) hits the max voltage you have set.   The sense line is used to show you voltage on the power supply display, so you see the voltage right at the cap.

[IanB]

Q1: Am I correct that I want a constant-current circuit to charge a supercap?
Q2: Am I correct that the charging voltage should start out at whatever the supercap's current voltage is, and then be dynamically raised high enough to maintain the constant-current feed as long as the voltage across the supercap never goes above the supercap's rated value?
Q3: Am I correct that when using a supercap as an energy source, I should always add a little resistor in series (or perhaps a current-limiter) to prevent the load from pulling too many mA?

Q1: Constant current with a max voltage limit yes.

Q2: You can't do anything except start charging at the caps current voltage.
If you push massive current into the cap you may notice you're charging at 2V but when you switch it off the cap is 1.5V.
What you're seeing here is losses in the wire between PSU and cap. The cap was never at 2V, it was 1.5V with 0.5V loses in wiring.
So you need your charging system to be measuring voltage for the cut-off right at the terminals of the supercap.

Q3: Depends, it's true that a supercap will not hesitate to smoke your wiring in an instant if you short it out. So i recommend a fast blow
fuse with a good margin above normal current.
You only really need to add some extra series resistance on a supercap when using it for a circuit that expects a battery.
For example, in a car where the alternator expects to see a battery it will often stall the car at idle trying to charge much too fast because the supercap voltage-vs-capacitor relationship is quite different to a lead acid battery.

Wow! That has to be the longest time between question and answer ever 

[Psi]

Yeah, i didn't realize the first question was so old until after posting it

[tautech]

Have you still got a set under the bonnet of your car Psi ?

[Psi]

Yep, still in the car, still working well
Am upgrading to a newer car soon, so will probably have to buy a battery for this car before i sell it.