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Take a look at this de-labeled smartphone Lithium-Ion battery:

Coil inside rechargable Lithium battery What is the coil used for?

Possibilities I thought of:

  • Wireless charging - unlikely because the outer metal foil would screen the magnetic field, too short anyway
  • Integrated active RFID tag - untypically short coil (not impossible though); same screening problem
  • Thermocouple-like temperatur sensor - I think this is the most likely use because those batteries need temperature sensing

The battery is Samsung brand.

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  • \$\begingroup\$ [re: 1st bullet] Non-ferrous metals don't screen magnetic field (at least nowhere near as much as they screen E-field). \$\endgroup\$ – Nick Alexeev Jul 13 '16 at 19:38
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    \$\begingroup\$ in case of Samsung, its another way to exchange small amount of data between phones (antenna for NFC near field communication) en.wikipedia.org/wiki/Near_field_communication \$\endgroup\$ – GR Tech Jul 13 '16 at 19:48
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Its a coil which can be used for NFC and/or wireless charging.

For optimal performance the distance between the coil and the NFC transmitter or wireless charging pad needs te bo as small as possible so some manufacturers put these coils on the battery on some models instead of making it a sticker that is attached to the battery cover of the phone.

The battery already has some electrical contacts and it is trivial to extend that connector with more contacts and this saves placing extra contacts elsewhere in the phone.

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