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Using a d cell battery to power a digital watch?

Discussion in 'Electronic Basics' started by [email protected], Mar 17, 2005.

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  1. Guest

    Hi, I am new to this group and have been playing around with unwanted
    (by others) digital watches. I have been powering them with wires
    soldered to the correct polarity on the watch contacts and using
    battery holders bought from radio shack to power the watches with d
    cell batteries (rechargeable alkaline in some cases). I have checked
    and the polarity/voltage (current is easily accepted especially
    compared to the special coin cell batteries these devices came with) is
    correct in my situation. For some reason some of the devices will reset
    or otherwise go blank. I have minimized this with one of my wanted
    timers by using solid wire (for some reason it makes a big difference)
    but when the wire is in certain positions the blanking still happens. I
    am suspecting that this low voltage/low current situation is caused by
    the battery wires acting like an antenna and the low current/low
    voltage is counteracted by this phenomenon.

    In summary the question is - how can I eliminate these digital devices
    from going blank seemingly randomly and if not then why not?

    Thanks, later!
     
  2. Rich Grise

    Rich Grise Guest

    It's probably intermittent solder connections. The battery clips in a
    watch are either stainless or nickel plated, both of which are hard to
    solder properly. I'd either go with a mechanical connection (i.e.,
    two pieces of copper foil on either side of a wood or plastic slug
    the size of a watch battery) or work on your soldering to those parts.
    You'll probably need a more aggressive flux, but then it will have to
    be cleaned completely so it doesn't corrode.

    And it's very, very unlikely that anything your leads would pick up
    antenna-wise would have anything to do with it - these voltages are
    measured in microvolts.

    Good Luck!
    Rich
     
  3. That battery holder could be part of the problem.
    Is there any movement of or shock applied to the
    battery holder when it is supposed to be helping
    power the watch? Brief opens would well explain
    the symptoms you report.
    I am reasonably sure that your solid wire preference is an
    example of the "Post Hoc" fallacy. If interested in that, see
    http://www2.sjsu.edu/depts/itl/graphics/adhom/posthoc.html

    If the solid wire actually did "fix" the problem, it was because
    you had to change your connection technique, and the new
    one works better.
    That is conceivable if you have made some huge loop,
    separating members of the wire pair. You didn't do
    that, did you? If not, RFI is not the problem.
    I think you just need to get power to them on
    a continuous basis, not even briefly interrupted.
    You're welcome now.
     
  4. Bob Masta

    Bob Masta Guest

    Besides the good advice others have given, consider that
    these watches typically use LCD displays that are not soldered
    in place but are simply held against a strip of contacts.
    It's pretty easy to dislodge this, and I've seen plenty of
    cheap units where the watch behaves just as you describe.
    I suspect that may be one reason why they are discarded
    by others in the first place!

    Best regards,



    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  5. Guest

    Thanks guys! It's better to be a brief open in this case rather
    than an antenna issue because this way I can fix it with a better
    electrical connection instead of trying to shield the power conductors.
    I am going to resolder the device I am most interested in so that the
    wires (will use stranded) are soldered directly to the device's
    internal wires instead of to the coin cell pads/connectors. My solder
    connections seemed perfect but will try the previously mentioned fix.
    The other devices I have can wait. I will probably post my findings
    here. It will be good to know if my stranded wire is good or not.
    Thanks again, later guys!
     
  6. Bill Bowden

    Bill Bowden Guest

    I have been powering them with wires soldered to the
    I think a AAA battery will last as long or longer than a "D" cell,
    since both have the same shelf life and the watch draws very little
    power. Probably not enough to drain a AAA battery during it's shelf
    life.

    Why waste a "D" cell when you can get the same performance from a AAA
    cell?

    -Bill
     
  7. Doug McLaren

    Doug McLaren Guest

    | >> I have been powering them with wires soldered to the
    | >> correct polarity on the watch contacts and using
    | >> battery holders bought from radio shack to power
    | >> the watches with d cell batteries (rechargeable
    | >> alkaline in some cases).
    |
    | I think a AAA battery will last as long or longer than a "D" cell,
    | since both have the same shelf life and the watch draws very little
    | power. Probably not enough to drain a AAA battery during it's shelf
    | life.
    |
    | Why waste a "D" cell when you can get the same performance from a AAA
    | cell?

    Agreed, a AAA cell would probably last years.

    However, you mentioned a rechargable alkaline? Big waste. I don't
    know what the discharge rate on those are, but a NiCd would be dead in
    six months, and a NiMH cell in under 3.

    I love my rechargable batteries and buy them in large quantities, but
    I also know that there's applications that they don't work well for
    due to their relatively high self discharge rate -- remotes, smoke
    alarms, emergency flashlights, etc.
     
  8. Bill Bowden

    Bill Bowden Guest

    However, you mentioned a rechargable alkaline? Big waste

    Actually, the OP mentioned it. But yes, recharagable batteries of any
    variety are a waste for a digital clock/watch application. Alkaline
    AAAs are the only way to go. Or maybe Lithium for longer shelf life,
    but those are more expensive.

    I ran a quartz wall clock for 3 years using 5 salt water cells in
    series. I used 5 small olive jars with a strip of copper and aluminum
    inside submerged in salt water. Gave me about 300 mV per cell at a
    couple milliamps, but I got tired of adding water to the cells every
    week or so to make up for the evaporation.

    -Bill
     
  9. Bob Masta

    Bob Masta Guest

    On a related note, my trick is to take 9V batteries
    that are "dead" at 8V, and make a tiny 3V regulator
    that sits on a battery clip. I have a menagerie of
    clocks, kitchen timers, calculators, etc, with 9V batteries
    stapped to them. It takes a ***long*** time to drain
    a "dead" 9V down below 3V... and dead 9V batteries
    are free!

    The regulator uses an LED with a large series resistor
    as a super-sharp low-voltage Zener, plus a simple pass
    transistor. Works great, and totally impractical... I love it!






    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  10. Guest

    Interesting stuff. I am pretty sure a rechargeable alkaline battery has
    a longer shelf (charge) life than rechargeable nicads or rechargeable
    nimhs. I am using a rechargeable "d" battery simply because it is
    handy. I don't like traveling across town in this tourist town risking
    my life every time with those other drivers. I appreciate the answers -
    especially the one about solder connections and nickel or stainless
    steel contact points. I soldered directly to the power supply wires in
    my aqua reminder and it works perfectly now and the same with the
    bicycle odometers. So it was a poor electrical connection, good
    mechanical connection that I had previously. This project is practical
    to me much like my other previous project I haven't finished yet. That
    was to make a rechargeable battery pack for a smoke alarm and before
    you scoff and say that wouldn't be reliable keep in mind I have seen
    fire alarms using that exact same method and that fire alarm was a
    first alert brand. The only reason I didn't buy it when I saw it is
    because it was used, I might very well do some searching right now for
    new units of those and might order one. The one I saw is directly wired
    to the house' power supply (~115 volts). Thanks for telling me that the
    solder connection was a poor electrical connection but a good
    mechanical connection when it comes to stainless steel and nickel, that
    solved my problem perfectly - Later!
     
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