Charging Supercapacitor - Page 1
Forum section: electronics / beginners

author: Undweeber
author_rank: Regular Contributor
timestamp: April 01, 2018, 08:08:55 pm
content: 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 0.4VWhereas it charges perfectly fine with Apple's 1A charger from zero to full voltage of 2.7VAlso, 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.

author: helius
author_rank: Super Contributor
timestamp: April 01, 2018, 08:41:37 pm
content: 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.

author: rs20
author_rank: Super Contributor
timestamp: April 01, 2018, 09:23:15 pm
content: 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 belowallthe maximum limits (voltage, internal temperature*, current) while charging, you'll be fine.* As calculated using the thermal resistance, electrical resistance, current, and ambient temperature.Quote from: helius on April 01, 2018, 08:41:37 pmYeah... like a USB charger, right?...

author: Undweeber
author_rank: Regular Contributor
timestamp: April 02, 2018, 04:51:33 am
content: 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?

author: digsys
author_rank: Unknown
timestamp: April 02, 2018, 05:44:01 am
content: 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

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 02, 2018, 06:15:51 am
content: I found a PDF, talking about charging super capacitor's:https://www.intersil.com/content/dam/intersil/whitepapers/switching-controller/supercapacitor-charging.pdfHope that it may help:-)

author: Eka
author_rank: Regular Contributor
timestamp: April 02, 2018, 01:42:20 pm
content: 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.

author: mikerj
author_rank: Super Contributor
timestamp: April 02, 2018, 06:29:20 pm
content: Quote from: digsys on April 02, 2018, 05:44:01 amThere 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.

author: ogden
author_rank: Super Contributor
timestamp: April 02, 2018, 06:45:26 pm
content: Quote from: Undweeber on April 01, 2018, 08:08:55 pmI am afraid that I am not aware of Samsung or Applesupercap chargers. Could you please share information about such?

author: Undweeber
author_rank: Regular Contributor
timestamp: April 04, 2018, 12:31:42 am
content: Quote from: ogden on April 02, 2018, 06:45:26 pmI am talking about phone charging bricks, i hope that wasn't meant to troll me@mikerjI 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.

author: ogden
author_rank: Super Contributor
timestamp: April 04, 2018, 12:52:31 am
content: Quote from: Undweeber on April 04, 2018, 12:31:42 amI 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.

author: David Hess
author_rank: Super Contributor
timestamp: April 04, 2018, 02:35:04 am
content: Power supplies often implement foldback current limiting as a safety measure which lowers the maximum output current as the output voltage decreases.

author: rs20
author_rank: Super Contributor
timestamp: April 04, 2018, 04:06:23 am
content: Quote from: Undweeber on April 04, 2018, 12:31:42 amYou'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: 170AInitial ESR: 3.2 milliohmESR after lifecycle: 3.2 milliohm + 100% = 6.4 milliohmPower dissipated if charged/discharged at 100A: I^2 * R = 100A ^ 2 * 6.4 milliohm = 64WThermal resistance: 10.9 degrees C/WInternal temperature if continually charged/discharged at 100A: 64W * 10.9 degrees/W + 25 degrees ambient = 722.6 degrees CAbsolute maximum temperature: 65 degrees CIs 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 secondsEnergy dissipated: 64W * 9.45 seconds = 604.8 JThermal capacitance: 60 Joules / KelvinTemperature 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...)

author: xani
author_rank: Frequent Contributor
timestamp: April 04, 2018, 05:05:08 am
content: You don't evenneed, it is right there in datasheet:Maximum Continuous Current(?T = 15oC) 21 A RMSMaximum Continuous Current(?T = 40oC) 34 A RMS

author: Undweeber
author_rank: Regular Contributor
timestamp: April 04, 2018, 10:48:10 pm
content: @ogdenThat 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.@rs20Okay 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

author: Seekonk
author_rank: Super Contributor
timestamp: April 05, 2018, 05:55:35 am
content: 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.

author: ogden
author_rank: Super Contributor
timestamp: April 05, 2018, 07:10:41 am
content: Quote from: Undweeber on April 04, 2018, 10:48:10 pmIt is. Very. The bigger capacity - the more danger.Further reading: Maxwellsupercapacitor documentation, chapter "7.1.1 Safety considerations about the voltage and current ".In short: RTFM