I've use battery backup/power banks when traveling or when I don't have access to a wall charger. The downside is that charging a battery with a battery can get pretty hot. If you are in a hot environment it can compound that. When I was traveling, I charged my phone with a 4xAA NiMH charger (Goal Zero) when out and about in 90 degree, humid weather and I was worried about damage to the phone and battery.
More recently I picked up an inexpensive Li Power Bank from Costco (it came with whatever I was looking for at the time). It was a no name, but it works fine. I generally get Anker when it comes to wall adapters and cables, but I don't know how well their power banks hold up. There are some chargers that double to charge other items with the battery. I think Nitecore has a small adapter that goes on it's larger batteries that both takes a charge and gives a charge if you are looking for something smaller.