This is the reason why I hate the fact that the 5to25 tickets are only available on the app, and why I'm still paying the £95 a month instead of £68.
As someone who can go through a full charge in half a day, it's not worth the the risk
As long as you don't mind the extra weight you can invest in a external power bank. A 20000mAh one is £20-30 on Amazon and gives the equivalent of around 5x or more full phone charges. I would never travel without one.