Trying to understand battery circuit

Discussion in 'The Projects Forum' started by dmarciano, Jun 17, 2011.

  1. dmarciano

    Thread Starter Member

    Oct 10, 2007
    21
    0
    At my job we have an instrument that runs off a custom battery pack and my company would like me to build a battery tester to be used in the field. So I opened up one of the dead batteries to see how they are step, however I don't fully understand what is going on.

    The battery pack consist of 5 rechargeable NiCad, 1.2V 3000mAH wired in series. There are three on the bottom with two stacked on top making a stem of sorts. There is two contacts on either side of the top of the stem (Tab 1 & Tab 2). These tabs are what connect to the instrument to run the instrument. On the very top there is a circular plug with four pins (Pin 1, 2, 3, 4) and this is where the battery is charged.

    The positive end of the battery series is connected to Tab 1 and Pin 1. The negative end of the battery series is connected to Tab 2 and Pin 2. This I understand and makes sense to run the instrument and to be charged through. It is the other two pins that confuse me. Pin 3 is connected to Pin 4 with what I think is a diode. When I opened of the battery this piece crumbled but it was quite small, cylindrical, and contained glass which is why I thought it is probably a diode and not a resistor.

    Does anyone have an idea what the purpose of Pin 3 and Pin 4 could be? I was thinking that when the charger is attached to all the pins and turned on, the charger sends a signal through these pins to see a battery is attached to the charger. If it doesn't get a signal back it thinks that no battery is connected and starts blinking the "Fail" light. When it does get a signal back then it turns on the actual charging circuitry.

    I quickly put together an image of the circuit to help visualize it. Please disregard that it isn't a professional schematic, and I just wanted to make something quick that would do the job. Any thoughts and comments would be appreciated, and any help would be greatly appreciated.
     
  2. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    The crumbled part might be a temperature sensor.
    Try measuring the characteristics from pin 3 to 4 on a good battery.
     
  3. dmarciano

    Thread Starter Member

    Oct 10, 2007
    21
    0
    Measuring the resistance I got about .5Ohms My meter read 0 for diode. If this is a thermistor, then it would just be measuring the temperature inside the battery pack. Now that I think of it, the older chargers did have a "HOT" light, although the newer ones don't. We haven't used the older chargers in quite a while and completely forgot about that light. Thanks, I feel pretty confident that is what it is now. Thanks :D
     
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