What is a good power bank ?

What I do with my iPhone is turn off push email. I check my email manually. That has at least doubled (if not more) my battery life.  I set another phone in the house to every 30 minutes and that saves that battery as well.

I turned off the EE notifications here in the EE app as well.

The more "every two minute" notifications you turn off, the longer the battery will go. Mine goes all day now without draining right away.