Ultra low leakage current supercapacitor brand / serie ? - Page 1
Forum section: electronics / beginners

author: JL34
author_rank: Contributor
timestamp: May 12, 2022, 12:10:26 pm
content: Hello,I am looking for a supercapacitor with an ultra low leakage current, around microamps or less, with a capacitance around 5F or more. This supercapacitor would serve as energy storage on a ultra low power solar node sensor.As sellers' websites do not show this data on their filters, and that datasheets do not always give this value, it is pretty difficult to find a good one.Maybe somebody know a serie of supercapacitor corresponding to my needs ? Or maybe give me a place to look ?Digikey, Farnell, Mouser and co do not give leakage currents on their filters.Thanks !

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2022, 03:06:08 pm
content: Keep the voltage and temperature low. Leakage is considerably worse near Vmax, and goes up exponentially with temperature. They're not much different from batteries, so have similar limitations. Likewise, there's not much differentiation between manufacturers, no such thing as high temp or anything (at least, as far as I know).So, if you need 3.3V for example, consider using two in series.At low currents, you can use the old fashioned CMOS backup caps (super high ESR), probably cheaper? Wider selection available to you, anyway. However if you're using highish peak currents, like to power a radio or whatever, a low ESR type is probably needed -- or just use a small Li ion cell, no worries. (BLE, Idunno, maybe low enough in operation?)Tim

author: TimFox
author_rank: Super Contributor
timestamp: May 12, 2022, 04:22:31 pm
content: I did a very cursory look at the "supercapacitors" in the Mouser search engine, and went for the first datasheet it cited.(Unfortunately, that one was an obsolescent part.)The datasheet cited (Kyocera) does list the specified leakage for the individual parts:https://www.mouser.com/datasheet/2/40/BestCap-2400650.pdfIn general, the search engine results from vendors usually have a link to the manufacturer's actual datasheet.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 12, 2022, 04:52:18 pm
content: Why not using a small Li-Ion or some other recheargeable battery instead of the supercapacitor?I think a rechargeable will be cheaper and last longer.

author: JL34
author_rank: Contributor
timestamp: May 13, 2022, 06:22:53 am
content: Thanks for your advices @T3sl4co1lThe MCU on my board, an EFR32BG22, will consume around 8mA when advertising datas, and few hundreds of nano amps in deep sleep. It will be woke up by a nano timer, that consume around 100nA. Those will be powered by the supercap, through a TPS61094 that have a 60nA quiescent current.So the board will have around 8mA peaks current, and consume under 1uA when idle.Moreover, I try to design a maintenance free node sensor, so I try to avoid coin cells or batteries. Supercap could reach >10years lifetime in this configuration.That's why I am looking for supercap or hybrid li-ion supercap.@TimFox thanks, but I know I could find leakeage current value in datasheets. But as it is not in search filters, I must open datasheets one by one. That's why I ask if someone know a supercap that correspond to my needs ;-)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2022, 06:59:26 am
content: Ah, neat. The "memory backup" kinds have ESR of, like, kohms, so probably too high for this then -- but yeh, check the datasheets, make sure it's adequate for your load. 8mA is probably too much for that kind, but obviously won't need any specialty crazy-low ESR types, so there you go.Probably no big deal wiring solar cells directly to it, as well; efficiency would be a bit worse than with a converter, but mainly only at low voltages. Do mind to check that you've got enough solar capacity, and storage, to deal with a few cloudy days inbetween. Amorphous cells (lower peak performance, but lower leakage, so they tend to perform better under dim conditions) may be an option, or the regular crystalline kind will be fine as long as direct sunlight is mostly available.There are converters for these sorts of applications -- usually something "energy harvesting", suitable for a variety of low-level sources including solar, piezo, etc. Not sure if any of them are really particularly cheap -- they might simply not be worth using!Tim

author: JL34
author_rank: Contributor
timestamp: May 13, 2022, 12:05:59 pm
content: Thanks again Tim about your solar cells tips.As small solar cells should be able to deliver sufficient energy, I will choose it later.Regarding this interresring article, a well dimensionned and choosed supercap and solar cell couple could permit energy harvesting whit only a diode. It is a cheaper and simplier way to harvest energy, avoiding using a 3-4€ EH chips, with quiescent current near harvested current.It still must calculate average available solar energy in a year, and determine which of this two solution is the best tradeoff :-Harvest energy only in full sun with a single diode. Cheapiest solution. But sufficient fullsun time is needed, and compatible supercap too.-Harvest energy via EH chip. Most efficient solution but costier.Anyway, it is a very interresting subject for an hobbyist

author: jeremy
author_rank: Super Contributor
timestamp: May 13, 2022, 12:33:33 pm
content: I have measured a lot of supercapacitors for this exact purpose. I have not come across one that is less than roughly 1uA per farad at 25C after 72hrs. Keep in mind that “leakage” is much higher (>100uA per F) at startup due to complicated physical effects where basically the charges are still finding their way into all areas of the capacitor; this also relates to dielectric absorption. This is why all of them only have leakage specs after being at a constant voltage for many days.If you put a heavy load on the supercap it can restart this high leakage time to some effect; it is very difficult to model in a general sense how the leakage of a supercap will respond to very dynamic loads. You pretty much just have to measure it in-situ and integrate the power over time to work out the energy.Cap-XX is a company which has good specs on their supercapacitors (including leakage). But to be honest in my measurements I have found that plenty of other manufacturers are competitive with these specs for a much lower cost, the other manufacturers just aren’t as good at putting all of the characterisation in the datasheet. Even lesser known manufacturers like CDA seem pretty good.My own experience is that supercap are *not* good for power sources when there is a long time (or infinite time) between charging; they are not a good replacement for a coin cell. Where they are ideal is where you have a weak but fairly consistent source of energy (vibration, thermal, solar, etc) that you can save up in the capacitor. Because the energy is always there, just weak, the leakage doesn’t matter too much.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2022, 03:41:53 pm
content: Yeah, basically the dielectric absorption is ludicrous: very long tail, a week or several. It's diffusion mediated, so, impedance goes as 1/sqrt(f) in the low-frequency region. Which also means C goes as 1/sqrt(f), an odd property of what is notionally supposed to be a capacitor of given energy storage -- the takeaway being, you can only use that extra little bit of energy over such long time scales, or alternately, for shorter time scales, that energy is inaccessible -- or spent as reduced efficiency.I've had some stay charged (at 25C) around 2-1.7V for some months. Not sure how much of that is due to absorption (i.e. apply 2.5V for some hours, then open circuit; voltage relaxes down over time) versus leakage (consistent current draw even after some days/weeks/months at same V); I didn't leave it under bias for weeks to check the latter.Tim

author: JL34
author_rank: Contributor
timestamp: May 13, 2022, 07:52:00 pm
content: Erratum : I have forgotten to give the article's link about how to charge supercap with just a diode. Sorry.This is the link.As initial design, I plane to have as a worse case, 5/10 days with bad weather. Rainy or cloudy days. A little energy could be harvested between some clouds, but not so much. So the embedded energy storage should permit to have a node sensor working for around 5/10 days with very little or no energy available. In a good scenario, a well dimensioned solar cell should permit to recharge the supercap in the next sunny day.There is here an important tradeoff to do, choose between the two solutions previously mentioned (charge with a diode or an EH chip). A more expensive EH chip could permit to harvest the little essential energy available in bad weather days. I think I could not make a choice without testing both...As the supercap would never be high-loaded, just with few milliamps, do you think I can neglect dielectric absorption effects and issues ?I am looking at hybrid capacitor side. Vinatech has developed an interesting series, VPC, with nice specs. The first one is VEL08203R8306G. Like thedatasheetshow, it has 30F of capacitance, 3.85Vmax, 1uA leakage current after 72 hours, and is available at 2,49€ at Farnell (by 10pcs). Specs and budget correspond to my design. The form is not as compact as a coin cell capacitor, but is not to big. A nice pretender.

author: jeremy
author_rank: Super Contributor
timestamp: May 13, 2022, 09:04:10 pm
content: My only experience here is in circumstances where MPPT is basically essential, so there is no choice but to use an active device. I have not used these Lithium hybrid capacitors yet, but from that datasheet I did notice one thing a little odd; the number of cycles is rated to a discharge of only 2.5V. That makes me a little suspicious, what happens to the lifetime if you discharge to 1.8V for example?