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How to calculate how long a battery will last

  1. Sep 20, 2011 #1
    Sorry if this is in the wrong section (it isn't homework). I am trying to form a basic understanding of electricity. Right now I'm attempting to figure out how to calculate how long a battery will last with certain devices.

    I might be completely off base but here's what I've got.
    12volt battery rated at 250Ah
    Device is rated at 115volts 10000 BTUs, that's 2931W-h

    2931 / 12 = 244 amps (97.7% = of battery capacity)
    Battery capacity 12 x 250 = 3000watts

    Using 2931 in one hour is 97.7 %

    Not sure if this is correct was picturing a car battery and a portable ac unit

    ......one other

    Battery rated at12volt and 12Ah
    Device 1420mAh (iphone)

    12000mAh battery at 12 volts

    12000/1420 = 8.45

    Does this mean I can charge the iphone 8.45 times off the other battery?

    Also is there any calculation required when using a power inverter? (I would be using a power inverter to go from dc to ac in these examples. (Not that id ever want to power a 10000btu ac unit in my vehicle lol)

  2. jcsd
  3. Sep 20, 2011 #2


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    The capacity rating (amp hour) for batteries is listed for a low current output. The capacity would be much less at a high load. I'm not sure at what current that 250 amp-hour rating would be realistic. The inverter would consume part of the power from the battery.
  4. Sep 20, 2011 #3


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    You must convert the voltage. In order to produce 1A at 115V you must use at least 115/12~10A of the battery. Simple customer grade voltage convertes rarely have efficiency better than 80% - so you must drain even more than 12 times higher current from your battery than you may make available to your device.

    You have awfull mess with units: you mixed units of energy with units of power.
    Last edited: Sep 20, 2011
  5. Sep 20, 2011 #4
    Thanks for the help! Still a bit confused but I'm getting there.

    Also noted about difference between units of energy and power thanks
  6. Sep 21, 2011 #5


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    There is another point about 12V lead acid batteries. They should not be discharged to much below half of their capacity rating or they will not survive. So a 100Ahr battery can only be used as a 50Ahr battery. Crazy but true (advertising standards seem to be ignored here) and it can be an expensive mistake to ignore this. There are some batteries, referred to as Deep Cycle, which can be take lower but you would need to read the actual spec about how much you could take at the rate you need.

    I totally agree with the foregoing comments about practicability and efficiency. the Ahr value is only a guideline and every other link in the chain will only serve to taqke things below this value. Tough. Do the sums and then spend much more than you calculated (as in all things).
  7. Sep 22, 2011 #6


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    You also have to keep in mind that real run-time is not linear with most lead-acid battery types.
    http://batteryuniversity.com/learn/article/calculating_the_battery_runtime [Broken]
    Last edited by a moderator: May 5, 2017
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