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Adding a *rechargeable* battery backup to a small device

Discussion in 'Electronic Design' started by nickjohnson, Dec 20, 2006.

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  1. nickjohnson

    nickjohnson Guest


    Imagine you have a small device which is normally plugged into a wall,
    such as an alarm clock. You want to put a battery back-up into it, so
    if the wall power should ever fail (unplugged, outtage, etc), your
    device still functions. But, at the same time, you want the battery to
    be rechargeable. Ideally, the device will recharge the battery while
    plugged in, but switch to battery power when necessary.

    So, I don't know too much about how a battery charger works, but here's
    what I was thinking:

    Wall power leads into a typical power supply, yielding 9Vdc. Those
    9Vdc supply a battery charger which is always charging a rechargeable
    9V. The terminals of the 9V go through a 5V regulator to power the
    rest of the circuit. There is no direct route from the 9Vdc power
    supply and the 5V regulator.

    Does this sound like it would work?

    Is there a more common-practice topology for this sort of design?

    Anyone have links on how to design a battery charger?

  2. GregS

    GregS Guest

    I bought my Sony clock many years ago because having a rechargable is the only
    way to go. Changing 9 volt batteries is not what I do. Imagine what that costs
    after 20 years.

    There are various voltages from 9 volt cells, depending on the number of cells
    used. That critical if driving signifant current into the cell.

    Sounds like that 5 volt thing would work, but is 5 volts really used

  3. Rich Grise

    Rich Grise Guest

    Design a trickle charger for the optimum trickle charge voltage
    for your battery plus .7V. Connect it to the battery via a
    1N4004 diode, and run your circuit off the battery.

    Good Luck!
  4. nickjohnson

    nickjohnson Guest

    Thanks Rich, this is exactly what I was looking for.

    Why the diode -- just to make sure the charger doesn't draw current
    from the battery?
  5. Tam/WB2TT

    Tam/WB2TT Guest

    You mention clock. Most clocks run off the AC line frequency; so, you need
    to supply timing to the circuit when the AC goes off. Most commercial
    digital clocks with battery backup use an RC oscillator to generate 60 (or
    50 ) Hz. These are typically set up to gain about 10 minutes/hour.

    Years ago I built a digital clock that ran off 11V, with a 9V backup battery
    The display ran off the 11V, the clock chip off a diode matrix that would
    use whichever voltage was higher. I included a crystal oscillator and
    countdown circuit to generate the line frequency.

    In the past, PCs used 5V with a 4.5V battery. Crystal was 32.768 KHz, which
    got counted down to 1 Hz.

  6. nickjohnson

    nickjohnson Guest

    You mention clock. Most clocks run off the AC line frequency; so, you need
    This is interesting; I would have just expected them to use a crystal.
    Do they do this to reduce product cost, or is the AC line a better
    clock than a crystal?
  7. Rich Grise

    Rich Grise Guest


    (BTW, we bottom post here)

  8. Tam/WB2TT

    Tam/WB2TT Guest

    AC line is a better clock than a cheap crystal. Power stations vary the
    frequency slightly to keep clocks accurate. Of course, that only works so
    long as you don't have a power outage.

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