Currently plugged into an Anker 10000 mah. Happy with it, it's about the size of a galaxy s5. http://www.amazon.com/gp/product/B009USAJCC?ie=UTF8&at=&force-full-site=1&ref_=aw_bottom_links Used to use a Satechi and was happy with it, but it went kaput after 2 years and I got Amazon to replace it. Next one lasted 4 months so switched.
I have the rav power pack. Know there are several others on here that use it also. Holds a charge really good. If I don't use it, it will go months without recharging. Will charge phones or tablets, also will charge my cameras slowly in a pinch. Can get about 4-5 full charges of my phone out of it.
If it's just for day use, and not a trip, then just go to a store like Wally world and pick up one of the $15-$20 units. They work, they're slim, and they're cheap. If it breaks or you lose it, you're not heartbroken over it. If you want to spend a couple of $ The Mophie units are good. They'll run you about $50. You can pick them up at stores like Best Buy.
New Trent iTorch Works great for me. I can charge my phone in freezing temps from 0 to 100% 2-3 times easy. Used it in Colorado to keep me going for 3 days at our spike camp.
I use the Rav as well. Works great and as advertised with my Iphone. Sent from my KFTT using Tapatalk 2
For just one time out hunting I like my iTek for $5 on sale at Menards. : P Charges quick and keeps my iphone going all day. You need to recharge it each night for sure, though. But for a longer term hunting trip, I'd buy one of these fancier ones for sure. And may soon. iTorch looks good.