Why do MOBILE BATTERIES have THREE terminals? A simple Explanation.

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In this video I am conducting a simple experiment that demonstrates the purpose of the 3rd terminal in cell phone Li-Ion batteries.
0:00 The rule of two wires in electronics.
0:43 How is mobile phone powered?
1:55 The purpose of the 3rd terminal.
3:08 Change of resistance.
3:50 Powering the phone from an external supply.
4:43 Why are 3rd party batteries dangerous?