If you don't already have a portable/external battery charger, you should consider getting one. Read on for more info and a current/decent deal ...
Note: It has been quite a while since I wrote this post. I won't call it "expired" since some of the info and links may still be good. But just be forewarned that the "deals" listed here might not still be the best deals out there ...
No big deal if you miss this deal (LINK HERE), as the prices on these gadgets has gotten so low that you can buy them for a good price (especially online - possibly LOTS more expensive at a retail store!). [Note: don't forget to use a shopping portal, as discussed HERE, if you buy this deal via Rakuten.com; but the other links below are from Amazon - which doesn't usually offer any shopping portal cash-back.]
The main thing you want to look for when you are comparing external batteries are 3 items:
a) the "mAh" rating - which basically tells you how big the battery is (and, thus, how many phone or tablet charges you can get each time you recharge your external battery)
b) whether or not the battery has a wall plug-in (most don't, so you need to plug it into a USB port on a computer - or, better yet, buy a USB-to-wall-outlet converter, if you don't already have one)
c) whether or not you're buying the unit on Amazon, go to Amazon.com and read the user reviews to make sure it doesn't have bad reviews (some reviewers point out that cheap models often times exaggerate their true mAh ratings)
The BIG units (like the 10,000 mAh unit linked above) can give you quite a few phone charges (apprx 5 full charges per 10,000 mAh) and a decent tablet charge (a 10,000 unit can probably nearly charge a dead iPad).
You might also want to consider a bigger (larger than 10,000) or smaller (2000-4000'ish) unit, depending on your needs. The bigger the rating, the more expensive & physically bigger & HEAVIER the unit is going to be.
Here's a link of a similar model (10,000 mAh) to consider, especially if you miss out on the above deal ... LINK
Here's a couple smaller models (well under 10,000 mAh) to consider ... LINK1 & LINK2
Here's an example of a BIGGER one (20,800!!!) ... LINK
And, finally, here's one (6000 mAh) that I have that's really convenient, because it has the wall charger built-into it (but it is heavier and less mAh-per-dollar!) ... LINK