Does a li-po/li-ion battery that has a pcm and ntc thermistor need 3 wires?

Discussion in 'General Electronics Chat' started by WLT, Apr 15, 2016.

  1. WLT

    Thread Starter New Member

    Apr 10, 2016
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    I want a battery with a thermistor for safety but I wanted to know if there are batteries with 2 wires and thermistor internal, instead of three wires connecting to a circuit. I know little about thermistor and cant find anything explaining this. I have searched and seen thermistors as a third wire on the battery but does this mean all 2 wire batteries don't have a thermistor?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Only batteries with three terminals or more will have thermistors inside, two terminals means no thermistor, however you can use a battery protection pcb.
     
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  3. paulktreg

    Distinguished Member

    Jun 2, 2008
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    Could you use a thermal fuse (like this)? This would only require two wires. You would of course have to select a temperature suitable for your application.
     
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  4. WLT

    Thread Starter New Member

    Apr 10, 2016
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    Thanks for the suggestion. If I were to hook up the 2 wire battery to a charger board, and then from the charger board to my circuit. Assuming the board has a thermistor, I wouldn't need the battery to have one right?
     
  5. WLT

    Thread Starter New Member

    Apr 10, 2016
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    Yeah but I hear it only protects from over/under charging.
     
  6. WLT

    Thread Starter New Member

    Apr 10, 2016
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    Also, I'm not knowledgeable on thermistors, but do the 3 wire batteries, does the thermistor on it need to be connected to a microcontroller? If not I would rather have one like you suggested.
     
  7. Dodgydave

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    Jun 22, 2012
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    Thr thermistor is connected to a microcontroller in the charger, it monitors the temperature of the cells, and if it gets too hot stops the charger.
     
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  8. WLT

    Thread Starter New Member

    Apr 10, 2016
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    which do you mean? the thermistor that comes with the charger board? or connecting the thermistor from the 3 wire battery?
     
  9. crutschow

    Expert

    Mar 14, 2008
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    A Li battery can explode and cause a fire (as many have) if they are overcharged and overheat.
    So the battery charger monitors the thermistor in the battery and terminates the charge when the temperature goes above the battery limit.
     
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  10. WLT

    Thread Starter New Member

    Apr 10, 2016
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  11. Dodgydave

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    Jun 22, 2012
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    They dont use thermistors, that board monitors the charge current and voltage, then disconnects the battery when charged.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you do use a thermistor (or thermal fuse) it must be in close thermal contact with the battery itself.
     
  13. recklessrog

    Member

    May 23, 2013
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    The point that is being missed is that the Thermistors resistance changes with temperature, so needs to be tight against the cells, the charge controller connected to the 3rd wire monitors the resistance so that if the temperature rises too much, the charge current is reduced or cut off for safety. Also,that and the charge characteristics of the cell are programmed into a micro processor which is sometimes built into the battery casing so you only get two wires. I recently opened up a failed camcorder battery pack which had the charge and monitoring circuit board all included in the pack but it still had three connections, one common, one for raw charge input to the controller, and the other was the voltage out from the pack which was monitored for overload to shut down the output in case of a short circuit. All clever stuff!!!
     
  14. Tonyr1084

    Active Member

    Sep 24, 2015
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    If you look close at the picture of the battery and charger you posted you should be able to see under the kapton tape a circuit board attached directly to the battery itself. Though there are only two wires delivering a charge to the battery, the on-battery circuit board monitors the condition of the charge and discharge rates as well as the temperature of the battery.
     
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