Lithium-ion battery

https://i.stack.imgur.com/RuNHX.png

ref:https://electronics.stackexchange.com/questions/10588/why-are-there-3-pins-on-some-batteries

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.