External battery power is an area where manufacturers are making continuous improvements. Usually those improvements are evolutionary -- basically every year a battery of a particular size will hold say 10% more power. This really adds up over time, so every few years it makes sense to upgrade, especially as these batteries are not terribly costly (around $20 for many of them). But recently there was a revolutionary change, at least for iPhone users. For years now it has been the case that Apple sells its phone with a charging unit ("wall wart") that delivers 1.2 Amperes of current. But, Apple sells iPads with a 2.4A charger, twice the energy flow. Users often ask if it is safe to use the double-speed charger with the iPhone. I am not a tech expert and please do not sue me if your iPhone breaks, but as far as I can tell all the experts seem to agree that there is no problem using the 2.4A charge speed with iPhone. If you like, go buy a 2.4A charger and use it for Phone and Pad alike. I did! But in truth I almost never use it anymore, because I always charge externally.*
And the big breakthrough is, many companies now sell external batteries that charge iPhone at double speed. My current favorites are these:
Anker PowerCore+ mini 3350mAh Lipstick-Sized Portable Charger (3rd Generation, Premium Aluminum Power Bank)
This is the smallest double-speed charger I know of. Holds about one charge for a normal iPhone and maybe 2/3 or something of a Plus charge. Super-sleek and nice design. Extremely easy-to-see light to show charge level, though it uses red-yellow-green so may not be ideal for those with serious color blindness.
Anker PowerCore 10000
This one has tons of charge and is still pocketable, plus charges at double speed. Uses a set of little blue lights to show charge, reasonably easy to see even for me!
If you buy one of the two above, you'll need wires to attach them to your phone or other devices. I've been extremely pleased with these inexpensive little Lightning wires:
Mini 6 inch Lightning Short Iphone Charger Cable,Puridea [5 Pack] Iphone Black Fast Charging Cord,USB Sync To Lightning Wire (Max 2.4A)
If you don't want to carry an extra wire, there's a nice solution by Jackery: The item below is about the size of the 10,000 mAh Anker and holds only around 2/3 as much power. But it has _built in_ Lightning and micro-USB connectors that fold away, which is super-convenient. And there's _also_ a slot for another connector wire, so, first, you can charge up to three devices at once and, second, if one of the built-in connector wires breaks you don't have to trash the charger, it can still charge anything using an external wire -- nice!
Jackery Bolt 6000 mAh Portable Charger
Finally, for true all-in-one convenience, I still haven't found anything to match my old Skiva:
Skiva MFi Lightweight Pocket-Friendly Super-Compact 2600mAh Portable power bank charger with built-in Lightning & Micro-USB connectors
The Skiva _doesn't_ charge at double speed and doesn't hold much for it's size as it's an old design. But it has a built-in full-USB connector, which means you can plug it into a computer and charge through it's built-in Lightning-out and micro-USB-out cords. So anytime you are near a computer (or wall wart) you can charge anything with no need for any other wires. If anyone knows about an up-to-date battery with this awesome feature, let me know and I will buy one!
*Why do I always charge using external batteries, rather than at the wall? Several reasons:
1) I love my iPhone and cannot bear to be parted from it!
2) My iPhone counts my steps and I can't live with taking steps and not getting credit for them. Yes, I am aware this is even more insane than 1)!
3) This way I'm always charging at double speed -- although I have a 2.4A wall charger, I also have several 1.2A wall chargers (one comes with every new phone) and I can never tell which is which. With the external battery I know I'm getting double speed.
4) This way I never go out and forget my phone. Mock me if you wish for my attachment, I deserve it, but let me just say that as scary as wandering around the world blind is, wandering around blind and phoneless is many times scarier!