How to charge iPhone 4S battery?

I got an iphone 4s in december and so far the battery life is phenomenal! I haven't had any issues. I charge the phone every night when i go to bed, and let it charge over night and take it off just before i leave the house for school. The battery will usually last until i'm ready to go to bed that night, at about 5% but i normally just let the phone die before i put the phone on the charger. I do this 3/5 times if not more out of the week. Or i will let it drop to 5-20% before charging. My friend told me that if i let my iphone completely die before charging that it takes 20mins off the battery life each time. IS THIS TURE? It concerend me when she said this because i do it very often. If i'm charging my phone wrong what do i do? How should i charge it?

Well when your iPhone dies it doesn't truly DIE. You can squeeze more juice out but it's safe to do this…

Let it charge up

Charge it before it gets below 10% just to be safe. And about once every three months let it fully die and fully charge to let the battery do a full cycle