My iPhone went from 0% to 100% battery in ONE second?

My iPhone's battery was completely DEAD (I was playing on it in bed and I literally saw its battery getting lower and lower until 2%, etc), so I charged it… It turned on straight away and it went straight to 100% charged in ONE second. So I took it out of the charger and now can use it. How is that even possible?

Needs a new battery.

Like the superman of the cell phone world.

Yeah new battery or update the software