Can I charge my iPhone 5s anytime or do I need to let the battery drain?
lithium ion batteries do not have a charge memory. Deep discharge cycles are not required. It's better for the battery to use partial discharge cycles.
Yes you can but once a week let it drain to less then 20% and then give it full charge without interruption. It's like maintenance for the battery.
yes miss_JT77 is right it is much better to let it drain before you charge it again ... or until it starts warning you
It is not advised to let your battery fall to 0% before charging it again. That has long term side effects on the battery life.
let the battery drain because if you charge it say at 50% then it could break your phone that is what happened to my moms iPhone 5