battery for RTC with longest possible shelf life? - Page 1
Forum section: electronics / projects

author: JBeale
author_rank: Frequent Contributor
timestamp: August 05, 2014, 06:21:14 pm
content: I see this posthttps://www.eevblog.com/forum/projects/rtc-backup-power/msg462619/#msg462619mentions running a low-power clock chip (RTC) from a primary lithium 3.6V AA cell and expecting 10+ years service life from it.I have a similar application, I want a battery to run a RTC for the longest possible time, in a room-temperature environment. This would be in a sealed enclosure with no access to any external power. Size is not a big issue, I could use 8x AA if that would help. I'm thinking about the Maxim DS1337 which draws 0.425 uA typ, 0.6 uA max at 1.3 V, and can run up to 5.5 V.I see the Energizer L91BP-8 lithium AA claims it holds power for 20 years when not in use (how did they test, is the product 20 years old?) but anyway, is this the longest-lived cell available? Does the claimed lifetime rely on an electrochemical passivation layer which requires true 0 current, or would 0.6 uA qualify as "not in use" ? I guess it is Lithium Iron Disulfide (Li-FeS2) at 1.5 V (but actually 1.8 V open-circuit).http://www.energizer.com/batteries/performance-lithium/ultimate-lithium/Pages/aa.aspxI think shelf life is the dominant factor in this application: according tohttp://en.wikipedia.org/wiki/AA_batteryI can expect 2700 mAh from a lithium AA cell, which apart from shelf life, could theoretically provide 0.5 uA current for 5.4 million hours which is 616 years.

author: mij59
author_rank: Frequent Contributor
timestamp: August 05, 2014, 07:00:41 pm
content: Hi,Maybe you can use a rtg (radioisotope thermoelectric generator) like used in the Voyager 1 and Voyager 2.

author: Kjelt
author_rank: Super Contributor
timestamp: August 05, 2014, 07:03:42 pm
content: I think you are definitely on the right track with Lithium batteries. They can last 15 to 20 years and are nowadays even used in pacemakers.The even better as in longer lasting battery is a plutonium 238 one used in the first generation of pacemakers in the early 70's and still running, they have a (half) lifetime expectancy of 88 years, but my guess is that just like in the BTF movie it is going to be very difficult to order it somewhere and you need lead in your enclosurehttp://uk.reuters.com/article/2007/12/20/health-heart-pacemaker-dc-idUKN1960427320071220

author: free_electron
author_rank: Super Contributor
timestamp: August 05, 2014, 07:04:41 pm
content: Tadiran Lithium Thionyl Chloride. cells. these can last 25 years +

author: JBeale
author_rank: Frequent Contributor
timestamp: August 05, 2014, 07:28:29 pm
content: Quote from: free_electron on August 05, 2014, 07:04:41 pmThanks, that looks like the winner. Considered hazardous material, but it looks like the best lifetime- and actually available from mouser.com, unlike the plutonium RTG :-)."Longer life...Tadiran batteries feature an outstanding shelf life and long term reliability. Over a ten-year period, Li/SOCl2 batteries stored at room temperature lose less than 10% of their total capacity." fromhttp://www.tadiranbat.com/index.php/tadiran-batteries-better-by-designand "...laboratory tests on devices that have been in the field for 20 years show that the batteries retain over 25% of their original capacity" fromhttp://www.tadiranbat.com/pdf.php?id=metering_international_20_years

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 05, 2014, 09:09:04 pm
content: Well, if shelf life means just sitting there with no load, you can't beat thermal batteries.

author: JBeale
author_rank: Frequent Contributor
timestamp: August 05, 2014, 09:52:51 pm
content: If you meanhttp://en.wikipedia.org/wiki/Molten_salt_batterythey are an interesting technology, indeed Wikipedia says 50+ years shelf life, but before heating it up to working temperature of several hundred degrees C the battery is completely passive, no voltage output so it is not useful for powering the RTC.If you mean a thermopile or series array of thermocouples, that might run forever so long as you have a temperature gradient handy, but I don't.

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2014, 03:35:12 am
content: I second Tadiran lithium thionyl chloride cells. Some models are competitive on price as well so there is no reason to go with a lessor chemistry.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: August 06, 2014, 03:52:30 am
content: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCAQFjAA&url=http%3A%2F%2Fwww.rayovac.com%2F~%2Fmedia%2FRayovac%2FFiles%2FProduct%2520Guides%2F42691_Lithium%2520Application%2520Notes%2520and%2520Product%2520Data%2520Sheets.ashx&ei=pqXhU_WHGpC0yAS_hoDABg&usg=AFQjCNEaGm3L5Hs5hUuP4pwvHVtBWMKB-w&sig2=mdKjKSYzISffXQIijWXN_Awtf is google doing to make links like that? - but it is some relevant info

author: mikerj
author_rank: Super Contributor
timestamp: August 06, 2014, 09:18:53 am
content: Quote from: f5r5e5d on August 06, 2014, 03:52:30 amIf you use Firefox then install the "Google Redirects Fixer" add-on to remove all the Google Garbage (tm).

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 06, 2014, 04:45:44 pm
content: Quote from: mikerj on August 06, 2014, 09:18:53 amThanks for the Google Redirects Fixer tip, and to keep on topic, I have had good luck with Panasonic ML series rechargeable lithium cells, but I rarely need to keep an RTC running for more than a few months without power.