Why Phone stays off while charging

[Android Question]

I put my Phone on charge while it was ON, but it turned off during charging. I could not turn it back on for calling while it was charging, but I failed. It could only be turned on when charging completed. WHY?

 

[Rukbat]

If the battery drops enough (and that's different with different phones), the charger can't supply enough poer to run the phone, so the phone stays off until the battery is charged.