Super cap self-discharge - Page 1
Forum section: electronics / projects

author: jenga
author_rank: Newbie
timestamp: January 25, 2024, 03:28:05 am
content: Hey all, long time viewer and EE designer here.I have a RTC and crystal that needs to be backed up with a supercap (and yes, it has to be a supercap)The backup circuit's parameters/requirements are:- 2.8uA current (kinda high, but for good reason)- Charge voltage: 3.3V-ish- Minimum voltage: 1.5V- Needs to keep the RTC alive (>=1.5V) for at least 3 days.I'm trying to estimate the life of a 1F supercap, which as it turns out, is quite a rabbit hole. Everyone seems to know super caps have relatively high self-discharge, but it's neglected by nearly every datasheet and source I can find. As most things in engineering go, the real answer seems to be 'it depends'.I've had a few cracks at figuring it out and I'm seeing huge discrepancies.Take the EDS105Z3R6 for example, it doesn't even list leakage or self-discharge current. Does that mean we can assume it's negligible? Seems like a bad idea to do that.https://www.cde.com/resources/catalogs/EDS.pdfLooking at other supercaps, such as the Lelon SCMDVLT5R5105ZVC205007E, it lists a 8uA leakage current. If we assume it's self-discharge current is always 8uA, that indicates the cap will completely discharge itself within ~3 days without a load. And that's unacceptable. Even if I assume the internal resistance is constant based on the leakage current (5V/8uA = 625k ohms), the expected lifetime still isn't great.But then in other sources say Kemet's 1F cap has a ~10uA leakage, but somehow can last 1500+ hours before dropping below 2V, completely contradicting the assumption that leakage current is even close to self-discharge:https://www.kemet.com/en/us/technical-resources/supercapacitor-leakage-self-discharge-characteristics.htmlThe conclusion I've come to (for now) is: I need to do some real-world testing. I've ordered a number of different parts, but they'll take a few days to come from Mouser. Plus it's going to take days on top of that to get a result. Getting this right is pretty important - some of you may even end up using the product this circuit is going in.So the question is:Does anyone know how supercap self-discharge really works? Does anyone have any real-world experience in using a supercap as a RTC backup source?

author: PSR B1257
author_rank: Frequent Contributor
timestamp: January 25, 2024, 08:50:37 am
content: QuoteWrong assumption.From the linked KEMET-web page:"Leakage current is a charge current measured from the pin-to-pin voltage across a charge resistor when the supercapacitor is charged for many hours and while the supercapacitor is on charge"QuoteDoesn't the manufacturer has already done extensive testing...

author: CaptDon
author_rank: Super Contributor
timestamp: January 25, 2024, 02:50:07 pm
content: I do know from experience the leakage current increases dramatically near the max rated voltage which for the ones we used was 5.5vdc. The size increases dramatically based on rated voltage also. If you are charging a 5.5vdc rated 1F capacitor to only 3.3vdc you may be able to entirely ignore self discharge. The charge circuit is more of a pain in the butt working with the equation 'faster charge settling time means lots of current'. Calculate the current requirements to charge 1F to 3.3vdc in 20 seconds. Fast charge of super caps gives poor results. They act a bit like a battery and need longer time to become 'fully charged' so to speak. If you bring them rapidly to your desired terminal voltage and then remove the charge path you will find they have not stored all of the energy predicted by their capacitance value. Charge to terminal voltage gives better results when spread out over perhaps two minutes.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 25, 2024, 04:19:28 pm
content: we use 1.5 farad at 5vdc, it take over 5 hrs to charge, and last 1 week for a rtc modulea supercap is not a battery, they will discharge over time, internal constitution ...even a battery, over the years will die

author: jenga
author_rank: Newbie
timestamp: January 28, 2024, 10:38:02 pm
content: Thanks everyone, that's some really great info.Quote from: CaptDon on January 25, 2024, 02:50:07 pmThat's particularly interesting, since most cap manufacturers will only list the behavior at the rated voltage. Some datasheets say something along the lines of 'capacitor behavior will change at lower voltages', without specifying. The parameters must vary a lot between different use cases and different manufacturers.Quote from: CaptDon on January 25, 2024, 02:50:07 pmThat's ok, this is a product that will be powered on for at 4-8 hours a day, so slower charge time is acceptable.Quote from: PSR B1257 on January 25, 2024, 08:50:37 amWhen the capacitor has been charged for many hours and the external charger is removed, then yes, self-discharge current should be nearly equal to the leakage current. But self-discharge current does not appear to be constant. Or if one assumes it is, the useable backup time is rubbish and most supercaps will completely discharge themselves in a day or so.What I suspected (and what CaptDon has alluded to) is the self-discharge current reduces as the capacitor voltage drops. But by how much? What does the curve look like? The datasheets do not to give that info, AFAIK.

author: tszaboo
author_rank: Super Contributor
timestamp: January 28, 2024, 11:24:12 pm
content: It's temperature dependent. One salesman was also convincing me that it would increase during the lifetime of the cap when it's always under voltage. But not theirs because it's hermetically sealed...It's also very difficult to measure.

author: jenga
author_rank: Newbie
timestamp: January 28, 2024, 11:59:30 pm
content: Quote from: tszaboo on January 28, 2024, 11:24:12 pmAnd that's why the requirement will specify backup time at 25 degreesOtherwise it'll be impossible to verify