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NiMH battery pack

Discussion in 'Electronic Basics' started by John Riley, Aug 15, 2006.

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  1. John Riley

    John Riley Guest

    I have a number of 4 cell, 4.8V NiMH mobile phone batteries that I
    want to string together to make a larger battery pack.
    When I pull one apart, I find that the centre of the three terminals
    on the case is the positive, the other two are straight negative, and
    negative with a very small component in the lead about 1mm in diameter
    and a few mm long. The voltage reading between pos and straight neg is
    5.19V and between pos and this other neg with the unknown component is
    5.13V. Can anyone tell me what this strange (to me) component is?
    Perhaps a diode, for charging purposes? Thanks for any help, regards,
  2. Puckdropper

    Puckdropper Guest

    Some batteries have fuses in them. They look quite a bit like a diode,
    but with no band. (Usually you'll see an amperage rating on them.)

  3. John Riley

    John Riley Guest

    This seems too small to be a fuse, and why the connection to the other
    terminal wirhout it? Seems to me it has to be something that needs to
    be different between charging and draining the battery. And this fuse
    would need destruction of the casing to change it. It is all glued
    together. This tiny little component seems to have some sort of
    marking like a diode with a triangle pointing in one direction. It is
    so small I can't figure it out with my strongest glasses and a
    magnifying glass out in the sunlight.
  4. Phil Allison

    Phil Allison Guest

    "John Riley"

    ** DO NOT TOP POST !!!!!!!

    ** Very likely it is a thermistor.

    A special resistor that changes value with temperature, possibly quite

    The phone uses it to sense if the battery is too hot and stop charging.

    ....... Phil
  5. John Riley

    John Riley Guest

    Thanks Phil. That seems to make sense. I will heat it up and see if
    the ohms rise accordingly.
    This will make little difference to my setup as I will just be
    shorting the two negative terminals together, as the bottleneck in my
    larger battery will be the fine wires used to hook the cells together.
    If one of the negs acquires a bit more resistance, then that's fine. I
    will be charging and discharging manually with temperature and current
    monitoring both ways. ps, sorry about the top posting, but I've been
    lulled into a sense of stupidity by Yahoo Groups :)
  6. John Riley

    John Riley Guest

    Before I heated the thing, I tried to measure its resistance.
    I couldn't get a reading. Open circuit?

    I then put it in series (both ways) with a battery and a globe
    that glowed. Nothing, and even with a meter in series as well,
    zero milliamps.

    But, measuring the voltage of the battery with this thing in
    series showed a slightly smaller voltage (~30 mV) than the
    direct voltage measurement.

    Could this be a capacitor? What would be its purpose?
  7. Rich Grise

    Rich Grise Guest

    Did you try it with the meter leads the other way around?
    No, it couldn't be a capacitor.

    It sounds like a diode, that's used as a temperature sensor by the
    smart charger. You won't read much voltage from it, and probably
    won't see much change in its forward drop, but the smart charger
    had a circuit that _does_ see the change, and uses it to decide
    when to stop charging the cells.

    Good Luck!
  8. John Riley

    John Riley Guest

    Thanks for your thoughts, Rich, but I measured the current and
    resistance both ways, and it acts exactly like an open circuit.
    What other component shows "infinite" DC resistance, but you can
    measure voltage through it? My extremely limited experience suggests a
    capacitor, which as you have suggested, makes no sense. It seems
    strange to me to put one component of a smart charging circuit right
    inside the pack against a cell, if it isn't there to monitor
    temperature. I was chuffed when it was suggested to be a thermistor.
    But this thing would allow no charge to flow. It could however monitor
    Aha, I wonder if its voltage reading varies with temp, and so it could
    tell the smart charger when to stop charging by its signal, rather
    than directly reducing charging current itself. I'll give that a try.
  9. John Riley

    John Riley Guest

    Hooley Dooley, I just put my meter in series with it and a
    battery, and as before, zero amps. I heated it and lo and
    behold, it started transmitting several amps.

    I tried the voltage and at room temp, it measured 4.99V and
    at hot, it went up to a steady 5.05 V which is the full
    voltage of the battery.

    Sooo, what sort of component, (group of components) has
    infinite resistance at room temp, and around 2 ohms when
    heated? Does this reconcile with the voltage readings?
  10. Guest

    John Riley,

    I'm sorta throwing this out and hope that it may be the right answer
    for you.

    I have a Newton which also uses a rechargeable pack which consists of
    (4) AA NiMH Cells. As part of it's construction it too has a thermistor
    installed. The themistor is present to monitor the temperature. This
    thermistor in the Newton Rechargeable Pack has a particular value which
    only activates at a predetermine temperature range. Since I'm not
    electronically inclined I can not describe all the mechanics, but in
    the Newton when the Rechargeable Battery Pack reaches this set
    temperature limit the Newton will stop charging the pack to prevent
    over-charge or possible explosion of the pack (which might be just a
    POP! and the cells crack open.)

    But since we don't know where you got this battery pack(s) from and
    what type of possible protective features or charging setup was
    implemented there's no real way to determing the use of this unknown
    item unless you contact the manufacturer whom might explain it's use OR
    knew first hand the item in question.

  11. Eeyore

    Eeyore Guest

    No. The thermistor is indeed there to monitor battery temp but this additionally
    gives vital inforamtion about its state of recharge.

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