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Creating a Battery Bank with Paraffin Wax as a Heat Sink

  1. Apr 5, 2016 #1
    I'm in charge of designing a battery bank (using 18650 batteries) that will produce large amounts of heat for a short period of time. The battery bank will be in a near vacuum (~800 Pa), so convective heat transfer will not be effective. Since the operation time is only ~80 seconds, my thought is to store the heat in paraffin wax. The battery bank will be placed into an aluminum box (coated with electrical tape spray to insulate it) and be raised from the bottom of the box with a polycarbonate sheet between the batteries and the aluminum. Paraffin wax will be poured into the box in direct contact with the batteries and allowed to solidify at room temperature. Initial calculations show that it will easily be able to store the heat generated.

    Now, I have concerns about putting the batteries in direct contact with the paraffin wax. Will the wax prevent the batteries from venting and possibly lead to an explosion? Or are there other possible issues with direct contact?
     
    Last edited: Apr 5, 2016
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  3. Apr 5, 2016 #2

    berkeman

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    Welcome to the PF.

    Are these batteries specified to be used in vacuum conditions?

    And you say that they deliver the power to heat something up -- how much do the batteries heat up when supplying this power?
     
  4. Apr 6, 2016 #3
    The batteries themselves aren't exposed to vacuum. The design currently is to place them in a box, fill it with paraffin wax (leaving a small air gap), evacuating the air, and reintroducing nitrogen gas at the top. The box is airtight to maintain the pressure even though the outside of the box is in a near vacuum. As the paraffin wax expands as it melts, the nitrogen gas is compressed, triggering a pressure relief valve if the pressure gets too high.

    The batteries are used to power motors. Because they draw so much current (~50 A), the batteries will internally produce 138 kJ of heat due to I^2 R losses.
     
  5. Apr 6, 2016 #4
    Lithium-ion batteries are prone to outgassing when charged incorrectly. This could happen if the charger malfunctions.

    In addition, as they age their internal resistance rises. This might mean recalculating the heat you need to absorb. It also means a strict replacement policy at end of life.

    Lithium-ion batteries have a reputation for burning. Usually the lithium doesn't burn and water helps the shorting cells to not burn. But I wouldn't swear the lithium couldn't burn and ignite the aluminum case. Metal fires are nasty.

    You didn't list any good oxidizers in your design, but light metals can steal oxygen from otherwise stable molecules. You will want to check on that.

    Without a way to vent heat, you will need to limit recharging after use until the project cools. Recharging will produce some heat, but the big danger would be an unintended short while the paraffin is still hot.

    Nightmare images of burning thermite and wax keep going through my head.
     
  6. Apr 6, 2016 #5

    berkeman

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    Um, does the pressure relief valve lead to the vacuum?
    No kidding.

    @skiandtea -- Why are you using batteries for this power source? Why not just use a conventional power source located outside of the vacuum environment? (BTW, this thread is close to being closed for safety reasons...)
     
  7. Apr 6, 2016 #6
    The pressure release valve would lead to vacuum; the reseating pressure will be considerably higher than near-vacuum to ensure the batteries operate in normal pressure ranges.

    Batteries have to be used because the system is moving very rapidly. We understand that the use of batteries constitutes a risk. However, we see no other viable alternative to get rid of the generated heat because the system is in a near-vacuum. Our only idea is to store the heat in a phase-change material. And this is with the mentorship of several professors from a prestigious engineering school. Safety will always be our first priority, which is why I posted here.
     
  8. Apr 6, 2016 #7

    CWatters

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    Check out the problems Boeing had with their 787 Li battery. Thermal runaway can be a real issue.
     
  9. Apr 6, 2016 #8
    You might also consider some other battery chemistries.

    The potential dangers and engineering costs of this approach would have me considering almost anything else. Fuel cells, small motors, solar fed by laser from outside, microwave, or even quantum tunneled power are options.

    Still, in a controlled environment with no people around, perhaps safety would be less of an issue. For example a moon rover might be happy to have the heat when the sun goes down for two weeks. (Though space applications typically spend more on engineering and less on wax.)
     
  10. Apr 6, 2016 #9

    NascentOxygen

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    Can you provide a link to the high discharge 18650s you plan to use? How many will you connect in series for each motor?

    Have you concluded that 18650s are the best for your purpose?

    After being used for 80 secs, how long before they will be called on to again drive the motors?
     
  11. Apr 10, 2016 #10
    These are the high discharge 18650:
    https://www.amazon.com/Genuine-18650-3000mah-Drain-Battery/dp/B010NUUQFK

    The 18650s are highly desirable because of their high discharge to draw large currents. I didn't do the research to say why else 18650s were chosen, though I assume there is very good reasoning. The 80 seconds of operation will only be done once for the final run. It's for a contrived situation basically meant to see how it will perform for a single use, not to run it repeatedly.
     
    Last edited by a moderator: May 7, 2017
  12. Apr 10, 2016 #11

    NascentOxygen

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    Those appear to be rated at just 4A steady discharge, in your context. Your application calls for 50A, what amount of voltage sag is being experienced?

    Contrast them with the Lipolys favoured by RC hobbyists for their enormous discharge, e.g., 100A or so for these 45C packs: https://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=11952

    I have no first-hand experience with Lipolys in such extreme conditions, but think they seem a better fit to your need.

     
    Last edited by a moderator: May 7, 2017
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