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Lithium-Polymer battery fire safety precautions

  1. Nov 4, 2011 #1
    Hello,
    I'm trying to learn about LiPo battery pack safety and was wondering if anyone could head me in the direction of some good sources of information.

    In particular I'm trying to learn enough to safely re-build and extend a laptop's 6-cell (3S2P) pack, but hopefully to extend it to a higher number of cells (eg 3S8P) as the extended run time (8+ hours) away from mains power would be particularly convenient for me and, as I understand it, the increased pack parallelism would be easier on the cells and extend their life-time somewhat.

    Cheers.

    edit: If possible I'd like to solder the new cells into the existing battery pack circuitry.
    Also, what about typical store stocked fireproof blankets?
     
    Last edited: Nov 4, 2011
  2. jcsd
  3. Nov 5, 2011 #2
    Please do not use Li-Poly, for your own sake and perhaps those around you.
    There is no such thing as a "3S8P" pack, and if there were, that would be 88.8 Volts that you would be pushing out. Is this for some funny experiment where you are trying to see how quickly you can melt a laptop?? It os obvious that you are quite ignorant in this, which is why I'm asking you to not even consider it.

    Plus, you do not add more cells to increase run time, this increases the Volts. To extend run time you run a pack with higher mAh. This is akin to having a larger fuel tank in your car, Volts increase the power. Furthermore, Li-Poly batteries are high capacity and high discharge. Most Li-poly packs today are 20C (current) and higher. Do you know what it means to pull that sort of current (Amps) at that voltage? Like I said, it will reduce your laptop to ashes.

    These batteries are used in high powered RC models to run brushless motors as their high discharge abilities make them far superior over the alternate nitromethane powered 2-stroke engines. However, like you said, fire is a very likely risk. Discharge a Li-Poly cell bellow 3 Volts and you have caused irreparable damage. Do this a few more times and it is then just a matter of time before it is in a ball of fire (yes, not if, but when). Over-charge a cell and, again, irreparable damage. Not only that but all cells must be maintained accurately balanced at all times... fail to do this, and you can guess where this is heading, you have a ticking time bomb sitting in your home. It's just a matter of "when."

    PS: Most laptops and cell phones use Lithium-ion, understandably so as it is much less unstable, not to mention that such devices do not need to pull 80+ amps. If your laptop does indeed use Li-Poly, just use an OEM battery. Do NOT play around.
     
    Last edited: Nov 5, 2011
  4. Nov 5, 2011 #3
    3S8P = Three cells in series (11.1 volts nominal). I've read of eight-parallel packs, but I imagine balance charging might be difficult. More typically, I've seen large paralleled battery packs divided into sections for charging, such as 2P or 3P.

    Outside of electric vehicle experimenters, the most active battery pack hackers are R/C and flashlight enthusiasts.
    http://www.candlepowerforums.com/vb/forumdisplay.php?9-Flashlight-Electronics-Batteries-Included
    http://www.rcgroups.com/batteries-and-chargers-129/
     
  5. Nov 6, 2011 #4
    Juval, you may want to learn about the subject yourself before you misread a post and lecture someone on neophytic errors they thought they'd made clear they were never going to make. LiPo packs exist, not least within the consumer electronics market, and it is clearly possible for their use to be at least as safe as many other daily activities.

    My primary interest with this post was to start learning specifics about LiPo safety, not to happy-hack a portable bomb. Furthermore the "C" rating is, as I understand it, an intended maximal rating ascertained by the manufacturer regarding a possible current draw, battery packs are not constant-current sources and they, for example, will not push hundreds of Amp through anything excepting an effective short-circuit scenario.

    To demonstrate; if we are to assume the C rating pushes current as you describe, even a single cell would be highly destructive as it too will push "20C" (or whatever the rating might be) through whatever it is connected to. This would be around 40Amp for a common 2Ah cell I've yet to hear tell of laptops being destroyed en masse by their ~5Ah battery packs discharging at the rate of ~200Amp through a non short-circuit load (i.e. a functioning laptop computer) as your interpretation of their behaviour could well entail. To me, that sounds far more ludicrous than even your misinterpretation of my post, not least because 88.8Volt from 24 cells is at least a physical possibility had I or anyone else been so inclined to hack together such a configuration.
     
  6. Nov 6, 2011 #5
    Thanks for the links pantaz, I'll probably contact some folk over there and see what they say about it.
     
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