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Lithium ion polymer battery: What is the 3rd (yellow) lead? what can I use for a replacement?

Discussion in 'Datasheets, Manuals and Component Identification' started by MidAtlantian, Sep 7, 2015.

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  1. MidAtlantian


    May 12, 2015

    I've got a BlueTooth keyboard that takes a 3.7v lithium-ion polymer battery. There are three leads coming from the battery: red, black and yellow. What is the function of the yellow lead, and is there a way to use a two lead battery in its place? I'm asking because I've not found a like for like replacement. The battery in it now is a 232535. It is 3mm x 23mm x 40mm. But there is plenty of extra space. It could accommodate a 40mm x 100mm, as long as it was not more than 3mm thick. To complicate things, I'm in the UK.

    Thanks for any help/suggestions.
  2. davenn

    davenn Moderator

    Sep 5, 2009
    its most likely for voltage or temperature monitoring and of those two most likely temperature
  3. MidAtlantian


    May 12, 2015
    Davenn: Thanks. I guess for Lithium-ion, given its record for fires, it's not a good idea to find a way around having that sensor. Would you have any idea where to look for a similar battery - one with a temperature lead - in the hopes that these types of batteries use the same sensing circuitry?
  4. 73's de Edd

    73's de Edd

    Aug 21, 2015

    Sir Midatlantian . . . .

    My suspicion is this:
    That the red and black be assigned to the + and - battery connections, with the yellow being a sense wire to provide corrective info to an off board charger. Typically the sense mode will be provided by variable resistance provided by a small ~10 k thermistor that is nestled against the battery case and is hidden by the shrink wrapping.(With the possible ecception of a tell-tale bulge.)

    To confirm:

    DC meter the + and - leads and confirm the battery voltage and then with meter negative still on NEGATIVE BATTERY wire, move the +meter wire to the yellow and confirm that there is being NO voltage presence.
    If so, it is then it is safe to go to ohms metering mode and confirm the K's of resistance between Black and Yellow.

    One step further then, might be to commandeer Mama Cass's blow drier and set to low heat and pass a couple of times over the side of the battery case . . . .mind you . . . not approaching a heat level which might blister the shrink wrap !
    You should then see that the thermistor resistance value will have shifted after the heat application . . . . . research case closed.

    The majority of cell phone, think pad, notebook computers and electronic devices that I see have the charger controller circuitry build onto a "very-very" small Pee Cee Bee. ("A. . .la. . .Elmer Fudd")
    and is attached to the battery, in which situation, thermal data is fed to that charger and controller circuitry and you never are aware of the sensors presence.

    See this illustration:


    If of high enough current capacity 1200 mah and of proper physical sizing, this one from Adafruit would be taking the RAW DC you feed to it for charging and any temp sensing is being done ON the small PCB shown at the left side and no need to send any temp sensing back to an off board charger via a yellow(sense) wire.

    73's de Edd

    .(Mod Note ... rubbish part deleted)
    Last edited by a moderator: Sep 8, 2015
  5. MidAtlantian


    May 12, 2015
    Hi "73's de Edd,

    The plot thickens. Yes, I can see that there is just that sort of mini-circuit-board included into the shrink-wrap envelope of the existing battery, to include two tiny din chips. I tried doing an ohm reading on the yellow-black pair, using a heat-gun, but got consistent zero readings, so it seems likely it likely produces a voltage. But the 3.7v battery is now producing only a few hundred milli-volts, so it might not be powering that wire even if it should. I can measure the charging circuit, and, when powered, that is giving me a very acceptable 4.2 volts. But in any case it seems that running without the yellow simply might not work. The truth is that I already have a replacement device, so I am not toooo dedicated to reviving this defective one. The battery you suggested is about 10x the capacity of the one that is there, which could be fine, but it is about 2mm too thick.

    In any case, thank you for your thoughts and suggestions.

  6. Alec_t


    Jul 7, 2015
    The battery should have protection circuitry to disconnect it if it gets discharged too far. Looks like it has activated in your case.
  7. MidAtlantian


    May 12, 2015
    What a minefield! If it disconnects, does that mean it also disconnects from the charging circuitry? With all of the on-board logic, why would it need an additional sensor wire?
  8. DavidSc


    Jan 31, 2019
    Hi MidAtlantian, sorry todo revive an old thread but I’ve the same problem, Bluetooth keyboard with a 232535 dead battery. Were you able to replace yours? And, if so, what about the infamous yellow cabe? Any advice is welcome. Thank you in advance. Dave
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    DavidSc likes this.
  10. MidAtlantian


    May 12, 2015
    Hi DavidSc,

    Sorry, but I cannot be of much help. I managed to source a proper replacement battery that had a yellow (sensor) wire.
    DavidSc likes this.
  11. DavidSc


    Jan 31, 2019
    Thank you Harald Kapp for your suggestion and thank you anyway MidAtlantian, I'll try to locate one with the yellow wire as well. All the best!
  12. Externet


    Aug 24, 2009
    If a single 4V cell, the extra wire should be temperature sensing for external management; if a two-series 8V battery, can be a center feed point for external management circuitry if has no protection/balancing board built-in.
    If a management board is built-in, can also be a 'enable' control switch.
    Explore also ---->^1.pdf
  13. Bluejets


    Oct 5, 2014
    It is conceivable that reusing the existing protection board with a new battery of similar capacity might be an acceptable alternative.

    Attached Files:

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