Newbie question - battery advice

Discussion in 'General Electronics Chat' started by ymotion, Jun 4, 2016.

  1. ymotion

    Thread Starter New Member

    Jun 4, 2016
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    Hi,

    I have a camera that runs on 7.2V battery. I'd like to use an external battery to run it.

    I have two options here and since I have no knowledge in electronics would need your assistance.

    1st option: 14.4V 95Wh battery
    2nd option: 7.2V 7800mAh battery.
    If I choose the first option I would need a regulator (not sure about the terminology here) to down-convert from 14.V to 7.2V. If I choose the second option I can connect it directly.
    Afaik the running time of a battery depends on the capacity, measured in mAh. i found out how to convert Wh to mAh:
    (mAh) = (Wh)*1000/(V)
    This means that the 14.4V 95Wh battery would give 6600 mAh. This is less than the 7.2V battery. So in this case, would it be better to use the 7.2V?

    (2) Some of the regulators convert the voltage to 8.4. Is it a bit higher because of loosing energy on the conversion (but actually you get 7.2V)? Would it still be safe?

    Would appreciate your advice here.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Welcome to AAC!

    I'd use the 7.2V battery.

    Whether operating at 8.4V will be safe depends on the camera. The output voltage from the AC adapter for my video camera is higher than nominal battery voltage.
     
  3. ymotion

    Thread Starter New Member

    Jun 4, 2016
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    thanks dl324! Good to be here :)
    So running time of the 14.4V in this case would be less than the 7.2V?
     
  4. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    WH is the correct way to compare batteries of different voltages. A battery with twice the voltage and half the current capacity of another battery has the same capacity in WH.

    So your 14.4V has 95WH, and your 7.2 has 7.2 * 7.8 = 53WH.

    The 14.4V battery therefore has more capacity. But you need a buck converter to supply the 7.4 your camera wants. These will be about 85 to 90% efficient. So now it is 95 * 0.85 = 81WH vs 53WH, still significantly better, and you might actually get more than 85% efficiency.

    Bob
     
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  5. ymotion

    Thread Starter New Member

    Jun 4, 2016
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    thanks Bob.
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
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    As with most things, it depends on the discharge profile of the batteries in question.

    If your 7.2V battery voltage is fairly flat until near EOL, the 7.2V battery would be a better choice. No regulator losses to contend with.

    If voltage drops off early (according to battery chemistry and/or health) and your camera is sensitive to low voltage, the higher voltage battery with a regulator might give you longer run time.

    This was assuming your amp hour calculation was correct.
     
  7. ymotion

    Thread Starter New Member

    Jun 4, 2016
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  8. dl324

    Distinguished Member

    Mar 30, 2015
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    Li-ion batteries have a flat output voltage; lower discharge rate gives flatter voltage profile.

    Do both of those batteries fit your camera? If not, the obvious choice is to use the battery that's compatible with your camera.
     
  9. ymotion

    Thread Starter New Member

    Jun 4, 2016
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    Actually none of them fit my camera :)
    but both have a dc out...
    So a dc cable to a dummy battery (or dc to a regulator+dummy if I take the 14.4V),
    I chose these options because the battery is relatively small and light. I'm not using a tripod and so they'll go into my pocket.
    Someone also recommended RC Lipo - but might be too much for my knowledge.
     
  10. dl324

    Distinguished Member

    Mar 30, 2015
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    I'd still go with the 7.2V battery. No regulator for you, or someone else, to forget to use and end up damaging your camera.
     
  11. ymotion

    Thread Starter New Member

    Jun 4, 2016
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    The 14.4V has a D tap outlet - so I'll need a special cable for that (which I can buy) - vs the 7.2V that has a DC cable. So mixing up won't be possible...But maybe the regulator option is a bit more risky, since it supplies a higher voltage.

    BTW, I'll need good DC cables to connect the battery to the dummy. what should i look for? I heard that some cable can introduce noise
     
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