It's quite ridiculous how much you depend on your smartphone in your daily life. In Denmark most of us have acces to our train pass through applications on either Android or IOS, which we are obligated to show to conductors whenever they ask. If you do not have a train pass you will be sanctioned with a train fine of about 749 Danish krones (DKK).
Last week I was on the move all day, and therefore used my iPhone for both map directions, music and important calls to clients. As you may guess this used quite a lot of my battery although it's a brand new smartphone with a reportedly great battery. At about 2pm my phone had about 6% power left, which is not quite enough for a 1 hour train ride and 3 hours until I finished work. With no power I could have been in a very vulnerable position if any train conductors where to ask for my train pass, since I would not be able to show it.
In Denmark they are quite strict about it, and you will get a fine no matter what if you can't show them a pass in the moment they ask for it.
Luckily I had my powerbank with me, which is an Power Add Apollo bought from Batteribyen.dk. The powerbank is so powerful that it can provide you with 2-3 charges, which allows you to recharge through the day or share with friends or colleagues.
It's quite a good investment, since a powerbank will only cost you around 200-600 krones, which is equivalent to 30-100 dollars. In the end it might save you from a train fine or a missed call, that potentially could be life determining.
If you do not own a powerbank yet, I can only highly recommend buying one from Batteribyen if you are from Denmark. They have quite a good selection of batteries, cables and chargers for every consumer electronics or vehicles of any kind.