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Lots of new batteries (for mobile devices, MP3 players, etc) have connectors with 3 pins. I would like to know what is the purpose of this and how should I use these three pins?

They are usually marked as (+) plus, (-) minus, and T.

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  • \$\begingroup\$ Where have you seen the third pin? Im interested in it. \$\endgroup\$ – Dean Feb 23 '11 at 10:14
  • \$\begingroup\$ Canon NB-4L for example \$\endgroup\$ – Martin Feb 23 '11 at 13:20
  • \$\begingroup\$ Roomba batteries too. Can't gut them and replace the internals without making sure you hook that third contact back up. \$\endgroup\$ – AngryEE Feb 23 '11 at 14:51
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    \$\begingroup\$ In addition to all the answers, sometimes it's just to provide a more stable mounting to the PCB. I deal in Varta Mempac memory batteries for an FM tuner, with two positive and one negative pins. They're large enough that two pins wouldn't be secure. \$\endgroup\$ – user207421 May 26 '13 at 1:50
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    \$\begingroup\$ I have a battery with + - C T, and I've seen a battery with + - D T. I wonder what the C and D stand for? \$\endgroup\$ – jpaugh Dec 3 '16 at 18:44
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The third pin is usually for an internal temperature sensor, to ensure safety during charging. Cheap knock-off batteries sometimes have a dummy sensor that returns a "temp OK" value regardless of actual temperature.
Some higher-end batteries have internal intelligence for charge control and status monitoring, in which case the third pin is for communications.

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  • \$\begingroup\$ Ohh that makes sense! I'm retracting my answer. \$\endgroup\$ – drxzcl Feb 23 '11 at 9:37
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    \$\begingroup\$ Yep. The "T" stands for "Temperature" (Or "Thermistor"). Normally, it's a thermistor. \$\endgroup\$ – Connor Wolf Feb 23 '11 at 11:26
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    \$\begingroup\$ What about the fourth pin? On some mobile phone batteries there is a terminal for identification. It is wired to a resistor, and by measuring the resistance, the phone know whether the battery complies. I do not remember where I read about that, however. \$\endgroup\$ – Vorac Jul 4 '12 at 14:13
  • \$\begingroup\$ Also a possibility: OneWire verification chip, to make sure the battery is one authorized by the manufacturer: maximintegrated.com/en/products/comms/one-wire/… (this example is for printer cartriges, but I'm sure others exist.) \$\endgroup\$ – TheDoctor Jul 27 '15 at 14:44
  • \$\begingroup\$ What is the return/resistor value of "Temp OK"? \$\endgroup\$ – Codebeat Dec 18 '17 at 20:30
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That third contact is connected to an internal thermistor, enabling the charger to measure the battery temperature.

enter image description here

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In mobile phones, some Li+ battery packs have 3 terminals. Two possibilities:

  • positive, negative, thermistor (as was already mentioned in previous answers)
  • positive, negative, 1-wire bus. The latter is a digital communication bus that’s connected to a gas gauge IC inside the pack.

If you want to explore what’s inside single-cell Li+ battery packs, look-up bq27000 gas gauge IC and associated application notes. Could be a good starting point.

Some packs have 4 terminals: positive, negative, SDA, SCL. The latter 2 lines are I2C or SMBus. Look up the bq27200 gas gauge IC (shares datasheet with bq27000).

EDIT: This was written as an answer to a duplicate question, which got merged with this one.

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  • \$\begingroup\$ Do you know any actual battery models which carry a BQ27000-like chip inside and expose a communication pin? \$\endgroup\$ – KT. Nov 22 '18 at 13:26
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    \$\begingroup\$ @KT. Here's a teardown of the iPhone 4 battery which has got a bq27541. Here's a write-up by Microsoft about adding a battery gauge to a power supply for testing and development purposes. \$\endgroup\$ – Nick Alexeev Dec 16 '18 at 22:17
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For Nokia batteries, one of the pins may be a BSI (Battery Size Indicator) pin, which contains a fixed resistor to ground, enabling the handset to identify which battery is connected. Examples of BSI resistor values include:

 - BMC-2  3k3  NiMH   640mAh
 - BMC-3  5k6  NiMH   900mAh
 - BLD-3  22k  Li-Ion 780mAh
 - BL-4B  68k  Li-Ion 700mAh
 - BL-5B  75k  Li-Ion 820mAh
 - BL-4U  82k  Li-Ion 1000mAh
 - BL-5C  82k  Li-Ion 1050mAh
 - BL-4J  100k Li-Ion 1200mAh
 - BL-5J  110k Li-Ion 1450mAh

See also: BSI - cpkb.org

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The third pin is usually found on Li-Poly, or Lithium Polymer batteries and is required in order to charge the battery safely. Because these batteries are usually multi-cell, the third pin is used for balancing the charge between each of the cells.

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    \$\begingroup\$ the only thing that sounds wrong here is that my balancing LiPo battery charger uses four pins, not one. How can they do the balancing with only one pin in cell phones? \$\endgroup\$ – Dave Feb 23 '11 at 18:19
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    \$\begingroup\$ Cell phones normally use single cell batteries, so it can't be for balancing purposes. \$\endgroup\$ – Connor Wolf Feb 23 '11 at 21:58
  • \$\begingroup\$ @Dave/@Fake Name - good points. I stand corrected :) \$\endgroup\$ – Richard Mar 1 '11 at 9:33
  • \$\begingroup\$ I have seen dual cell LiPo batteries that had the balancing connection. However these would normally not be labelled T. The OP does not specify only cell phones so we cannot work from that assumption. \$\endgroup\$ – KalleMP Dec 27 '17 at 18:51

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