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Going to Disney World? On a cruise? Other vacation or experience where you'll need your cell phone to work all day without access to a charging outlet? There are times when having an external battery or two is vital. I bought a lipstick style external battery at the Atlanta airport Brookstone store about a year ago. Bad experience. That external battery was poorly constructed, would not make a reliable connection, and charged exceedingly slowly. I put my phone into airplane mode to minimize its power consumption and still had to limp carefully to be able to get photos throughout the day while infrequently checking for messages or missed calls. For much of the time I had my phone and battery in my backpack carefully settled so it would hopefully keep charging without losing the connection. So I didn't always have it available immediately for photo opportunities.
For our next trip I didn't want to take a chance on another lipstick style battery, so I got two Anker Astro Slim2 4500mAh batteries. They seemed perfect. A built in short micro-USB cable plus another USB port to charge another device at the same time. Flat, fits in my pocket back to back with the phone. But there was a rather significant problem. It is not compatible with my LG G2. I still don't know what makes my phone special for that charger, but I ended up having to use the 2nd port with a separate USB cable which sometimes did work on my phone.
Trying another lipstick-style battery--this Intocircuit Power Mini 3000mAh External Battery. It comes with an 18" USB cable, but I used a shorter 3" cable so it wouldn't tangle in my pocket. This battery can be plugged into my LG G2 smartphone and still fits comfortably in my pocket. In my testing it charged my phone at rate of about 15 mAh per minute. That is the same charge rate of the larger Anker. But this Intocircuit external battery actually works; it is compatible with my LG G2. Maybe that means it is more broadly compatible with other devices as it should be. The Intocircuit battery has a rating of 3000mAh vs. 4500mAh for the Anker. But it is so small (and works on my phone) that I would prefer to have two or three of these charged and ready to go in my backpack.
The Intocircuit battery also has a built in flashlight. Since it is often needed at night, this is quite handy for double use. Hisgadget.com has a bit of humor in its specs as it says the flashlight is "good for night and darkness use." Good to know. Unlike the Brookstone model, this battery is solidly constructed. It has a 3-light status indicator of its charge level. I've asked Hisgadget to add specs to their external battery pages that give dimensions, weight, and output power ratings. As to the output power rating, my tests indicates that it charges a device at a rate that is consistent with the a/c charger and the Anker. As I noted though, some models are not good quality. This battery feels well-constructed.
In my tests both the Intocircuit and Anker batteries actually delivered about 63% of each device's rating which is in the normal range. 30-40% is lost during power transfer as combined circuit losses from resistance and voltage conversions.
February 14, 2015: This device charged my phone at a rate of 9 mAh/minute (not counting charging circuit losses).
Put your phone in airplane mode when you really don't need connectivity, e.g., to take photos, and its charge will last much longer.
This phone does have a button to turn on charging (and to turn on the flashlight).
One thing to keep in mind is that this Intocircuit external battery (as with the Anker and probably others) will not charge another device at the same time it is being charged.
One reviewer complained about the case flexing when pressing the button. I do not have this problem with my Intocircuit. In fact, mine is solidly built.
Considering my experience with three different brands of external batteries, and knowing now what I can expect in my real-world conditions, I am very pleased with the Intocircuit Power Mini 3000mAh external battery. And having the built-in flashlight, too, is a nice bonus