RTC battery backup

Discussion in 'General Electronics Chat' started by jonisonvespa, Feb 5, 2013.

  1. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    hi,
    just wondering whats the best way to protect my real time clock in regards to a power loss, my rtc runs at 200na

    i really dont like the idea of using a non rechargeable battery.

    id like to go the route of connecting a 3.3v rechargeable battery to it but really unsure on the best method to charge a battery / keep it maintained or even which is the best battery technology to use.

    would this circuit be ok or do i need to limit the current to the battery?

    any help greatly appreciated

    thank you
     
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    Last edited: Feb 5, 2013
  2. Mad Professor

    Active Member

    Apr 15, 2009
    133
    1
    How about using a supercap?
     
  3. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    yes i did think about that but my space is really limited to sm components
    cheers
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    A cell such as the CR2032 will keep a RTC running for 5 to 10 years all by itself.

    Why knock yourself out to recharge a battery that ain't gonna discharge until after your device is obsolete anyway?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Don't computers automatically correct their time today from the internet?

    My computer is 6 years old. I have NEVER set its clock and today it is exactly the same to the second as the clocks on TV.
    My computer has never been disconnected from mains electricity (60Hz).
    Maybe the extremely accurate 60Hz is the timing for the RTC?
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    767
    I think the main board is used a 32768Hz crystal oscillator to generate 1 Hz timebase, you can check your main board that beside the CR2032 has a small crystal oscillator.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Windows automatically syncs with the public atomic clocks daily. A typical computer clock will lose as much as 3 minutes a day if the system is heavily loaded with running apps.

    When the power is on, usually boot time and at 1am each day, the OS runs the clock and sets the time, rather than the Dallas RTC Module. The on-board battery (Almost always a CR2032, also saves NVRAM) is backup for when the power is unplugged.

    The 60Hz line frequency isn't used at all for computer clocks the way it is for, say, oven clocks for exact second sync, they simply ping time.nist.gov to set the correct time each day.

    There are very, very few clocks that are WORSE than a PC Realtime Clock while the system is running. Even a $5 no-name watch usually doesn't drift more than a minute a day.
     
  8. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    i see where your comming from ok will try to work out how long this will last, im a bit old school (probable more like tight) i like my things to last though, if it lasts for 10 years + i will use this battery or maby find a larger capacity one

    or use 2 maby?, ok found a 500ma and 1a battery coin cell that'l do it

    thanks very much for the solution
     
    Last edited: Feb 5, 2013
  9. Audioguru

    New Member

    Dec 20, 2007
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    My LCD watch has a quartz crystal. It is a high quality Seiko one.
    Its timing changes a little with temperature changes. It gains a second or two when I wear it and it loses a second or two when I do not wear it.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
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  11. crutschow

    Expert

    Mar 14, 2008
    13,035
    3,241
    A CR2032 has a capacity of 225mA-Hrs so it will theoretically run your RTC at 200nA for 128 years. This means that it's really the shelf life of the battery (which is perhaps 10 years) that determines when it dies, not your RTC load, so it wouldn't help to add more or larger batteries, only one with a longer shelf life.
     
  12. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    I've put coin cells for RTC's in several commercial products before and when I do the calculations, even being very conservative, what Crutcschow said always turns out to be the case. The shelf life of the battery is the limiting factor, not the battery capacity.

    Having said that, a supercap is viable and a decent solution. Of course it takes more circuitry (therefore more $$) because you'll need to recharge it. And if you do, pay attention to the voltage that you charge it up to. Even slightly exceeding the rated voltage is bad for a super-cap and can reduce its life to less than the coin cell.

    Once, our manufacturing engineer got a call from the warehouse saying that batteries were getting hot sitting on the shelf. He went down there and found that they had taken hundreds of the coin cells out of their trays and dumped them into a one big bag - to save shelf space in the warehouse! Imagine 500 or more coin cells randomly in large bag shorting against eachother! I went down there and just as they said, the whole bag was warm. We had to throw all of them out.
     
  13. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    many thanks didnt think of that will bear that in mind, when selecting the cell
     
    Last edited: Feb 5, 2013
  14. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    I had one supercap here from 1986- actually corroded. No charge anymore. From a Mitsubishi VCR. So their life is also limited. While otherwise this VCR was made of very good components, small electrolytics still work fine, and polyester caps. without cracked cases.

    Don't rely on components to last forever. I saw cheaper electrolytics from the late 1980s failing completely already.
     
  15. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    thanks to everybody for sharing your knowledge
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    The new Lithium battery in a brand new Boeing 787 Dreamliner jet was the cause of smoke in Japan. It had thermal runaway. Shame on Boeing for using it.

    Canada Post refuses to accept Lithium batteries (but many products have them hidden inside and are accepted).

    I have many Li-Po batteries that were used a lot and now have poor capacity. Most are swollen (they might explode or catch on fire?). I store them in a tin can but I should take them to the hazzardous waste dump site.
     
  17. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    Last few times I needed a RTC I used a discrete ic device that not only has the crystal built in but also monitors it's own temperature to remove changes from delta T.

    Let me know if you want the part numbers I can get them later.
     
  18. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    that sounds usefull what is the part number?
     
  19. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    All Lithium Ion batteries are potentially dangerous due to the volatile nature of Lithium and the physical construction of the cells. Overcharge, or overvoltage will create a hazardous conditions. This is why they put a safety circuit inside the battery pack to monitor the cells and disconnect them from the load when one of these conditions, or an overtemperature condition occurs.

    If you try to buy the larger capacity LiIon cells such as 18650's (laptop cells) directly from a legitimate source like Moli or Panasonic, they won't sell to you unless you are registered with them and have your safety circuitry approved by them.

    I worked for a company that made handheld computers and we used an outside battery vendor to design our battery packs which used 18650 cells. I was doing the power design and could have easily designed the safety circuit inside the packs. I was familiar with them and had tested plenty of them from the battery vendor we used. But the company just wasn't willing to take on that liability. LiIon batteries are great, they have the best energy density of any chemistry available. But they are not somethng to play with. I get really worried when I see posts from hobbyists experiementing with LiIon cells and not understanding the consequences. Please, be careful with these things. They can and will catch on fire or explode if mistreated. Look at what AudioGuru mentioned about Boeing and the smoking LiIon battery. And Boeing has teams of engineers and extensive design reviews before releasing something.
     
  20. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    Actually I am going to have a CR123 Li-ion option on a new PCB design. I was thinking, these holders are more solid than to use a mobile phone or helicopter pack.

    And I will have a specialized Li-ion charging IC for that battery.

    The PCB isn't meant to be to deal with shorts of any kinds, but the world is foolish at times. Should I add a polyfuse to the PCB?

    I am currently still in the design phase and I am using the forum to gather ideas and see additional problems. Well here's one: Li-ion safety. One shorted wire, and an explosion may happen, at best only producing battery goo everywhere, at worst happening in 2 years in the wooden holiday home of a customer.
     
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