Are there any mobile battery packs that exist that can be used to power phones and other mobile devices?
Is there anything specific I should look for or know before buying and using one
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Yes, there are many different manufacturers of lithium-ion battery packs. You'll need to be aware of the power drain of your device. Most modern smartphones that charge via USB (which is pretty much all of them now) pull about 1 amp of power from the power source when charging. (This is why they usually charge more slowly when connected to a computer -- computer USB ports usually only supply 500mA.)
iPads pull 2.1 amps when charging, and they can charge and be operated at the same time. I suspect most other tablets are somewhere in that vicinity. So, if you'll need to charge AND use an iPad or other tablet, you'll need to make sure your battery pack supplies enough amperage.
Obviously something else to be aware of is the actual capacity (mAH, or milliamp-hours) of the battery, as this will tell you how long it will charge/power the device. Usually the packaging will tell you something like "Power your iPhone for up to 4 hours!" or something similar.
Here are a couple examples (not necessarily recommending them):
Certainly. There are lots of portable USB chargers out there. Product recommendation questions are off topic for most Stack Exchange sites and you haven't specified any particular requirements, so I'm giving general guidance on how to choose a portable USB charger rather than giving a specific recommendation.
I personally use a Sony CP-ELS (2000 mAh), but there are loads of choices, from compact low-power units (like mine) to expensive, high-performance power supplies with capacities upwards of 10,000 mAh and capable of fully charging a smartphone several times.
Whatever your choice, make sure it has enough capacity to meet your requirements and can supply enough current for the devices you want to connect to it. Some of them can be unreliable, so you'll want to pay attention to the customer reviews before you buy.