Electronics > Beginners
charge a super capacitor
nickeevblog10:
Hi, I purchased a 2.7V 500F capacitor to use as an experimental spot welder for battery tabs. I will not spot weld many battery tabs and I have no idea if this Youtube project will really work. After some looking at the specifications it seems this capacitor can store something like 30Amps and so may have a good little whack of energy needed to melt light aluminium strips for tabbing AA AAA and other batteries.
My question, I have a variable power adapter which has 12-3V settings and charges @ .500A and puts out 4.4V. Can I safely charge this capacitor using this power adapter setting.
I also understand I need to monitor the rate of charge with my multimeter so the capacitor does not overcharge and dangerously explode or something like this.
This capacitor has no dangerous chemicals according to is specification .pdf file.
http://www.samwha.com/electric/product/list_pdf1/DB.pdf
This capacitor was dented during transit, the packaging was damaged but because there is no damage other than a closed dent I am going to use it. I also asked for a replacement and the person offered me a refund because he no longer had any left. I took the refund, not knowing if it would work or not. I am not sure what I can do about this now.
Psi:
If you put a diode in series with the 3V from the wall adapter this will drop the voltage by ~0.5V and prevent any capacitor over-charge.
Something like a 1N4007 diode or similar.
You also must protect against drawing too much current from that wall adapter because i doubt that one has a current limit.
It may overheat if you don't limit the initial current somehow. I recommend you find some sort of dummy load, like a resistor or light bulb which draws the max current available (500mA at 3V).
Put that resistor/lightbulb in series too, it will prevent over-current when the capacitor is first connected in a flat state. It will also prevent the diode from being damaged by large currents.
Ideally you should find a better power supply that has adjustable voltage and current limit.
If you set a proper lab power supply to output 2.7V with its current limit set to the max rated current of your cap (30A) it will charge up the super cap as fast as possible. It will also be safe from overcurrent or overvoltage.
NOTE: A psu that can do 2.7V at 30A is going to be hard to find. Most lab power supplys do 0-30V at 0-5A or something like that.
You can use a 5A one, it will just take longer to charge the cap at 5A than 30A.
The important thing is to set the voltage to match the cap voltage of 2.7V
I also have my doubts you are going to get enough energy from that cap to spot weld a battery tab.
I tried spot welding with a 2.5V 2600F maxwell supercap and couldnt get it to work very well.
mariush:
I simply used a 1117 adjustable linear regulator, configured at 2.65v with 2 resistors: https://www.digikey.com/short/p4b890
Some have 0.8A max current, others 1A... and internal current limit at around 1.2-1.5A and input voltage must be ~1v higher than output, so it works fine with 5v from USB, phone chargers etc...
If you want you could use regulators with fixed 2.5v output, it's a super common voltage... and sometimes cheaper ex 15 cents for 1117-2.5v : https://www.digikey.com/short/p4b85r
...or 11 cents for the MIC5504-2.5 : https://www.digikey.com/product-detail/en/microchip-technology/MIC5504-2.5YM5-TR/576-4880-1-ND/5209408
This one's 300mA max but has only 0.38v drop at 300mA so you could use the 3v output and get 2.5v out.... but charging 500F would be slooow.. like 12000s at ~300mA (6000 seconds to 2v)
At 0.8A, 500F would charge to 2.5v in ~4200s (half to reach 2v)
nickeevblog10:
Just now realizing I can measure Amperes with my multimeter @ 3V reads 0.06A. This is a DC power adapter so I think the voltage and current are both going to be steady and slow charging, very slow if you like.
So charging the capacitor seems to me to be possible safely if I monitor the charged capacitor so the charge does not go over 2.8V which is the absolute maximum rated voltage, but I may just like to charge up to 2.6V to keep things safe.
What do you think? I do not mind even letting the charge go slowly as this is my first time at this.
Could I even charge at a higher Voltage. I notice the Amps double @ 4.5V.
This is an experiment and the metal for the tabs is very thin, thank you China.
Psi:
You can't measure amps when the red DMM probe is in the volts socket.
But it's good that you did that wrong because you had the DMM probes connected directly across the power supply!
If you had used the DMM amps socket you would have shorted the power-supply and maybe damaged something.
You can only measure the amps that a particular load draws. You can't measure the amps of a powersupply
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