Charging a supercapacitor - Page 1
Forum section: electronics / beginners

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: April 18, 2013, 02:54:11 am
content: I've decided to start a little side project that involves using a supercapacitor instead of a rechargable battery. While I have read up on using supercaps, I'd just like to confirm a few concepts before I start. (For my experiments, I'm using a 400F 2.7V supercap.)Q1: Am I correct that I want aconstant-currentcircuit to charge a supercap?Q2: Am I correct that the chargingvoltageshould start out at whatever the supercap's current voltage is, and then be dynamically raised high enough to maintain the constant-current feedas long asthe 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?Any advice is appreciated!

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 03:00:25 am
content: Datasheet? It differs from one to the next. Some don't require any limiting at all for discharge, as they can't deliver enough current to damage themselves, and only require limiting for charge if you need to avoid the inrush. Even if it does require limiting, I think that a CC circuit is probably gilding the lily a bit for most implementations. A resistor ought to be fine.

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: April 18, 2013, 03:06:15 am
content: The supercap I'm working with is theIOXUS RSC2R7407SR.

author: BravoV
author_rank: Super Contributor
timestamp: April 18, 2013, 03:10:52 am
content: Though just an EE wannabe, take my advise with a grain of salt ..Quote from: SoftwareSamurai on April 18, 2013, 02:54:11 amThe need for constant current is just merely to avoid overloading the supply part, cause without a controlled charging condition (constant current), the supercap to the power supply is like a short circuit that will fry the charging circuit.Quote from: SoftwareSamurai on April 18, 2013, 02:54:11 amIf the supply part voltage is "lower" than the supercap voltage, then it will discharge it's energy to the supply circuit, assuming there is no diode to block the backward current.Quote from: SoftwareSamurai on April 18, 2013, 02:54:11 amSupercap capable of sourcing huge current, the purpose of limiting the current is just merely a controlled energy discharge, like using a resistor as you mentioned or using a regulator circuit for more precise discharging mechanism.

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 03:16:02 am
content: Jesus, that's one fancy datasheet...With that kind of short circuit current I'd definitely be using a current limiting resistor, or at least a polyswitch. You could start a car with a few of these... Depending on how long of a charge time is acceptable to you, I'd either use a series power resistor, or a CC circuit delivering a relatively high current for faster charge. A 2A charge current will charge in 9 minutes, and a 0.27 Ohm resistor will give you roughly the same charge time.With such a high charge current and power dissipation, you might also consider a buck converter. It's pretty easy to rig an MC34063 to give constant current. Assuming you're charging it from a 2.7V rail, you'll be dissipating over 3W over the first 2 minutes.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 18, 2013, 08:12:56 am
content: I'd re-wind a MOT, rectify it, put the rectifier on a big-ass heatsink, charge the cap with that

author: Nickk2057
author_rank: Regular Contributor
timestamp: April 18, 2013, 12:53:01 pm
content: haha... you guys are nuts when it comes to those supercapsbut i guess those things are great to use (or abuse) in the work environment

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 12:55:05 pm
content: Quote from: Nickk2057 on April 18, 2013, 12:53:01 pmHow so?

author: smashedProton
author_rank: Frequent Contributor
timestamp: April 18, 2013, 02:08:05 pm
content: Just make a charge pump ..

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 18, 2013, 02:34:22 pm
content: I need to go back to the tool storage , and look for something I bought from Snap On tools while I was a mechanic , and open it up again .It was a small light jump-start box and used caps for jumpstarting , 12vdc up to 1600 amps , in order to use , it had to be connected to a supplied wal-wart to keep it charged , so not good for keeping in trunk , unless it was connected to the car's charging system , but again that wouldn't work , because if it was parked and needed , the unit wouldn't be charged , so shop use only .

author: c4757p
author_rank: Super Contributor
timestamp: April 18, 2013, 02:42:31 pm
content: Quote from: smashedProton on April 18, 2013, 02:08:05 pmI don't see how a charge pump is going to help you efficiently charge a 400F capacitor.

author: AlfBaz
author_rank: Super Contributor
timestamp: April 18, 2013, 02:48:54 pm
content: Quote from: KD0CAC John on April 18, 2013, 02:34:22 pmIf the cap leakage is that bad you'd want to avoid having it in the boot, electron stains are really hard to get out of the carpet

author: Nickk2057
author_rank: Regular Contributor
timestamp: April 19, 2013, 01:42:30 am
content: what I mean is that is one of those short circuits it will blow up in your face... and or cause a fire.... and like KD0CAC John said... "shop use only"... which sounds like a good warning since it will have a good fire extinguisher nearby..

author: c4757p
author_rank: Super Contributor
timestamp: April 19, 2013, 02:16:14 am
content: Eh, just stick a fuse on it. I'd go for a polyswitch - it'll have enough internal resistance to drop the short circuit current even before it trips. Not a big deal unless you go messing around before the fuse.

author: Ferroto
author_rank: Frequent Contributor
timestamp: April 19, 2013, 03:40:11 am
content: Be careful supercapacitors can deliver alot of current at relatively low voltages, here is a video showing how.Useful calculator.http://www.circuits.dk/calculator_capacitor_discharge.htm

author: Nickk2057
author_rank: Regular Contributor
timestamp: April 19, 2013, 04:56:02 am
content: Quote from: Ferroto on April 19, 2013, 03:40:11 amnow see... that is what I was talking about.... just so much energy in those things.... a lot of sparks from just a capacitor (or capacitors)

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: April 19, 2013, 11:57:34 am
content: I decided to try to charge up my 400F supercap and see what happens when I disconnect it and leave it alone for a while.To charge it, I used an old PC power supply and the 3.3V rail and a tiny current limiter (300mA). I connected my meter across the cap and watched it slowly creep up to 2.5V. (Took a several hours.) Then I disconnected it completely. I've been periodically checking it now for some time.@ 0 hours, 2.490V. (Just disconnected from the charging circuit.)@ 8 hours, 1.936V.@ 19 hours, 1.685V.@ 34 hours, 1.640V.@ 44 hours, 1.481V.Q: Do supercaps really drop that much voltage over a ~48 hr period?

author: Flávio V
author_rank: Regular Contributor
timestamp: April 19, 2013, 06:31:24 pm
content: The supercap should have retained near 2.2V in those 44H....when i charge my 350F maxwell ones after 8 hours they go from 2.7V to near 2.5-2.6V...so those super caps might be faulty or it is normal leakage...Ioxus can leak at 1mA...the maxwell ones leak at 300uA(max or both)

author: Nickk2057
author_rank: Regular Contributor
timestamp: April 21, 2013, 02:03:25 pm
content: hehe.... did it go boom yet?anyways.. if it hasn't... congratz

author: chris.jarboe
author_rank: Newbie
timestamp: April 22, 2013, 02:50:59 am
content: Samurai,The initial leakage current for a supercap is typically high. The datasheet for the Ioxus part specifies 1.2mA for the leakage current, but this is "72hr leakage," or the leakage current after being charged for 72 hours. So yes, this is normal when they are new.As for your original questions:1) Absolutely correct.2) Absolutely correct.3) Ideally you want as little series resistance as possible, so I would start without it.Good Luck!

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: April 22, 2013, 11:52:41 am
content: Quote from: chris.jarboe on April 22, 2013, 02:50:59 amThanks Chris.So, when you say this is normal when they are new, do you mean that it will leak less after many charge/discharge cycles? (i.e. As it gets "older"?)

author: chris.jarboe
author_rank: Newbie
timestamp: April 22, 2013, 10:37:17 pm
content: Yes. The leakage current will eventually be what you expect from the datasheet.

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: April 22, 2013, 11:36:27 pm
content: Interesting. That's not what I would expect from a capacitor.I did contact IOXUS and they would like me to repeat my experiment and take measurements every 8 hours to double-check the discharge rate. Looks like I know what I'll be doing next weekend!

author: jh15
author_rank: Frequent Contributor
timestamp: April 23, 2013, 04:08:04 am
content: I bought a Coleman capacitor screwdriver just to have the concept in hand of capacitor power.If I did not buy this product, I would think it was all vaporware. Great to demo to people. I drive many screws, then complain, oh gee my screwdriver needs to be recharged... they expect it to take hours, it takes about 30 seconds.

author: Psi
author_rank: Super Contributor
timestamp: April 23, 2013, 04:32:48 am
content: Quote from: SoftwareSamurai on April 19, 2013, 11:57:34 amNo, that's not normal.My 2.5V maxwell 2600F caps take 1-2 months to drop from 2.5V to 2.0V and after that the loss is much slower.I've got one here which hasn't been charged in ~6 months and its still at 1.8VYou either had something connected to the caps which was drawing current or your caps are faulty (perhaps small internal leakage).

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: April 23, 2013, 01:56:01 pm
content: 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.

author: duracell
author_rank: Contributor
timestamp: February 13, 2017, 10:59:29 pm
content: 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 VWhen 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

author: Psi
author_rank: Super Contributor
timestamp: April 15, 2017, 05:11:20 am
content: Quote from: SoftwareSamurai on April 18, 2013, 02:54:11 amQ1: Constant current with a max voltage limit yes.Q2: You can't do anythingexceptstart 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 blowfuse 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.

author: Psi
author_rank: Super Contributor
timestamp: April 15, 2017, 05:15:21 am
content: Quote from: duracell on February 13, 2017, 10:59:29 pmAll 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.

author: IanB
author_rank: Super Contributor
timestamp: April 15, 2017, 05:18:23 am
content: Quote from: SoftwareSamurai on April 18, 2013, 02:54:11 amQuote from: Psi on April 15, 2017, 05:11:20 amWow! That has to be the longest time between question and answer ever

author: Psi
author_rank: Super Contributor
timestamp: April 15, 2017, 05:19:22 am
content: Yeah, i didn't realize the first question was so old until after posting it

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2017, 05:21:32 am
content: Have you still got a set under the bonnet of your car Psi ?

author: Psi
author_rank: Super Contributor
timestamp: April 15, 2017, 05:21:59 am
content: Yep, still in the car, still working wellAm upgrading to a newer car soon, so will probably have to buy a battery for this car before i sell it.