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Battery Calulation Check for TV

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    Battery Calulation Check for TV

    I’m looking to run a tv at a tailgate for 6 hours. I have been interested in building a 18650 battery project for awhile now and decided to start with a portable pack. I’m confused on how many 18650 3500mah batteries I’ll need. so here are my calculations so far.

    The tv runs at 120v ac @ 120w.

    120vac @ 1amp draw through converter = 12vdc @ 11.04amps dc

    11.04amps dc x 1000 = 11,040 mah

    6 hours x 11040 mah = 66240mah

    66240mah / 3500mah = 18.92. In this case I’ll say 19

    using 4.2v batteries I planned on doing a 3x19 configuration
    resulting in 12.6v going to the converter.

    Any thoughts, questions or concerns would be appreciated!

    TVs are DC devices, so why not hack into your TV and just supply the correct DC voltage downstream of the TV's built-in power supply! It would be more efficient than going from DC to AC and back to DC.
    For your calculations, you need to take into account the efficiencies of your inverters as well.


      Hacking the tv won’t work as this will be used for other 120v items.

      Per efficiency: I have over calculated the usage by 33% initially by stating 6 hours. The actual requirements would be 4 hours. But I would rather be left with energy left over then not enough. Hence going with 6 hours to design.

      Thanks for confirming the design. If anyone has inverter recommendations or any other comments I am all ears!


        Math check on TV.
        If TV is 120W, 6 hour x 120W is 720 watt-hour.

        Math check on converter.
        12V x 11 A = 132 watts
        132 watts x 6 hours = 792 Watt-hour

        Math check for battery
        19 cells ? This is 19 x 3.5AH *12V = 798 WH.

        A minimum of 800 watt-hours is needed,

        That 3500 AH per cell is based on running the cell to its highest rated to lowest rated voltage, i.e, from 4.2V down to 2.5V. This is the manufacturer's spec, but in real life, this shortens the cell life and raises odds of cell damage.

        In the ebike world, motor controllers are usually set higher for a lowest voltage of 3.0V. This only uses 90% of the battery, but it's done for safety. Many of us never run our ebikes flat, by the way.
        Meanwhile 12V inverters are usually intended for lead acid batteries and those also have a minimum operating voltage. Check what your intended inverter has for a low voltage DC cutoff. Betcha it's around 10V. That would mean running a 3 series lithium cell battery down to about 3.3V, which is around 70% of the battery.

        If you're building this pack, that means you have to size up your cell count to compensate. Probably at least 30%. off the top of my head.