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power consumption for portable heater

Discussion in 'Electronic Basics' started by [email protected], Apr 23, 2007.

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  1. Guest

    I have a very simple electrical question. I have a portable heater
    which is rated at 12 V, 12.5 amps, which I have attached to a 12 V, 73
    amp hour battery. I'm wanting to get a rough idea of how long this
    heater could run before completely draining the battery.

  2. 73/12.5 = ?

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  3. Less than 6 hours - maybe a lot less.
  4. Sjouke Burry

    Sjouke Burry Guest

    And then assume an efficiency of about 80 percent,
    so (73/12.5)*.8
  5. The amp-hour capacity of a battery is usually given assuming a 20 hour
    discharge time - your 73 AH battery could deliver 3.65 amps for 20
    hours. Discharging much faster than the 20 hour rate will reduce the
    total amp-hours that you can get.

    According to a table I have, you should get about 78% of the capacity,
    or 57 AH, which, at 12.5A, gives you 4.5 hours.

    This also depends on your definition of "completely drains". The
    official definition is a discharge to 10.5 volts.

    Peter Bennett, VE7CEI
    peterbb4 (at)
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  6. Tom Biasi

    Tom Biasi Guest

    At that rate of discharge with a 73AH battery your effective heating
    efficiency will fall very rapidly.

    Peter's calculations describe the math but in real life the battery will be
    getting hot, the internal resistance will go up rapidly and the power to the
    heater will fall quickly.
    You didn't say what kind of battery. Are you really concerned about how
    quickly it will drain the battery or how long you can get useful heat?
    My SWAG is 2 hours max. Maybe a little more if you cycle the on time to let
    the battery cool a bit.

  7. Phil Allison

    Phil Allison Guest

    "Sjouke Burry"

    ** So the missing 20% turns into what ?

    IR light ?

    Cosmic radiation ?

    Fairy dust ??

    ......... Phil
  8. Tom Biasi

    Tom Biasi Guest

    Heat in the battery and chemical changes and maybe a little fairy dust.
  9. Phil Allison

    Phil Allison Guest

    "Tom Biasi"

    ** You missed the point of the ambiguity - fuckwit.

    Plus all the wrong assumptions re the cell type in question.

    ....... Phil
  10. Chuck

    Chuck Guest

    As Peter suggests, look up Peukert's
    Law. You will get a lot fewer than 73
    amp-hours from the battery when
    discharging at that rate.

    In addition, keep in mind that the
    battery voltage will drop beginning
    almost immediately, which means that the
    heater will actually draw fewer than
    12.5 amps given the fixed resistance of
    the load. This will work in the opposite
    direction of Peukert's factor, but the
    heater will give less heat as the
    voltage and current drop.

    Another consideration is that most
    lead-acid batteries cannot withstand
    more than a few total discharge cycles
    before they are permanently damaged. It
    would be good to avoid discharging to
    more than 50% capacity if you intend to
    keep the battery.

  11. BobG

    BobG Guest

    A heat pump would move more BTUs than a resistance heater... look for
    a DC RV AC
  12. Tom Biasi

    Tom Biasi Guest

    Thanks Phil,
    Your well thought out comments are always welcome.
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