I would just recommend buying yourself a portable charger. There's a lot cheap ones out there that should make your smartphone run for twice as long. I paid 8 euro for mine and it can store up to 3500mAh. So for a smartphone with 4000mAh, that would mean you could fully deplete your smartphone, then almost fully charge it, and use it as much again. That way, you'd have 7500mAh, and even more if you choose to buy a better portable charger. There are some that store more than 20.000 mAh.