The trend with external batteries is to make them smaller and more portable while packing in as much capacity as possible. The new second generation Anker Pro2 doesn't so much eschew that trend as it takes the capacity thing way to the extreme. This external battery has a whopping 20,000mAh capacity, which obviously impacts the size. Still, it's somewhat slim for having so much juice. Should you make the room?


  • Capacity: 20,000mAh
  • Output: USB: 4.5A (max), DC: 9V / 2A or 12V / 1.5A
  • Input: 12V / 2A
  • Weight: 510g / 18oz
  • Size: 113 x 168 x 16mm / 4.4 x 6.6 x 0.6in
  • Price: $79.99 (Amazon)

The Good

  • Huge capacity
  • Roughly 80% efficiency
  • Solid aluminum build
  • Three USB ports
  • DC output
  • LCD battery gauge

The Bad

  • LCD is a little underutilized
  • A few included DC adapters would be nice
  • Very low drain devices might stop charging
  • Big and heavy


The Anker Astro Pro2 is not as svelte as many of the external batteries out there – you're not likely to haul it around on a daily basis. However, it's still pretty portable. If the measurements above don't quite get the point across, the Anker Astro Pro2 has a surface area slightly larger than a short mass market paperback book. The battery is hefty, weighing a little more than a pound, which means it's quite dense for the size.


On the front is a small monochrome LCD screen that shows you the current charge level when the battery is switched on. On the left side is the single button to operate the device, which is clicky and easy to find by touch. On the top edge are three smart USB ports that will automatically output the maximum current for the device plugged into them. There is also a DC in for charging the battery (via the included adapter) and a DC out for charging non-phone devices.


There are a dozen small torx screws flush with the surface of the brushed aluminum housing holding the whole thing together. It feels like a really premium product.

How It Works

To start charging a USB device, just plug it in and turn on the battery with the button on the side. The phone or tablet should charge up normally and then trickle charge to stay full. When you disconnect, there is no way to turn off the charger immediately – it just goes to sleep after a few minutes. This setup usually works just fine, but I've found that some very low drain devices (I tested with an old MP3 player) won't trickle charge like you might expect. There isn't quite enough draw for the battery to know it's still connected and it eventually goes to sleep. This isn't an uncommon issue with external batteries, so not really a deal breaker.


The DC out is a cool feature, and one you don't see very often. The Anker Astro Pro2 can output 9v or 12v, which you can toggle by long-pressing the power button. This allows you to juice up many tablets and other small electronics (like routers or cameras) that run in that voltage range. You'll need to pick up a set of adapters to make the most use of it (it only comes with one DC output cable), but it's a nice extra nonetheless. Charging the battery itself via the included DC adapter is fast (about 4-5 hours for a full charge), but the tradeoff is that you can't just attach it to a microUSB to juice up like you can smaller batteries.

So, what about charging speed? The Anker Astro Pro2 claims it can output 4.5A of total current to connected devices, which should be able to match a wall charger easily. I found that the Anker Astro Pro2 does get pretty close to that, even if you have three devices plugged in and charging. With just a single phone or tablet plugged in, there's no question it's at least as good as a wall charger. The average current drops off a tiny bit as you add devices, but I'm still impressed with the speed.


A 20,000mAh cell ought to be enough to charge a Nexus 5 about 8 times, but that's just a number. The efficiency of most external batteries is between 70 and 80% due to voltage conversion and circuit resistance. Some cheaper batteries are well below that mark, though. Astro says these second gen batteries are closer to the 80% threshold, and my testing agrees with that. It looks like the Anker Astro Pro2 gets very close to the 80% efficiency level, which places it at the top of the heap. The LCD screen is supposed to be more accurate (within 3%) with this new generation, and I suppose I believe that.


You're looking at $79.99 for this battery on Amazon, which, admittedly, is a lot for portable power. This isn't the product for someone who just needs a little extra juice on a short flight or to give their phone a little boost at the end of the day. This is a battery for a serious traveler, or for someone with three or four devices, some of which need DC input. The Anker Astro Pro2 also makes sense of you're sharing a battery with a few others. Maybe you can go halfsies on it.

wm_A1 wm_A2

Left: backlight on, Right: backlight off

There is very little negative I can say about this battery. For almost all intents and purposes, it's a great unit. It charges devices very fast, has industry-leading efficiency, and seems very well-built. One of my few qualms is that the LCD seems a little underutilized – it could do more than just showing the charge level. I can see a few more icons in the LCD, but I can't for the life of me figure out how to make them do anything, and the manual was no help.

If you need a very large battery, this one should definitely be at the top of your list. There are some slightly cheaper alternatives, but you get what you pay for.

Ryan Whitwam
Ryan is a tech/science writer, skeptic, lover of all things electronic, and Android fan. In his spare time he reads golden-age sci-fi and sleeps, but rarely at the same time. His wife tolerates him as few would.

He's the author of a sci-fi novel called The Crooked City, which is available on Amazon and Google Play. http://goo.gl/WQIXBM

  • Sean Lumly

    I'm in love. It's a tad shorter, wider, and thicker than the Nexus 7 2012, but would easily fit into a small bag and has a decent weight. This sort of thing is perfect to alleviate battery anxiety for devices that already may have decent life.

    The only thing that would make this better would be wireless charging. Imagine slipping your phone/tablet into a protective side pocket (directly adjacent to this external battery) and the device automatically starts charging. Hopefully this is next-year's model. :)

  • axel

    I wish it had Qi charging like my Powerbot battery pack.

    • Sean Lumly

      Wow! I didn't even know that mobile Qi was a thing. It shouldn't be too hard to combine these two devices in a bag for a no-hassle charging solution.

      • andy_o

        Exactly. I also have a Qi powerbank. As long as it can charge your phone a couple of times or more (mine certainly does), you can just charge it every couple of days and the power loss isn't an issue at all. Affixing it to a bag's pocket shouldn't be too hard, it's a no-brainer.

        • Sean Lumly

          I couldn't have said it better :)

    • TY

      I won't use qi charging on a battery pack, because (at least) 30% energy would be lost. With the battery's limited energy storage I want to squeeze out every bit of it.

      • axel

        That's true. There IS a greater amount of power lost, but I love the convenience. At least the way I use it. I leave it at my desk and just place my phone on it. Some might say that a regular qi charger would do but there are no plugs, and I mainly do it to keep my battery up since I use my phone and move a lot. Having yo plug and unplug would be a hassle. I guess "first world problems" lol

        • vn33

          If you have a Sony Xperia with magnetic charging (I have an Z Ultra), I would use that option. Not as convenient, but easier than prying the waterproof USB port cover & plugging in the cord.

          • axel

            Nah, I have a Nexus 4 but I do wish it had the magnet like the Nexus 5 because the glass is kinda slippery, but it never falls off the charging pad, so I don't have a problem

      • Sean Lumly

        Even 56% of 20Ah (~11Ah) is still a whole lot of energy! It's overkill, actually, and would likely allow you to use your phone for many days without an outlet.

        For me, this is enough and convenience easily wins out. With a convenient Qi charging pad, I would be able to charge my phone at a bedstand (without need to plug in) or in a handy (aligned) pocket in a bag/backpack.

  • TY

    Actually at 1A output, many quality external batteries have at least 80% efficiency, with the best ones going up to around 90%. Charging at a faster rate will lower the efficiency, though.

  • aatifsumar

    Is there an adapter to charge ultrabooks? Would love being able to charge my Air from this.

    • aedwards

      The DC out port on the battery can output 12v, so you'd need an adapter cable that goes from the DC port to a car 12v socket, and use a 12v car charger for the MacBook Air.

      Check the maximum current the battery can give you on it's DC port, though - it may not be happy powering a laptop charger.

  • FrillArtist

    Looks like it could power up a small house.

  • AOSPrevails

    Just finished a 3 week trip with my 13,000 mAh Astro E4, had to use it less often than I envisioned because on the long flights I had a USB charging port at my seat and the short flights are not really a big drain on the battery.

    • Nathan Blume

      Multiply the Ah rating by 5V to get Wh rating.

  • Merlin

    I've been charging my Nexus 5 only from this battery as an experiment. Light/moderate usage: 1-1.5 h browsing over Hspa+ and local music; a call or two but not every day. This battery charged my phone for 11 days. Impressive huh?

    Also fully charged Nexus 7 2013 and there is still 67% battery left. It stayed about a week after I charged it before charging Nexus 7, so part of the battery probably drained by itself.

  • RXG9

    Removable battery FTW

  • tehboogieman

    My 10,000mAh 3E Anker battery can charge my nexus 4 at least 4 times and this battery seems to be twice that battery. (2x capacity, 2x weight, 2x price) Hopefully they have increased the efficiency so it could more than twice as effective.

  • CoreRooted

    I have an Anker Astro 15,000 mAh external battery that I keep charged in my backpack and has been a life saver more times than I can count. It can charge my G2, my N7, and my iPhone 5S quite a few times before draining. I would love to have a 20,000mAh external battery that could also charge my Chromebook if needed, but it isn't a neccesity right now. I think that if Anker lowered the price the way they did with the Astro, this would literally sell itself though.

  • Tucknology

    If the battery pack is dead, can I charge it using AC while ALSO charging my (dead) cell phone?

  • Michael

    I got one from RAVPower last year. It generates 15000mAh and lighten than this one. It works great with my ultra book. I'm happy with it.

  • Angela Berry

    wow,big capacity,but not easy to carry everyday,i don't think i'll use it as my everyday kit..I prefer kinkoo power bank..sleek and easy to carry.

  • jtz109

    I am not much of a traveler...but i got this bad boy for when my family and I do take trips and weekends away. We all have portable devices to charge...and 20,000 mAh with three ports is idea. NOTE: I also bought a 6000 mAh portable Anker device for the every day use.

  • horizon

    i bought a Kinkoo
    8000MAH external battery charger last year and i love it. it's very easy to
    carry around for its light weight and small size. it' s been nearly one
    year and i still can charge my iphone for 4 times.