Bought two of these. Anker is pretty good quality. What we found with this model is that is DOES hold a large electricity supply, but it has very poor shelf life. That is, if I fully charge it, then keep it on my desk for a week or two without using it, it DISCHARGES completely. While in use, yes, it can power a phone or two quite well. But it is not reliable as a backup if it has been sitting around more that a few days.
EDIT:
Dec. 9, 2016: The seller noticed my poor review and felt that they wanted to help me out. They sent me another battery FREE, postage prepaid. It works perfectly-- good shelf life, not like the first two. When I told the seller, then they sent me ANOTHER one... free. So now I have two GOOD ones which do hold the shelf life, from a company who really cares about making the product right!

  This battery pack is ideal for your purse, pocket or backpack and it is one of the most portable batteries that I have tested. It had good test results. This is an exceptional charger with a feature that everyone needs. The feature is PowerIQ charging technology for the charging port on this battery charger. That means that you can plug in any type of device and this smart port will set the charging current to match the maximum charging rate that your device can safely handle. The port is rated at 5 VDC and up to 2 Amps but in my testing I was able to charge and Apple iPad Air at 2.1 amps.

Many people don't realize that device manufacturers play games by wiring their devices and cables differently so that a charger that is designed to charge an Android device will not quickly charge an Apple device. Manufacturers like Apple, Samsung, Android devices and Chinese smart devices have different circuitry in them and if the charger doesn't match their circuitry they can charge at a much slower rate. The IQ port doesn't care what type of device you plug into it because it adapts its charging rate to match what the device wants by sensing what type of device is plugged into it. I was so impressed that I put together a video of the unit and the test results for your review.

The construction of this battery pack is simply beautiful. The exterior shell is glossy black. The battery weighs in at 4.23 ounces. The battery pack has one smart IQ output port that output 5 VDC at variable current to meet the charging requirements of the device you have plugged into it. It is called a Smart Port or an IQ port. It uses a chip to help select the proper current to allow the maximum safe charging current for the fastest device charging. The port can output as much as 2 amps but the battery pack in my testing output up to 2.1 Amps while charging an Apple iPad Air.

This battery pack has enough power to charge an iPhone 5S 217%. To start your devices charging connect them with the proper cable. (You will probably have to use your own charging cable that came with your device) and charging starts after you press the on button on the side of the battery. The charging will stop automatically when your device is fully charged or the battery pack is depleted.

The battery pack has 4 blue LEDs on it and when it is charging they work like this:

* One LED blinking = less than 25%
* One LED on and one blinking = 25 to 50%
* Two LEDs on and one blinking = 50 to 75%
* Three LEDs on and one blinking = 75 to 99%
* Four LEDs on and the battery pack is fully charged

The Lithium battery can be recharged over 500 times during its life cycle. It comes with an 18 month warranty. It is recommended to charge the battery at 1 Amp and 5 VDC. It will take 5 hours to charge the battery at that charging rate. You can use your USB 2.0 port of your computer it takes 10 hours at ½ Amp input. You can even use an AC powered USB wall charger or a USB car charger and using up to 2 amps will not hurt the battery.

The battery pack does have a power on switch to turn on the charging cycle for your device or to allow you to check the battery's remaining power charge. .

Items in the box:

* The battery itself
* An USB to Micro USB charging cable that is 27 inches long

Anker 5200 mAh second generation battery pack test data:

iPhone 5S unit one 74% - battery fully charged with 4 LEDs

* 30 minutes 96% - battery 4 LEDs
* 50 minutes 100% - battery 4 LEDs

* 26% x 1560 mAh = 405.6 mAh

iPhone 5S unit two 32% - battery 4 LEDs remaining

* 30 minutes 63% - battery 3 LEDs
* 60 minutes 91% - battery 3 LEDs
* 90 minutes 100% - battery 3 LEDs

* 68% x 1560 mAh = 1060.5mAh

Samsung Galaxy Tab 4 at 54% - battery 3 LEDs remaining

* 30 minutes 66% battery 2 LEDs
* 60 minutes 77% battery 2 LEDs
* 90 minutes 89% battery 1 LED
* 120 minutes 97% battery dead

* 43% x 4450 mAh = 1913.5 mAh

Summary of all items charged:

* 405.6 mAh + 1060.8 mAh + 1913.5 mAh = 3379.9 mAh / 5200 mAh = 65% efficiency

* This is a good result and gives this battery a 5 star rating especially after charging a larger tablet in the testing.

Device internal battery requirements:

* iPhone 4 battery 1,420 mAh battery capacity
* iPhone 4S battery 1,432 mAh battery capacity
* iPhone 5 battery 1440 mAh battery capacity
* iPhone 5S battery 1560 mAh battery capacity
* iPad 2 battery 6,944 mAh battery capacity
* iPad 3 battery 11,560 mAh battery capacity
* Kindle Fire battery 4400 mAh capacity
* Kindle Keyboard battery 1750 mAh capacity
* iPad 4 battery is 11,486 mAh 3.7 VDC at 42.5 Wh
* iPad Mini battery is 4490 mAh at 3.72 VDC 16.7 Wh
* iPad Air battery is 8820 mAh capacity
* iPad Air 2 battery is 7340 mAh capacity
* Samsung Galaxy Tab 4 - 8 inch Tablet 4,450 mAh

As you can see the Anker 5200 mAh will not fully charge a totally dead iPad 3 or 4 because it contains only 5200 mAh of capacity. That battery is ideal for recharging your smart phone or smaller smart tablet.

You have to consider the losses in the charging circuits of both the battery pack and the device you are charging so this result is outstanding! When using an external battery you have to allow approximately 30 % of the battery capacity will be lost to heat, voltage conversion, powering the batteries electronics and the electronics of your electronics device's charging circuits. In addition you have to consider that your device is not truly off like a cell phone is still connected to the WIFI and phone networks and probably receiving emails, etc. This is one of the most powerful rechargeable battery packs I have tested.

Real world examples of charging capability with this battery pack:

* iPhone 4 approximately 238% of a full charge out of a fully charged Anker 20000 mAh battery pack
* iPhone 5S approximately 217% of a full charge
* iPad 2 approximately 49% of a full charge
* iPad 3 or 4 approximately 29% of a full charge
* iPad Mini approximately 75.3% of a full charge
* Kindle Fire approximately 76.8% of a full charge
* iPad Air approximately 38% of a full charge
* iPad Air 2 approximately 46% pf a full charge
* Samsung Tab 4 eight inch tablet approximately 76% of a full charge

* A normal estimated usable storage is 70% of capacity or .70 * 5200 mAh = 3640 mAh charging capacity available for your electronic devices.

* In my testing I was able to use approximately 3379.9 mAh of capacity or roughly 65% of the 5200 mAh capacity. This is a good result for a battery of this size. This was based upon the first full charge of the battery pack and the results should get better after the battery is charged several more times.

I was supplied a sample for test and evaluation. A review was not promised to the supplier in exchange for the sample but I stated that if I provided a review that it would be fair and honest.

No wonder this is the top rated portable power bank. I've had this thing for a few months now and wow am I impressed. The charge that this power bank holds is incredible given its size! I use an iPhone SE and I've gotten 2 full charges out of this with extra battery to spare! It's obvious that the folks at Anker really care about giving you a quality product at an unbeatable price. The prices of these things elsewhere are insane. Before purchasing this I looked at Walmart and the ones they had at this price looked horrible. The charge cord was very sturdy and the carrying bag fits the charger and cord perfectly. I also love that it has a battery level indicator. I carry this with my every single day. Would recommend for literally everyone. There's not a smartphone user on this earth that wouldn't benefit from having one of these.

It's a battery and does what it's supposed to do: let you safely carry around electricity until you need it. This battery is compact and appears to have the stated capacity based on the number of times it could charge my son's phone before needing to be recharged itself. So, battery-wise, you'll have no regrets if you buy this battery.

I bought this battery on November 11, 2015 (so Amazon tells me at the top of the page). It stopped charging other devices after about 13 months...so a week ago or so. The battery has an 18 month warranty so I wrote to Anker on the night of December 7 and explained the situation. I had a reply in less than 12 hours and in under 24 hours since my initial e-mail I had a tracking number for a replacement battery. The battery was delivered yesterday (December 10) which to me is stunningly great service...I had the replacement battery less than 72 hours after first writing to Anker.

I've been an Anker fan for years...I buy all our chargers, cables, and batteries from them because their products are excellent. This is the first time I've needed to contact support and I'm happy to see their support is as good as their products!

I was out of state and spent a lot of time out of the hotel charging areas. I had an iPhone 6, not suppose to be affected by the new IOS upgrade, but that proved to be wrong. It would go from 70% to 20% in seconds, then shut off. With this condition, if you plug it into a power source for as little as 10 seconds, it will return to the 70% level. The charger worked flawlessly, brought my phone back to life on several occasions, and since it is small, i could leave it in my pocket connected to the phone for a complete charge.
On one occasion, before I had this unit, my wife and I were waiting for an Uber Driver around 10p and the phone went dead, so we lost the ride. We had to rely on a regular taxi, at over twice the price. If I had this charger on that occasion, the savings would have paid for the unit.
I'm on an iPhone 7 now, and have had wonderful results with the battery life, however, if I know I'm going to out and about in an extended period of time, you'd better believe I'll have it with me. You Never Know!