batteries - Why does Li-Po battery voltage drop under load?

I have very low background in electronics.

I have a Li-Po battery connected to an electric motor. The question is:

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  Why does battery voltage drop under load?

  
  


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  Why is each cell of a Li-Po battery limited to 3.0 V (discharging)?

  
  


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  Why is each cell of a Li-Po battery limited to 4.2 V (charging)?