Lithium-ion battery

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.