Replacing a wall adapter with a battery pack

Discussion in 'The Projects Forum' started by geosteve, Aug 9, 2010.

  1. geosteve

    Thread Starter New Member

    Aug 9, 2010
    I have a small electronic component (a transmitter) that runs off a plug in wall adapter. The adapter is rated to provide 9V DC at 200mA. I'd like to replace that adapter with a battery pack.

    I've got the plug and wire, just want to figure out the best strategy for what type and how many batteries to use, and if I need any other components. I really don't want to fry the transmitter. :)

    So, from what I understand, I can simply plug in a 9V battery, match the polarity on the wires, and it should work, correct?

    I can also plug in 6 cell batteries (AA, C, or D) for a total of 9V. FWIU, The difference would be that these batteries have considerably more capacity (500 mAh for a 9V vs 12000 mAh for a D, according to Is that, and size, the only difference?

    If I use 6 D cells in this case, with my 200mA device, should I expect to get 60 hours of use (12000 mAh / 200mA), or 6*60 hours of use?

    Finally, do I need any resistors or any other components between the battery and the transmitter? Do I need to do anything to achieve the 200mA, or is that just an indication of what will flow through the device when it's given 9V?

    Thanks for the help!!
  2. JDT

    Well-Known Member

    Feb 12, 2009

    Larger batteries will last longer. That is basically the only difference.

    Providing you supply the transmitter with its correct voltage it will take whatever current it needs. So no extra components needed.

    The problem with batteries, compared to the wall adapter is that the voltage will fall as the battery discharges. Wether this is a problem or not depends on your transmitter device. If you wanted to be sure that the transmitter is receiving an accurate 9V supply then it would be best to use a 12V battery and an electronic voltage regulator. This drops the variable battery voltage to a steady 9V. It could also give an indication when the battery voltage drops to the "fully discharged" level. This would require some fairly simple electronics, however.
  3. geosteve

    Thread Starter New Member

    Aug 9, 2010
    Thank you for the quick reply! It's good to know I'm on the right track. :)

    As far as battery usage goes.. if I used 6 (or 8 w/ a voltage regulator) D-cell batteries (each with 12000 mAh rating), and my transmitter operates at 200mA, should I expect 60 hours of use, or 6*60 hours of use?

  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    typically battery life is calculated as follows (only a ball park formula)
    (Battery string mAh/device current drain) x.7 = battery life
    The .7 is because you can never drain a battery all the way down as the voltage will drop too.

    So for your setup you have
    ((6x12000)/200)x.7)= 252 hours approx.
  5. geosteve

    Thread Starter New Member

    Aug 9, 2010
    Perfect, thanks. You both have been a huge help!