Why do some mobile phone batteries have 4 terminals? I want to use a mobile phone battery for my project.
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The pinouts of phone batteries are different between all types. You can almost see these pinouts in many cases: All About Battery connector pinout of mobile phone and also maybe this link will help you: Why are there 3 pins on some batteries?
I think two of the pins are (-) for the battery + protection circuit, one is for the (+) of the battery + protection circuit, and the fourth is for a 10K thermistor to the (-) side. See this compatible battery image. T = Thermistor.
It would be easy enough to probe out with a multimeter (check that volts are zero before trying to measure ohms).