There is a plethora of options out there, but they roughly break into three main styles.
1. The slim line 'lipstick' or pocket back up battery (normally charge for a full iphone battery)
2. The iPhone battery/protector case (protective case with the battery built in behind)
3. The 'brick' or multi-port charger (multiple ports and multiple charges for an iPhone/iPad)
Over the years I've tried a few of these, of varying shapes and sizes but what I've found works for me is one or even two of the slim line backup batteries that have roughly 2,000 mAh, along with a brick that gives 14,000 mAh.
The main advantage of the slim line models is they work very well on the run just sitting in your pocket. Heading out to a restaurant for dinner, and don't want to lug around your bag? No problem, just plug in your USB charger cord and pop it in your pocket and your sorted.
Where the brick comes in is when you have multiple devices and possibly an iPad that requires a higher amperage - maybe you are going off grid and won't be seeing a powerpoint for a while. With at least 6 full iPhone charges up your sleeve you just don't have to worry about it.
The model I bought was the New Trent PowerPak Ultra, what I liked most about this one was the capacity (largest I could find at the time), along with being sturdy/waterproof, and having one of the ports compatible to charging an iPad. After about 6 months or more in use I haven't been disappointed - it seems to hold its charge and seems to have no problem giving at least 4 full iPhone charges (although I wouldn't say I've been meticulous in testing - it could be more).
Like having local or roaming SIM cards, the real saver is when you least expect (you don't plan to be stranded without power or mobile data), and a bit of prior planning at a small cost can end up saving you a lot of headaches down the track.
Got a tech tip or know of some specific models we should be mentioning here? let us know in the comments!
Written by Peter Cain
Peter Cain is the Founder and CEO of dataGO.co, a company committed to making it easier and cheaper to stay connected whilst travelling. You can find him on Twitter and Google+. You can also find dataGO on Facebook, Twitter, LinkedIn, and Google+ for all the latest info on local and roaming SIMs and data hotspots.