Need help with simple battery circuit !

Discussion in 'The Projects Forum' started by mountainsun, Dec 14, 2010.

  1. mountainsun

    Thread Starter New Member

    Dec 14, 2010
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    0
    I am attempting to make a pair of battery powered heated gloves, as my hands get extremely cold in the winter. I purchased some 30 AWG Teflon coated stranded Hook up wire from Newark Electronics, was told that the wire has a resistance of 93 Ohms/1000'. I am using 2-1.2 volt 2500 maH Enercell rechargeable batteries, inside a Radio shack battery box, with a slider switch. I am using approximately 2-3' of this wire in each glove. I have only made one glove yet, but I'm having issues.

    The glove has a high heat for approximately 25 minutes, and then the batteries are either drained and no more heat output.

    The more important issue is that the batteries get very warm, one of the shrinkwrap labels on one started to warp. I seem to have the glove wired correctly, but need to fix the length of time and hot battery problems.

    The 1.2 volts determines how much heat is output(vs. a 9 Volt), and the 2500 maH determines how long heat is output, is this correct ?

    Any help is much appreciated !
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    93 Ohms per 1000 feet means 93m Ohms per foot; if your total wire length is three feet, then the resistance is 0.279 Ohms (279m Ohms).

    If your 1.2v batteries are wired in parallel, then you will have about 4.3 Amperes current flowing through the wiring in your glove, and 5.1 Watts of power dissipation. This is considerably more current than your batteries were designed to output.
    The 2500mAH rating is usually derived by a constant drain current for 20 hours; in this case 125mA, as 2500/20=125; or 0.125 Amperes x 2 = .25 A. Basically, you're putting about 35/2=17 times the load on the batteries that they were designed for.

    If you have wired the batteries in series, then it's no wonder they are getting hot. It's somewhat surprising that they haven't ruptured forcefully from the internal heat and resulting pressure.
     
  3. #12

    Expert

    Nov 30, 2010
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    I think Wookie is saying you used the wrong wire. It allows too much current. You can't get batteries with smaller voltage, so you have to get wire with more resistance. Wookie did the math. Look there to calculate what you really need.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    Find an old toaster and remove the wire from it. It's Nichrome wire and has approx 1/2 ohm per foot. connect your battery to various lengths and select a length that gives off the right amount of heat. Probably be about 3 to 4 feet.
     
  5. #12

    Expert

    Nov 30, 2010
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    Good idea! Most toaster wire is flat, unlike that curly stuff in space heaters. PS, you will have a tendency to allow too much heat in a test condition because you're working rather quickly. Be conservative. Heat builds up over time. You can always cut the wire shorter if it doesn't heat enough, but it's hard go back later and cut it longer.
     
  6. mountainsun

    Thread Starter New Member

    Dec 14, 2010
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    0
    I didn't think toaster wire was coated ? Wire will be in direct contact with fabric of glove.
     
  7. #12

    Expert

    Nov 30, 2010
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    True. Are you expecting to get an electrical shock or thinking about having the wire so hot that the fabric bursts into flames?
     
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