supercapacitor aging and leakage current - Page 1
Forum section: electronics / beginners

author: peterthenovice
author_rank: Regular Contributor
timestamp: November 07, 2015, 01:26:35 am
content: hii am looking for a 1 farad or more supercap@5.5 volts that has good aging characteristics and low leakage current for a rtc backup. I want to use a maxim ds3231m.the ds3231m is a ds3231 with a MEMS resonator. i would like to have at least a 1000 hr backup time.

author: dacman
author_rank: Frequent Contributor
timestamp: November 07, 2015, 01:42:44 am
content: A search at Newark came up with this:http://www.newark.com/panasonic-electronic-components/eec-f5r5u105/capacitor-double-layer-1f-5-5v/dp/96K9364?MER=PPSO_N_C_EverywhereElse_Noneand thishttp://www.newark.com/panasonic-electronic-components/eec-f5r5u155/capacitor-double-layer-1-5f-5/dp/25M9121?MER=PPSO_N_C_EverywhereElse_NoneFeatures:Endurance of 1000h at 70°CRated voltage of 5.5VDC

author: helius
author_rank: Super Contributor
timestamp: November 07, 2015, 01:47:13 am
content: The rated lifetime of the capacitor in thousands of hours has little to do with the length of time it can power a circuit.You need to find the load's characteristic current draw as a function of voltage and see if there is enough charge in the capacitor between the minimum and maximum voltage levels. Because it is not a battery, voltage will drop as a linear function of charge used, if the capacitance is constant (which it isn't exactly).

author: dacman
author_rank: Frequent Contributor
timestamp: November 07, 2015, 01:53:14 am
content: I found no leakage spec for those capacitors. I have seen 72 hours, but it would depend on the current draw.

author: peterthenovice
author_rank: Regular Contributor
timestamp: November 07, 2015, 01:55:34 am
content: are there low leakage supercaps or will 470uf ceramic caps have lower leakage current in parallel

author: dacman
author_rank: Frequent Contributor
timestamp: November 07, 2015, 02:12:07 am
content: At 3 µA, 1F will discharge from 5.5 to 2.3 V in about 300 hours. (A 470 µF in about 8 minutes.) And that is not considering leakage.

author: peterthenovice
author_rank: Regular Contributor
timestamp: November 07, 2015, 02:14:03 am
content: i found this cap on mouser.http://www.mouser.com/ds/2/88/ckh_cke-611140.pdf

author: dacman
author_rank: Frequent Contributor
timestamp: November 07, 2015, 02:33:18 am
content: Leakage current of 0.01CV or 3 µA whichever is greater means that at 5.5V and 1F the leakage would be 55 mA max (0.01 * 1 * 5.5). If you need 1000 hours of backup time then I would suggest a 9V battery. Which current draw will you need (3 µA or 100 nA)?

author: peterthenovice
author_rank: Regular Contributor
timestamp: November 07, 2015, 02:41:47 am
content: i need 3.5 uA max@5.5v max for the ds3231 to keep time.

author: dacman
author_rank: Frequent Contributor
timestamp: November 07, 2015, 04:25:36 am
content: My calculations tell me you would need 4F if it had no leakage.Here are some caps, but their leakage spec would indicate that they would be discharged to 2.3 V in less than a week:http://www.digikey.com/product-detail/en/MAL219690102E3/4707PHBK-ND/5015891http://www.digikey.com/product-detail/en/MAL219691214E3/4706PHBK-ND/5015890http://www.digikey.com/product-detail/en/MAL219691204E3/4705PHBK-ND/5015889

author: elements
author_rank: Newbie
timestamp: November 07, 2015, 04:36:56 am
content: I have used a supercapacitor to replace a battery for a project recently. I did some calculations for autonomy and tested it in practice a few times. This 283-3524-ND (digi-key) is 5F @ 5V and according to your current consumption of 3.5uA, would last ~1900 hours without leakage taken into account. It's way lighter then a 9v battery but more expensive!at what temperature is the circuit functionning ?

author: dacman
author_rank: Frequent Contributor
timestamp: November 07, 2015, 11:06:12 pm
content: Quote from: elements on November 07, 2015, 04:36:56 amI come up with that one should last 5 days (25 µA leakage nominal).A 200 mAh CR2032 would last 6.5 years, if it could actually source 200 mAh in that amount of time.