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 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.
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Look up the (shares datasheet with bq27000). EDIT: This was written as an answer to a, which got merged with this one.