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Battery Backup — Parallax Forums

Battery Backup

mctriviamctrivia Posts: 3,772
edited 2009-01-19 00:42 in Propeller 1
Does anyone have any idea on what would be required to make a battery backup system for the prop and let the prop know when it is on wall power or battery?

I would like to be able to use a standard 9v or 3v button cell battery to keep my system clock running. All other cogs can be suspended to save power but when the power comes back on I want it to remember the last state it was in and still know the correct time.

Comments

  • SRLMSRLM Posts: 5,045
    edited 2009-01-17 23:32
    I was just researching that this morning... [noparse]:)[/noparse] Here's what I found:

    http://picnote.blogspot.com/2008/10/battery-backup-for-microcontroller.html
  • mctriviamctrivia Posts: 3,772
    edited 2009-01-17 23:40
    that is so sympel and would work great if the propeller was 5v drive
  • mctriviamctrivia Posts: 3,772
    edited 2009-01-17 23:49
    that gave me a great idea though

    use the following schematic
    2220.jpg
    360 x 163 - 9K
  • SRLMSRLM Posts: 5,045
    edited 2009-01-18 01:41
    The problem with the circuit you posted is that it requires a chip and a few other specialty devices. I like the other one because it only needs two diodes.
  • T ChapT Chap Posts: 4,223
    edited 2009-01-18 01:46
    Look at the DS1307 which has the battery backup switching built in:


    www.maxim-ic.com/quick_view2.cfm/qv_pk/2688
  • mctriviamctrivia Posts: 3,772
    edited 2009-01-18 01:49
    SRLM said...
    The problem with the circuit you posted is that it requires a chip and a few other specialty devices. I like the other one because it only needs two diodes.

    true but the voltage at the prop would only be 2.3v when on battery using a 3v.
    TChapman said...
    Look at the DS1307 which has the battery backup switching built in

    I will have to remember this. Not sure if I can get away with just keeping the clock running but that is a great option.

    Post Edited (mctrivia) : 1/18/2009 1:57:56 AM GMT
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-01-18 02:03
    mctrivia,

    "that is so sympel and would work great if the propeller was 5v drive" --- So why not drive it at 3.3V

    from the diagram that SRLM posted... replace D1 with a resistor to trickle charge a 3.3V battery. Keep D2 but keep your wall supply regulated at about 3.9V

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • mctriviamctrivia Posts: 3,772
    edited 2009-01-18 02:59
    good idea. This won't cause any trouble when the battery is fully charged? never worked with rechargable batteries.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-01-18 03:04
    mctrivia,

    "This won't cause any trouble when the battery is fully charged?" ... not if you trickle charge it about 1/10th the normal charging rate.· The old style "over-night" 10-14 hour chargers are what I'm referring to.
    ·

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • hippyhippy Posts: 1,981
    edited 2009-01-18 15:32
    Beau Schwabe (Parallax) said...
    from the diagram that SRLM posted... replace D1 with a resistor to trickle charge a 3.3V battery. Keep D2 but keep your wall supply regulated at about 3.9V

    Won't D1 replaced by an R ( large enough for trickle charging ) limit the current for the Prop ?

    Would it not be better to keep D1 and add an R from battery +Ve to VDD/+5V/Vin ?

    I've used the diode-mixing technique and it works well because it's so simple. I have been told that very long term trickle charging could cause problems for the battery but I'm no expert. One solution would be to use two, remove one and recharge as required externally.

    It's a shame the Propeller doesn't have zero-extra component on-chip ADC because that makes it easy to add voltage monitoring for the batteries. IMO that's the only real weak spot in the Propeller design compared to alternatives.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-01-18 18:48
    Try this design...

    When the battery is at 3.0V, the trickle charge current is limited to about 70mA
    When the battery is at 3.7V, the trickle charge current is almost 0mA

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
    TRICKLE.JPG
    973 x 673 - 152K
  • mctriviamctrivia Posts: 3,772
    edited 2009-01-18 20:53
    what battery could be used with this? have a 3V Lithium rechargable. Digikey does not sell rechargable lithiums over 3.1V
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-01-19 00:42
    mctrivia,

    Typical Lithium-Poly or LiFePO4's·are 3.7 Volts per cell... just make sure that the regulated voltage is at least 0.3V above your battery.

    The problem is, that the Schottky diode will drop the voltage about 0.3V, so If you start out with a 3V battery, you won't have much operating overhead for the Propeller.· A 3.7V battery or more is a better choice.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.

    Post Edited (Beau Schwabe (Parallax)) : 1/19/2009 12:50:10 AM GMT
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