You should only charge your phone when it's completely dead......Not true. You can charge your phone as often as needed.
Here’s an essential piece of information: Lithium-ion batteries have limited charging cycles, and for an iPhone, it’s typically around 500. A cycle is a full charge from 0 percent to 100 percent. So, if you only charge your phone when it’s completely dead, you’ll exhaust the charging cycle pretty quickly. But if you charge the phone from 90 percent to 100 percent, you would have only used 1/10 of the cycle.
This is why experts recommend keeping the charge between 40 percent to 80 percent so you can charge your phone multiple times a day, getting the most out of one cycle. This practice extends your battery life and increases performance.