Intocircuit 2nd Gen Power Castle 15000mAh Portable Dual-Port 5V 3.1A (2.1A+1A) External Battery Charger
From the product page on Amazon.com:
List Price: $129.99
Sale: $39.99 & FREE Shipping
Update 10 July 2016: This unit now has a bad failure mode. If I use it to charge my smartphone overnight (my phone is an LG G2), at some point it stops charging and seems to switch to charging the charger from the phone. I've left it charging like this twice, observing it successfully charging my phone, then checked it the next morning and my phone is down to 3%. Fortunately, my other chargers do not exhibit this problem. In light of this I am changing the rating on this device.
Update 2 May 2015: I noticed that I've had this charger sitting fully charged without use for over a month and it is still reporting 100% charge. Nice!
This charger has a rated capacity of 15,000 mAh. Before using this charger for the first time, I charged it to full capacity using one of its sister products, the "iClever 50W / 10-Amp 6-Port Fast Charging USB Wall/Travel/Desktop Charging Station AC Adapter with SmartID(tm) Technology" which I reviewed here earlier. To give you an idea of charging time and efficiency, this Intocircuit 2nd Gen Power Castle 15000mAh charger charged from 80% to 100% (3000 mAh) in 129 minutes or 23.3 mAh per minute.
Then I charged my 3950 mAh Nexus 7 (2013) tablet on the 2.1A port. The Nexus charged 33% (1304 mAh) in 86 minutes or 15.2 mAh/minute. This used 1950 mAh of the charger's capacity. So there was a 33% loss in the combined circuit during charging which is about what is expected.
My 3000 mAh smartphone charged from 49% to 100% in 60 minutes or 25.5 mAh per minute. The charger ended up at 83% capacity for about 40% circuit loss (so my phone must have a lower charging efficiency than my Nexus tablet). Extrapolating, this means I would be able to charge my phone by 50% about six times with this charger. Compare that to getting just a little more than one 50% charge from a 3000mAh lipstick size battery.
At the weight and size of this charger it is possible to fit it in a man's pants pocket, but I doubt most people would want to. It is easily stored in a purse, backpack, or fanny pack. When I travel and am using my phone all day with no convenient time or place to wait for it to charge, I like to have a smaller charger to use in my pocket (so the phone and charger fit ok).
A single small battery charge is insufficient when I am very actively using my smartphone all day, especially if I need to keep it out of airplane mode. This was the case when we went to Disney World--out all day, taking a lot of photos and videos, keeping up with emails and the occasional call, and no convenient time or place to charge my phone. I wish I had had this charger with me. The 15,000 mAh battery in my backpack would ensure that I have all the power I need for my phone for at least a couple of days.
There is a built-in flashlight. With the capacity of this charger, the flashlight should be usable for a very long time, but since it is too bulky and heavy to carry around in one's pocket, the flashlight will be less convenient that the one built into the Intocircuit 2nd Gen Power Mini 3000mAh lipstick-sized charger.
This charger comes with a handy backlit LCD display to show the remaining charge percentage. On a humorous note, be aware that you need to look at the display with the device held with the display at the bottom to read it correctly. I held it at what seemed to be right side up with the display at the top of the device, pushed the button next to the display, and saw that it appeared to be at 89% charge. So I started charging it and saw a little later that instead of charging, it seemed to have dropped to 66%. Checking the cables to make sure they were in the correct charging configuration, I realized that the correct way to view the display is to hold it oriented so the display is at the bottom of the device. As it turned out, it had actually charged from 68% to 99%. Brief confusion from this coincidence.
There are two charging ports, a 2.1A port and a 1A port, allowing you to charge two devices at once. The LCD indicator informs you which port(s) is charging a device and the voltage applied.
Construction is solid and has a quality feel. On a minor note, one end cap of my unit was not completely flush with the metal case.
I have been very pleased with this charger.