EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Undweeber on April 01, 2018, 08:08:55 pm
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Why does charging supercapacitor using 1A Samsung charger does not do anything if the supercap is fully discharged, but only starts working when the supercap is at about 0.4V
Whereas it charges perfectly fine with Apple's 1A charger from zero to full voltage of 2.7V
Also, attached is the spec sheet to my supercap, does it include graphs for how to charge it properly? help me understand it because i don't get it.
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If you apply the ESR in the datasheet to the definition of resistance, you get a short-circuit current of 844A which is above the datasheet maximum current limit. In practice any practical power supply will have limited ability to supply current and should be within the specs in the datasheet.
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If the charger provides 1A, the voltage across the cap will initially be 3.2 milliohm * 1 Amp = 0.0032 Volts. From the point of view of the charger, this looks exactly the same as being shorted out with a wire, which is obviously well beyond the normal operating conditions for a USB charger. There are two possible, perfectly reasonable ways in which a USB charger might handle this condition:
- (Samsung?) Consider it a fault and turn off
- (Apple?) Behave as a 1A constant current power supply (given that a lot of chargers communicate their max power by simply switching to CC mode, allowing the phone to detect the sag in voltage, this is more likely than you might think)
Supercapacitors don't have complicated charging strategies like batteries, so there's no particular instructions in datasheets. As long as you stay below all the maximum limits (voltage, internal temperature*, current) while charging, you'll be fine.
* As calculated using the thermal resistance, electrical resistance, current, and ambient temperature.
In practice any practical power supply will have limited ability to supply current and should be within the specs in the datasheet.
Yeah... like a USB charger, right?...
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but that datasheet states nothing on how to charge it, I see what it can put out but i dont see the maximum voltage current i can charge it with so I have been being safe by charging it using 1A, it takes 21:30 minutes to fully charge from "0V" but if I don't want to wait I use the 2.4A 13W iPad charger which charges it in 6:30 minutes, one time I overcharged it by not monitoring the voltage and the supercap got warm and it reached 3.2V so I had to conduct an emergency discharge by shorting it out and making sparks, then let it cool down for the night, so i don't want to go through that again.
What is the max voltage and current I can use to charge it based off the data sheet?
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There is no technical limit to charge rate, unlike a battery. You can charge at 100A if you can supply it (and the cap is in good condition).
ANY over voltage, however, MAY reduce it's life #cycles, or even destroy it. They usually specify a very short term max of 5.0V and a "kaboom"
voltage of 5.5V
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I found a PDF, talking about charging super capacitor's: https://www.intersil.com/content/dam/intersil/whitepapers/switching-controller/supercapacitor-charging.pdf (https://www.intersil.com/content/dam/intersil/whitepapers/switching-controller/supercapacitor-charging.pdf)
Hope that it may help:-)
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For mains powered circuits, I charge super capacitors though a resistor and diode string to set a maximum charge current, and voltage. The combined diode voltage drops allows me to charge it from a 5VDC source safely. The diodes also keep it from back feeding the source after power has failed. You need to use very low leakage current diodes so they don't draw heavily on the supper cap after power is out.
A 1F super capacitor has the energy to keep the DS3231 clock chip I normally use going for a few days of power outage. More than enough for most power outage situations, and I don't have a lithium backup battery to make shipping a pain. Unfortunately it does cost more.
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There is no technical limit to charge rate, unlike a battery. You can charge at 100A if you can supply it (and the cap is in good condition).
ANY over voltage, however, MAY reduce it's life #cycles, or even destroy it. They usually specify a very short term max of 5.0V and a "kaboom"
voltage of 5.5V
There is a limit to charge rate; the datasheet clearly states an absolute maximum current of 170 Amps and a maximum continuous current of 34 Amps for a temperature rise of 40C.
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Why does charging supercapacitor using 1A Samsung charger does not do anything if the supercap is fully discharged, but only starts working when the supercap is at about 0.4V
Whereas it charges perfectly fine with Apple's 1A charger from zero to full voltage of 2.7V
Also, attached is the spec sheet to my supercap, does it include graphs for how to charge it properly? help me understand it because i don't get it.
I am afraid that I am not aware of Samsung or Apple supercap chargers. Could you please share information about such?
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I am afraid that I am not aware of Samsung or Apple supercap chargers. Could you please share information about such?
I am talking about phone charging bricks, i hope that wasn't meant to troll me
@mikerj
I don't understand, are you sure that's 170A is the charge limit or is it the discharge limit? Also, what if I use one of those microwave oven motors that produce 120V if you crank it, but very small current.
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I am afraid that I am not aware of Samsung or Apple supercap chargers. Could you please share information about such?
I am talking about phone charging bricks, i hope that wasn't meant to troll me
I asked in hope that you start to think and dig into reasons of my question.
Voltage of discharged supercapacitor is 0V, but for phone charger it essentially is short circuit. Phones never short chargers, most of them draw current while voltage is at nominal 5V. Some phone chargers have built-in short circuit protection. If so, then such cannot be used for supercapacitor directly w/o additional circuit that does not allow short circuit protection to trip.
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Power supplies often implement foldback current limiting as a safety measure which lowers the maximum output current as the output voltage decreases.
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I don't understand, are you sure that's 170A is the charge limit or is it the discharge limit? Also, what if I use one of those microwave oven motors that produce 120V if you crank it, but very small current.
You're thinking too much like this thing is a battery and not at all like it's a capacitor. It's a maximum current limit; it doesn't care whether it's "charging" or "discharging".
Like I said before, you just have to make sure that you obey all of the stated limits. From the datasheet:
Absolute maximum current: 170A
Initial ESR: 3.2 milliohm
ESR after lifecycle: 3.2 milliohm + 100% = 6.4 milliohm
Power dissipated if charged/discharged at 100A: I^2 * R = 100A ^ 2 * 6.4 milliohm = 64W
Thermal resistance: 10.9 degrees C/W
Internal temperature if continually charged/discharged at 100A: 64W * 10.9 degrees/W + 25 degrees ambient = 722.6 degrees C
Absolute maximum temperature: 65 degrees C
Is it OK to continually charge/discharge this thing at 100A: 722.6 > 65, so No.
Time taken to charge once at 100A: 350 farad * 2.7 volts / 100 amps = 9.45 seconds
Energy dissipated: 64W * 9.45 seconds = 604.8 J
Thermal capacitance: 60 Joules / Kelvin
Temperature rise due to being charged once at 100A : 604.8 J / (60 J/K) = 10 Kelvins (i.e., 10 degrees C above ambient)
Is it OK to charge this thing once at 100A: 10 + 25 < 65, so Yes.
As mentioned by others, your main concern is avoiding A) short circuiting the thing or B) over-volting it, even slightly and briefly.
My point is, you don't need special permission or special guides to answer your questions. It's all there in the datasheet, you just have to do a few simple calculations using the numbers given in the datasheet. Might be intimidating at first, but honestly if you do your calculations in Google Search and you get the correct units coming out at the end you almost can't go wrong (I mean, the units for thermal resistance and capacitance are degrees C per Watt and degrees C per Joule -- they're almost self-documenting...)
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You don't even need, it is right there in datasheet:
Maximum Continuous Current
(?T = 15oC) 21 A RMS
Maximum Continuous Current
(?T = 40oC) 34 A RMS
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@ogden
That is what Samsung 1A does, it cuts out if the charge is below 0.4V. Apple charger however, neither 1A one, nor 2.4A 13W iPad, care that the cap is at 0V, they charge directly without any additional circuitry, 2.4A one takes 6:30min, and 1A takes 21:30min.
@rs20
Okay I understand that I can put any current into it, but current is not a concern, I don't have anything that could put so much current unless I used the lead acid motorcycle battery I keep in my refrigerator. My issue is that I want to use my 21 VAC motor i harvested from a working microwave, I will hook it up to a rectifier chip which is full wave bridge I believe, I am aware that I am not supposed to go over the voltage of 2.7V, but I have been charging the thing using iPhone charger which is 5V, and stopping the charging manually at 2.7V, monitoring everything with a multimeter.
My main question at this point is whether it is dangerous to charge it with high voltage, like what is the highest voltage power supply can I use and how would it affect the charging, this AC motor is a low amperage high voltage type thing
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I imagine the charger with current limiting may be the one that charges. These small wall warts have a winding on the transformer that powers the chip after initial start up trickle charge. This is that high value resistor from the B+. Short the transformer and it will only get these tiny bumps a couple times a second. The one with current limiting has a higher output resistance as the shunt resistor has to develop at least .7V to work. I imagine if you put a 1/4 ohm in series with the one that doesn't work, it might.
Could be just the components that were chosen. I use a number of wall warts on 50V DC. The low voltage dropout of some chips is much lower than others.
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My main question at this point is whether it is dangerous to charge it with high voltage
It is. Very. The bigger capacity - the more danger.
Further reading: Maxwell supercapacitor documentation (http://www.maxwell.com/images/documents/PG_boostcap_product_guide.pdf), chapter "7.1.1 Safety considerations about the voltage and current ".
In short: RTFM :)