This battery pack is one of the most powerful rechargeable battery packs that I have ever tested. It is also matches the highest quality design and construction that I have seen in any battery pack. 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 brushed black anodized aluminum that looks fantastic. The battery is the size of a 7 inch tablet and it weighs in at 1 pound 1 and 1/2 ounces. It is assembled with machine screws and the overall construction is the best I have seen. The battery pack has three USB smart output ports 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. It can also output 12 VDC at 1.5 amps and 9 VDC at 2 amps to charge many other higher voltage devices. You press the ON button and hold it for 2 seconds to select the 9 or 12 volt output voltage for specialty charging so make sure that you set it to match the needs of your device. This is important.

This battery pack has enough power to charge an iPad 3 fully and in reality it could charge it 1.33 times according to my tests. I was able to charge my electronic devices for over 13.5 hours on a single charge of the battery pack which is amazing. During the testing the temperature of the battery pack never even exceeded ambient room temperature. I used the USB port to charge my devices during this testing. To start your device charging connect it with the proper cable and power adapter (You will probably have to use your own charging cable that came with your device.), set the voltage, if required, and press the battery pack ON power button and charging starts. The charging will stop automatically when your device is fully charged or the battery pack is depleted.

The battery has a beautiful backlit LCD display that measure the battery's capacity from 0 to 100% and there is a battery symbol shown that also has 5 bars for a fully charged battery. The display will also indicate which higher voltage is set. The battery display is not always linear to the amount of the battery's remaining capacity and I think that is due to the fact that the circuit is measuring voltage and not true mAh capacity remaining.

The Lithium Polymer battery can be recharged over 500 times during its life cycle and if you are a power user of a tablet, laptop or any electronic device that uses these voltages then this is the battery pack to own. It comes with an 18 month warranty. The unit comes with a powerful AC wall charger that quickly recharges the battery pack. The specs claim that it can recharge the battery pack in about 4 to 5 hours and in my testing that is what I saw when the battery was fully depleted. This product is one of the best I have tested and I rate it at 5 stars. If you are a business person or a student and have to depend on your tablet or laptop for long periods of time without the ability to get to an AC power source then get this battery.

Items in the box:
* Powerful AC wall charger adapter that will operate on 100 to 240 VAC at 50 to 60 Hertz.
* The battery itself
* A charging cable for your laptop and higher voltage devices that is 4.3 x 1.6mm to 5.5 x 2.5mm multi-voltage cable with pin connectors. This cable is 31 inches long.
* An USB to Micro USB charging cable that is 28 inches long
* A user instruction manual
* A velour carrying bag
* The 18 month warranty card

If you are looking for a good Lightning cable to use with this battery here is one released by Anker that works very well with my iPad Air and iPhone 5S.

[Apple MFi Certified] Anker® Lightning to USB Cable 3ft / 0.9m for iPhone 5s / 5c / 5, iPad Air / mini / mini2, iPad 4th generation, iPod 5th generation, and iPod nano 7th generation

Charging my devices:
I charged the battery pack fully before I started my test.
Anker 20,000 mAh Battery Pack efficiency test:

iPad 2 at 4% and the Anker battery pack was fully charged with 5 bars - 100%

* 30 min iPad 2 at 14% - battery 100% 5 bars
* 60 min iPad 2 at 23% - battery 97% 5 bars
* 90 min iPad 2 at 32% - battery 93% 5 bars
* 120 min iPad 2 at 41% - battery 89% 4 bars
* 150 min iPad 2 at 50% - battery 85% 4 bars
* 180 min iPad 2 at 59% - battery 81% 4 bars
* 210 min iPad 2 at 68% - battery 78% 4 bars
* 240 min iPad 2 at 76% - battery 74% 4 bars
* 270 min iPad 2 at 85% - battery 70% 4 bars
* 300 min iPad 2 at 90% - battery 68% 3 bars
* 330 min iPad 2 at 94% - battery 66% 3 bars
* 360 min iPad 2 at 100% - battery 65% 3 bars

* 96% of 6944 mAh = 6666.24 mAh

iPhone 5S at 21% battery pack at 65% with 3 bars remaining

* 30 min iPhone at 56% - battery 62% 3 bars
* 60 min iPhone at 91% - battery 58% 3 bars
* 90 min iPhone at 100% - battery 57% 3 bars

* 79% of 1560 mAh = 1232.4 mAh

iPad Air at 38% - battery pack at 57% with 3 bars remaining

* 30 minutes iPad Air at 51% and the battery at 50 % and 3 bars
* 60 minutes iPad Air at 64% and the battery is at 43% and 2 bars
* 90 minutes iPad Air at 77% and the battery is at 35% and 2 bars
* 120 minutes iPad Air at 87% and the battery is at 25% and 1 bar
* 150 minutes iPad Air at 96% and the battery is at 18% and 1 bar
* 180 minutes iPad Air at 100% and the battery is at 16% and 1 bar

* 62% x 8820 mAh = 5468.4 mAh

iPhone 5S at 34 % battery at 16% one bar

* 30 minutes iPhone at 65% battery at 12% one bar
* 60 minutes iPhone at 93 % battery at 8% one bar
* 85 minutes iPhone at 100% battery at 7% no bars

* 66% x 1560 mAh = 1029.6 mAh

iPhone 5S at 73 % battery at 7% no bar showing

* 30 min iPhone 5S at 97% battery at 4% no bar
* 60 min iPhone 5S at 100% battery at 4% no bar

* 27% x 1560 mAh = 421.2 mAh

iPad 2 at 61% battery at 4% with no bar showing

* 30 min iPad 2 at 69% battery dead

* 8% x 6944 mAh = 555.52 mAh

Anker battery test summary for efficiency:

* 6666.24 mAh + 1232.4 mAh + 5468.4 mAh + 1029.6 mAh + 421.2 mAh + 555.52 mAh = 15,373.36 mAh / 20,000 mAh = 76.9 % efficiency
* This is an excellent result and gives this battery a 5 star rating.

Limitations of device charging of some common electronic devices:

* 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

As you can see the Anker 20000 mAh will fully charge a totally dead iPad 3 or 4 because it contains 20000 mAh of capacity.

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 1083% of a full charge out of a fully charged Anker 20000 mAh battery pack
* iPhone 5S approximately 985% of a full charge
* iPad 2 approximately 221% of a full charge
* iPad 3 or 4 approximately 133% of a full charge
* iPad Mini approximately 342% of a full charge
* Kindle Fire approximately 349% of a full charge

* A normal estimated usable storage is 70% of capacity or .70 * 20000 mAh = 14000 mAh charging capacity available for your electronic devices.

* In my testing I was able to use approximately 15373.36 mAh of capacity or roughly 76.9% of the 20000 mAh capacity. This is an excellent result for a battery of this size.

I was supplied a sample for test and evaluation. A review was not promised to the supplier in exchange for the sample.

This is for the Anker 2nd Gen Astro Pro2 20000mAh battery pack.

There are a few different products in the reviews for this product so you have to read carefully.

This is designed very well, beautiful finish and easy to operate.

This can achieve 80%+ efficiency when you are charging a device that charges at 5v 1000ma. Better efficiency can be achieved when you are charging devices that draw 2000ma. However, if you are charging a device that only charges at 500ma, the battery pack does not last nearly as long due to several factors. It's effective charge potential can be as low as 2,500ma if you are running a 50ma LED light for example. (50 hour run time). The reason for this is common in the marketplace for these style batteries due to the 3.7v to 5v/12v circuits. It is rated at 3.7v @ 20,000 mAh or 74 watt hours. You can subtract 1 watt (270 ma) per hour the unit is on and it will be completely depleted if on for 74 hours with load. (Auto-shutoff prevents this). My LG phone draws about 30ma and peaks to 75ma every 15-20 seconds so the unit does not turn off after the phone is done charging. If my phone has 98% battery and put it on charge with this battery when I go to bed, it will use 7-10% of the battery in 8 hours. If I left the phone unplugged, it would still be on 98% in the morning. I bought this one for the flexibility of using 12v for charging the unit and misc items I have like a 12v rechargeable lantern. The Charging circuit uses about 270ma + (charge mA * (Output Voltage / 3.7). I.E. If you are charging a 500ma device, it draws 945ma (675ma + 270ma) off the battery. When charging 500ma devices, You will get about 20 hours of charging before battery is dead. This example is far below the 80% efficiency it claims. Including the power conversion factor, it is 69% efficient using the example above.

As the market and technology goes, this unit is par with others on the market. All of the multi-volt units draw about a watt due to the circuitry required. Cost wise, this unit is worth the money. I gave it 3 stars because of the deceptive marketing Anker and many companies use. I would much rather have all the information above disclosed and not have to discover it myself. Just like UPS companies, Buyer beware because the numbers are designed to look much better than they are. If the output power is 5v and 12v, capacity should be disclosed for each voltage. This is rated by the 3.7 battery. While you might get close to the same wattage - 1w/hour charging, you won't get anywhere near 20,000mAh. Again, the entire industry does it this way. Some are starting to disclose this information. I wish they all did.

mAh output for each voltage:

5v = 14,800 mAh - (200ma * hours used)
12v = 6,166 mAh - (83ma * hours used)

Power used for conversion from 3.7v to 5v is minimal. The unit itself does get warm when charging so if you charge via a 12v solar panel, it is best to keep it out of the sun.

First lets start off with, this thing is massive. Like almost most 4 packs of smokes "2 side by side, 2 below them". Its also heavy for a mobile product. On the good side its built like a tank. It also does indeed charge anything you want period. Would i buy it again knowing what I know now,, no cause its just to heavy to lug around. So i guess i will keep it, never know when i will need to jump start a car.