super capacitor question - Page 1
Forum section: electronics / beginners

author: luddite
author_rank: Newbie
timestamp: March 11, 2021, 10:19:18 pm
content: HI allI received a Green-Cap 2.7v 500f today and am a bit puzzled.I put 2.7v in and my power supply duly stopped charging it, I checked and it showed 2.1vI then dialled it up to 3.1v and when done, it showed 2.6v and immediately starting discharging.I know the multimeter will draw a little so just put the capacitor to one side and went back to it after 30 minutes.It had dropped to 2.4v and after 2 hours down to 2.1vIs that normal please?

author: liviu.gelea
author_rank: Contributor
timestamp: March 11, 2021, 10:42:08 pm
content: Unfortunately yes, all capacitors letSOMEcurrent through. It's why they're rarely used as long term energy storage. I think they leak more when you aproach the maximum voltage rating as it is proportional to the insulation they provide

author: JustMeHere
author_rank: Frequent Contributor
timestamp: March 12, 2021, 12:50:40 am
content: QuoteWhen it's AC then they pretty much let it all through. This makes me wonder if switching power supplies would have a harder time charging a super cap?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 12, 2021, 01:56:26 am
content: There are a couple of things going on here. Yes there is leakage current. But I would bet there are polarization effects going on. Think of it this way. Even though you are only applying a couple of volts across the capacitor, the spacing between the plates is incredibly small, and the electric field is then extremely high, on the order of hundreds of thousands of volts per meter. Things change under such fields and effectively the plates get even closer. So now you have even higher capacitance with the same charge so the voltage is lower.Two experiments that you can do to confirm this. Charge your capacitor up to a "high" voltage (my recommendation is something under the spec, say 2.5 volts). Monitor the current, it should go to a very low value as the voltage reaches your supply. Now disconnect and measure the time to drop to 2 V. Then recharge to the initial voltage and again disconnect. If the problem is purely leakage the two times will be the same. If polarization is involved the second time will be longer.The second experiment is to discharge the capacitor. As soon as the voltage reaches zero (stops dropping on your meter, since it theoretically never actually gets to zero) disconnect your discharge circuit. Now wait a few hours and measure the voltage across the capacitor. It will likely have gone up, sometimes by a very impressive amount. That voltage results from the polarization relaxing over time.

author: JimRemington
author_rank: Regular Contributor
timestamp: March 12, 2021, 03:01:04 am
content: The performance of at least some supercaps improves after being fully charged and maintained at constant voltage for 24 to 48 hours.After this treatment you may notice a large drop in the leakage current and hence, the charge retention time for a disconnected capacitor.My positive experience of this nature was with much smaller supercaps (e.g. 0.47 - 1.0 F) so try it with your monster, and let us know.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2021, 05:10:31 am
content: What do you mean by "power supply stopped charging it", and why was the terminal voltage different from the setting? What power supply is it?I ask, because the sequence of events is unlike any experience I've had with a power supply. I doubt either that you are using a normal type of power supply, or that you are using it in the usual way.In either case, the impression I get is that the capacitor's terminal voltage was set to something (regardless of current flow in that instant), and you're seeing its voltage drop slowly over time?If that is the correct sequence of events, then:Yes, that is normal behavior. EDLCs (electric double layer capacitors) exhibit considerable dielectric absorption. The physical mechanism is ionic diffusion. The electric double layer, is a layer of charged ions in a solvent, surrounding the electrodes. The double-layer is very thin (some molecules), so it can be packed very tightly; the capacitor is constructed using porous materials with extremely high surface area (activated carbon), and this is why the capacitance can be so massive. However, there is very little flow of ions through those pores. So, the open faces of those electrodes can charge quickly, but that doesn't amount to much charge; then charge has to move into the spaces just behind them, and so on, deeper and deeper into the pores.The overall effect is that, for a step change in voltage, the current flow goes as 1/sqrt(t). Or the ESR or capacitance, measured at a given frequency, goes as sqrt(F) or 1/sqrt(F) respectively. This works down to extremely low frequencies (~µHz), which is to say, you need to wait a very long time for the current to finally drop to zero (for a constant charging voltage) -- days, weeks even.There is also some leakage current, due to chemical reactions that activate above some voltage threshold. In my experience, under 2.1, maybe 2.3V, charge can be retained for months; near the rating however, a leakage of many uA (~mA even, at high temperature) is typical. (And again, you can only measure this accurately, after soaking at constant voltage for many days.)Tim

author: luddite
author_rank: Newbie
timestamp: March 12, 2021, 09:04:33 am
content: Thanks to everyone who replied.Power supply is rockseed 305pbecause of the variation between volts IN and actual volts shown on the Fluke I monitored it after that second charge.It showed 2.6v as mentioned after putting 3.1vafter an hour it was 1.92vAfter 12 hours (overnight) it shows 1.7vIs that still what it should be or is it faulty?

author: luddite
author_rank: Newbie
timestamp: March 13, 2021, 12:11:43 pm
content: Will do JimI have a rockseed 305p, I assume I can set this on 2.7v and just leave it?What does concern me is that I charge to 2.7v & it only shows 2.1v (is that because of what you mention regarding conditioning charging or is your suggestion to do with the voltage drop over several hours)thanks

author: mariush
author_rank: Super Contributor
timestamp: March 13, 2021, 12:48:12 pm
content: A discharged capacitor will pull a lot of current, so that may cause voltage drop on the power supply AND in the cables. You may configure the power supply to 2.5v but on the terminals of the capacitor you may get only 2.1v due to voltage drop in the wires and inside the power supply (due to exceeding the maximum output current)