1. Welcome Hearth.com Guests and Visitors - Please enjoy our forums!
    Hearth.com GOLD Sponsors who help bring the site content to you:
    Hearthstone Soapstone and Cast-Iron stoves( Wood, Gas or Pellet Stoves and Inserts)
    Caluwe - Passion for Fire and Water ( Pellet and Wood Hydronic and Space Heating)

Thomas Anderson's batteries

Post in 'The Green Room' started by Joful, Oct 16, 2012.

  1. Joful

    Joful Minister of Fire

    Joined:
    Mar 7, 2012
    Messages:
    5,891
    Loc:
    Philadelphia
    Off-topic thread started here: http://www.hearth.com/talk/threads/who-burns-24-7.92094/page-3#post-1208258

    I don't really understand the question, as that typically depends on our neighbor's usage. One transformer will feed several houses in a neighborhood. In the case of our house, we use so much electric, one might assume we're heating with it.

    The internal resistance of a typical marine or RV battery is higher than that of a car battery, but still frighteningly low. The 2500 amp number came from a manufacturer of marine batteries. That number cannot be maintained without overheating the battery (recall, exploding / boiling battery acid flying about), so you could never run a 60 kilowatt continuous load, or discharge in the time you suggest.

    Fuses have one trip mechanism: thermal. There is a time vs. load curve for any fuse, as they do take time to overheat and burn open. Circuit breakers typically have two trip mechanisms, magnetic and thermal. Magnetic trip is designed for short circuit or severe overload protection, whereas the thermal trip mechanism is designed for simple overload protection, and has a slow time vs. current behavior. In either case, you're bound for some excitement in the event of a true short circuit, but I do suspect you will be safe from an overheating / exploding battery situation... unless the short happens upstream of the fuse!

    There must be something to that story beyond what you heard or relayed. The effects of current on the human body have been thoroughly studied over the years. OSHA, notoriously conservative, puts minimum current for respiratory arrest at 17 mA. Most sources are closer to 100 mA. Developing even 17 mA from a 1.5 volt battery would require a net resistance of 88 ohms, most likley in a path thru the victim's chest cavity. Simply put, there is no possible way anything close to that is happening.
    hilbiliarkiboi likes this.

    Helpful Sponsor Ads!





  2. jharkin

    jharkin Minister of Fire

    Joined:
    Oct 21, 2009
    Messages:
    3,195
    Loc:
    Holliston, MA USA
    Agree with that. Your scenario (2500A from a 550Ah cell) is a 5C discharge rate - pushing it for a lead acid but not impossible. What will happen is the cell voltage will rapidly drop to around 1v and then go flat in just a few minutes, releasing less than 50% of its rated capacity (rated capacity is C/5)... and get very hot... but you will definately get some big amps for a short time if you short that thing.

    Be glad its lead acid and not lithium-ion-polymer or lithium Iron Ferrite. Either chemistry can support continuous discharge rates of 30C or more these days with minimal voltage drop and the LiFePO4s can burst over 60C for a few seconds. Lithiums have nasty explosions too......
  3. Thomas Anderson

    Thomas Anderson Member

    Joined:
    Sep 3, 2012
    Messages:
    149
    Loc:
    Central PA
    The point was that someone had said having that much power stored up in batteries gave him the heebee jeebees. But a mains transformer is essentially attached to a power plant and is at tens of thousands of volts potential. So that should be scarier in my estimation than a pair of 24V battery banks. I said if the batteries give you the heebee jeebees, you probably don't want to think about the transformer. And then you said that you're an electrical engineer so you don't have to think about it... with the implication that you would then tell us about it.

    I have a fast-response 60A fuse right on the positive lead of each battery bank, so a battery melting short would have to be caused by someone directly placing a conductor across the battery terminals (which have rubber sleeves over top of them). As I think I related before, I did in fact cause a short accidentally when I was initially installing the batteries, when I dropped a screwdriver. It vaporized a chunk of the screwdriver in a split-second flash.

    Now that I think about it, I think the story wasn't that he licked his fingers, but rather pricked his fingers so there was blood contact. I'll have to try to find the source for that.
  4. Sprinter

    Sprinter Minister of Fire

    Joined:
    Jul 1, 2012
    Messages:
    1,889
    Loc:
    SW Washington
    And with batteries we're talking DC. Unfortunately, it turns out that the peak frequency for initiating fibrillation of the heart is right at 60 hz. Fibrillation threshold is much higher with DC current. Too bad they didn't know that when 60 hz was adopted as a standard.

    Disclaimer: This is not to say that DC from high current batteries is safe! Tape up your screwdrivers and remove your rings!!!
  5. Thomas Anderson

    Thomas Anderson Member

    Joined:
    Sep 3, 2012
    Messages:
    149
    Loc:
    Central PA

Share This Page