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Attached you find photos of a Li-Ion battery's PCB. I'd like to know what this component is called and what its use is.

The component in question is the three pin stretched silver "bridge". The white blobs seem to be glue, while the black things you see are ICs. There is nothing beneath the brownish center of the component.

enter image description here enter image description here

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    \$\begingroup\$ I was wondering if this might be a thermistor of some kind. It is in close proximity to those two black ICs but it is surrounded by the silastic glue which would act as a thermal insulator so I'm not certain. \$\endgroup\$ – user98663 Aug 31 '16 at 7:58
  • \$\begingroup\$ @Wossname thermistors don't need such big terminals, as you want to pass as little current trough them as possible (current would heat them up and offset the measurement). \$\endgroup\$ – Dmitry Grigoryev Aug 31 '16 at 8:06
  • \$\begingroup\$ Good point. Although thick pins would increase thermal mass. But I admit it's probably not a thermistor, as you say. \$\endgroup\$ – user98663 Aug 31 '16 at 8:10
  • \$\begingroup\$ I notice that the two black ICs under the mystery object have all their pins completely covered in the silastic, is that a deliberate thing for elimination of creepage perhaps? \$\endgroup\$ – user98663 Aug 31 '16 at 8:14
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This looks like a resettable PPTC fuse with an additional control terminal. The third terminal is used for external activation: when abnormal conditions like overcharge or overheat are detected by the battery controller, it can force the fuse to open even if the current stays within limits:

enter image description here

(source)

The component sits on the glue blob rather than the PCB because it needs to stay hot when latched up. PCB would act as a heatsink and could reset the fuse before the short circuit is removed.

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  • \$\begingroup\$ Thanks for your fast reply. How common are those? \$\endgroup\$ – bot47 Aug 31 '16 at 7:55
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    \$\begingroup\$ Pretty common. You'll find those in most laptop battery packs for example. \$\endgroup\$ – Dmitry Grigoryev Aug 31 '16 at 7:56
  • \$\begingroup\$ Do you have a manufacturer part number? \$\endgroup\$ – user98663 Aug 31 '16 at 8:01
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    \$\begingroup\$ @Wossname No, sorry. I'd upvote an answer providing one. \$\endgroup\$ – Dmitry Grigoryev Aug 31 '16 at 8:10

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