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Using a Small Lithium-Ion Battery to Heat Wire

oseania

Hi,

 

Hi we have a very small capacity battery (around 40 mAh) which we use to heat a wire (10 ohms). For wire heating we use over 100mA current. I started to wonder would it be beneficial for the battery lifetime if the wire would be a part of LRC-oscillator or some other form of resonant circuit. At the moment we see quite a fast drop in the battery voltage when we drive the wire with 1kHz PWM signal (duty cycle around 90%). I started to think about the inductive stoves and their efficiency.

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  • 0

    Basically what you are describing sounds similar to what is in disposable ecigarette vaporisers.

     

    The heat developed in a heating element used resistively is basically related to the RMS current through the wire, as it is  basically 100% efficient. Pulsing the wire would reduce the net current by duty cycling - you are not running the wire all the time. Sure, the battery voltage may be slightly higher as a result of the net reduction in average current but the same effect might be had with an increased resistance wire.

     

    I suspect you might see rapid voltage drop purely due to internal resistance in the small battery which was probably not designed to discharge at >1C rates. Reducing current draw by PWM may be acceptable but the peak current still might be high enough to cause internal battery heating and damage.

     

    Inductive stove efficiency is due to a completely different reason - AC is necessary to induce a current in the metal of a pot itself through rather than send heat into the pot through conduction where more of the heat is lost to the air.

     

    - Gough

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  • 0

    Basically what you are describing sounds similar to what is in disposable ecigarette vaporisers.

     

    The heat developed in a heating element used resistively is basically related to the RMS current through the wire, as it is  basically 100% efficient. Pulsing the wire would reduce the net current by duty cycling - you are not running the wire all the time. Sure, the battery voltage may be slightly higher as a result of the net reduction in average current but the same effect might be had with an increased resistance wire.

     

    I suspect you might see rapid voltage drop purely due to internal resistance in the small battery which was probably not designed to discharge at >1C rates. Reducing current draw by PWM may be acceptable but the peak current still might be high enough to cause internal battery heating and damage.

     

    Inductive stove efficiency is due to a completely different reason - AC is necessary to induce a current in the metal of a pot itself through rather than send heat into the pot through conduction where more of the heat is lost to the air.

     

    - Gough

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  • 0 in reply to Gough Lui

    Thanks very much for your response. I was wondering if supercapacitor could be used to decrease the stress imposed to the battery. Basically slowly charging the supercap and then rapidly discharging it to the wire.

  • 0 in reply to oseania

    In theory, yes, but depends on how long the operation time is - regardless the battery still needs to supply *at least* the average current required.

     

    E.g. you have 100mA current heater, duty cycle 50% - the battery sees 100mA/0mA/100mA/0mA pulses of equal length. Assuming you have an "infinite" capacitor, the battery will seem to be loaded with 50mA *constantly*. But in truth, there are small losses in the ESR of the capacitor, so the current on the battery will still have "ripple" - i.e. not perfectly averaging.

     

    This technique is used to run short-burst devices (e.g. radio transmitter ICs) from small coin-cell batteries to prolong life (e.g. push-button remote central locking remotes).

     

    - Gough