How to determine max charge rate for a supercap? - Page 1
Forum section: electronics / general

author: krupski
author_rank: Newbie
timestamp: March 20, 2024, 01:10:51 pm
content: Hi all,Is there a formula that can specify the recommended charge rate for a supercap? I have some 5.5 volt, 1.5 farad caps and wish to charge them without damage. Any info will be appreciated. Thanks!-- Roger

author: Zero999
author_rank: Super Contributor
timestamp: March 20, 2024, 01:18:26 pm
content: It depends. Quite often the ESR is the only limiting factor.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: March 20, 2024, 02:43:37 pm
content: Do you know the part number and manufacturer. Can you get the data sheet. How fast do you want to charge them?

author: shadow.dark
author_rank: Contributor
timestamp: March 21, 2024, 03:38:09 am
content: It might be possible to determine this by measure the temperature riseing rate of the capacitor, essentially measuring the thermal effect of the ESR. Or the current could be determined by your product's temperature rise tolerance for the supercapacitor, which would also depend on the conditions of heat dissipation.Actually, one thing I haven't been able to figure out is what are the main factors that contribute to the life of a supercapacitor. Derating used MLCC capacitors has almost infinite life, but batteries are another matter entirely.Translated withwww.DeepL.com/Translator(free version)

author: ahbushnell
author_rank: Frequent Contributor
timestamp: March 21, 2024, 09:12:47 pm
content: Quote from: krupski on March 20, 2024, 01:10:51 pmchrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.eaton.com/content/dam/eaton/products/electronic-components/resources/data-sheet/eaton-kvr-supercapacitors-coin-cells-data-sheet-elx1175-en.pdfHere is a data sheet that might be similar to your capacitor.

author: f4eru
author_rank: Super Contributor
timestamp: March 22, 2024, 07:13:21 am
content: Voltage and temperature are the ennemies of your supercaps :https://www.analog.com/en/resources/analog-dialogue/raqs/raq-issue-179.htmlMake a lifetime evaluation.

author: radiolistener
author_rank: Super Contributor
timestamp: March 25, 2024, 07:43:21 pm
content: Quote from: krupski on March 20, 2024, 01:10:51 pmYou can buy some amount and perform the test. Then analyze statistics which voltage damage it.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 30, 2024, 03:06:50 pm
content: Short answer is that there's no straightforward way to estimate max charge/discharge current.The limiting factor is internal temperature, which should be kept well below the boiling point of the electrolyte. Many supercaps use ACN in their dielectric which boils at 82C, and they typically rate the max operating temperature at 65C.For "standby" applications where the supercaps are rarely charged/discharged, such that the case temperature always relaxes back to the ambient temperature before the next cycle, you can make a very rough approximation as follows: Your 1.5F cap has a mass of 9.1g. Maybe 3g of that is dielectric. The specific heat capacity of ACN is ~2.2 J/g/k. Raising its temperature by one degree C requires 6.6J of energy. If your ambient temperature is 25C, and you limit the internal temperature to 65C, your max temperature rise is 40C, which takes 264J or energy. This is a lot more than the capacitor is even capable of storing (45J).If you actually want to charge it that fast, that begs the question "what the heck are you using the supercap for". Otherwise, charge/discharge current for standby applications is effectively unlimited (at ambient temp of 25C, anyways).For "cycle" applications where charging/discharging happens frequently enough that their case temperature remains elevated above ambient, you have to estimate the case temperature (which replaces ambient temperature in the math above). Vendors are not going to provide this. It's too dependent on factors outside their control (orientation, proximity to other components, available airflow, etc). It basically turns into a fluid thermodynamics modelling problem, which I'm not even going to try and attempt here. If you're serious about developing and verifying such a setup, you will need to verify it experimentally yourself.