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Useful alarm clock modification

Discussion in 'Electronic Repair' started by James Sweet, Nov 29, 2003.

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  1. James Sweet

    James Sweet Guest

    I've got a number of digital clocks around the house that use a 9V battery
    to keep time during a power failure. Seems that the batteries only last one
    or two power outages and then they're dead which ends up being most of the
    time. I found a simple modification to fix this, simply get a 9V NiCd
    rechargeable battery, they're actually 7.2v but that doesn't matter. Now
    open the clock and follow the wires from the battery snap until you get to
    the diode in series with it, this is usually easy to find. Solder a 2.2k
    resistor across the diode and measure the voltage at the battery snap,
    should be floating somewhere between 10-15V, install the rechargeable
    battery and you're good to go. The resistor passes enough current to keep
    the battery charged but the diode still allows the full current to pass the
    other way during a power outage, no more dead batteries. Depending on the
    voltage of the power supply anything from 1.2k to 3.3k or so can be used
    just fine, a higher value will result in longer charge time but less risk of
    overcharge, too high and it won't fully charge at all. 2.2k has worked in
    every clock I've tried so far.
  2. JW

    JW Guest

    Assuming there's room, I wonder if a good size capacitor wouldn't do the
    trick as well?
  3. mike

    mike Guest

    I'm often tempted to put rechargeable batteries in stuff that wasn't
    designed for 'em. The thing I worry about is when I got a new one, I
    forgot about the charging, sent the old one to goodwill, someone bought
    it for a quarter, put alkalines in it and ends up with caustic goo all
    over their
    nightstand, or worse an explosion or even worse, the baby touched it and
    got it in his eye...When you're dead, that nightstand might be YOUR

    If you gotta do stuff like this, at least hot-glue the resistor to the
    battery and run a wire inside the box. That way, it will get
    disconnected if the battery is replaced.

    A lot of tragedy "sounded like a good idea at the time".
    Something to think about.

    Bunch of stuff For Sale and Wanted at the link below.
    laptops and parts Test Equipment
    Honda CB-125S
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
  4. James Sweet

    James Sweet Guest

    I do label the battery compartment, but realistically I'll never get rid of
    those clocks until they break, and when that happens I'll remove anything
    useful and throw the rest in the trash. A leaky battery won't kill anyone
    anyway, it'll just make a mess.
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