It is really annoying when you buy a new phone and the battery won't last anymore as it was brand new in a long run. I have the same problem, i bought my iPhone 6s six months ago and the battery is not that good anymore and I usually charger twice a day because I am a heavy user. I was thinking to buy a power bank but,I was hesitant because I am intending to sell my phone and buy new one if I can afford too.
There are lots of choices in terms of extending the battery usage of your phone. There is the one that battery case, where you can use it as normal case on but with battery in it to extend the usage of your phone battery. There is also type of power bank where you can connect your phone through usb connector, it has different mAh so its up to you how much you really wanted. These two type is all up your need which one is you really want to use and becomeshandy.