Choosing an External Battery

I recently investigated the external battery market. As always, a backpacker needs to become a minor expert in a subject in order to see through the marketing fluff and make a wise purchasing decision so I thought it might be useful to pass on some of my new-found knowledge.

What Is An External Battery?

Charging a phone

Charging a smartphone

Physically an unremarkable metal and plastic oblong probably about the size of a thick smartphone (the thicker ones probably have more capacity, i.e. store more juice) or some smaller ones are lipstick shaped. It is basically a big battery which you can recharge hundreds of times from a wall charger or via a USB cable from, say, a laptop or personal computer.

It stores the electricity (which I shall call ‘juice’) for several months, losing only a little over time, and when you need to recharge one of your devices (ebook reader, phone, tablet, camera, GPS unit, headlamp, etc.) you connect it with an appropriate cable (you often have to supply your own, such as the one that came with your device) and it charges your device. It may or may not do this very quickly and it may or may not charge it all the way up and it may or may not be able to repeat this process depending on the respective capacities of the device and the battery.

It is worth noting that they are useful for many, but not all, mobile devices. A colleague just bought a Toshiba Excite tablet and, unlike say a Google Nexus or Apple iPad, it requires a traditional laptop-like ‘brick’ transformer outputting 3 amps and therefore isn’t really ‘portable’ despite its form factor and ‘tablet’ name. Such devices are beyond the purview of the external batteries reviewed here.

So, if you carry an external battery you will be able to recharge your compatible devices ‘in the field’, both metaphorically and literally, i.e. away from a wall charger. This makes them potentially useful to backpackers who wish to carry electronic devices (phones, GPS units and personal locator beacons in particular) that will otherwise run out of power too soon. Day hikers probably don’t need to carry one.


What Do I Need To Know In Advance?

You have to be clear in advance which devices you want to charge with this battery, what power input port each has (all devices have only one power input port ), what the input rating for that port is (in watts) and what the capacity of each of these devices is (in “milliamp hours”). You should also check the input voltage for your device is around the output voltage of the external battery, usually 5V.

And of course you have to decide yourself how many times you want to be able to recharge your device – is an emergency 20% top-up sufficient, do you want to recharge it fully twice, or whatever you want. Obviously, the more spare juice you want the bigger a battery you will have to carry.

Aside: some devices quote their capacity as ‘watt hours’ instead of milliamp hours. To convert watt hours into milliamp hours you divide the battery watt hours by the battery voltage and then multiply by 1000. In practice it is easier to just Google “‘my device name’ battery in mah” to get the answer. E.g. the iPad Air (2013 version) is only listed as 32.4WH on but Wikipedia soon tells you that it’s about 8800mAh.


What Are the Key Features of the External Battery?

  • Capacity - aka how much juice it can store. Luckily this is expressed in the same way for all batteries and appears to be a reliable and trustworthy value. It is ‘milliamp hours’ or mAh and the number is often found in the product name itself but is always found in the product description or specifications list (‘specs’). A very small value is 1000, a medium size is 4000-5000 and a large one is, well, bigger than that.
  • Weight – this is always a key spec to consider for all items when backpacking.
  • How many output ports – batteries always come with at least 1 output port but some come with 2, 4 or even more. Decide how many you need. Having an extra one or two you won’t use is not a problem other than the extra weight and space they will have taken up.
  • What kind of output port(s) does it have? Micro-USB, full-size USB, Apple 30-pin or Lightning? Typically a 2-port unit has 0 or 1 micro-USB ports and 1 or 2 full-size USBs but check. To use an Apple device from a full-size USB port you just need to use the cable that came with your Apple device. While you can buy an Apple micro-USB converter it probably isn’t worth it; it’s easier to get a battery with a full-size USB.
  • Input rating – how much charge it can take into itself dictates how quickly the battery itself recharges. This is a very important feature that is little-discussed and can be hard to find but will impact you enormously when out backpacking with very limited access to wall sockets for recharging.
  • Output rating(s) – how much charge it can give out to your devices. What charge can each output port give out? How is that charge affected when more than one device is plugged in (is it shared)?
  • You might think that ‘ruggedness’ is a key feature a backpacker looks for but actually this feature is so rare as to render a shortlist self-defining and the battery would be so heavy that it makes more sense to pop a battery in a light waterproof bag and just be careful not to drop it.

Note: all external batteries supply only a fraction of their rated capacity to the charging device due to loss to heat and other inefficiencies. Typically this runs from 70% to 80% so before purchase you have to accept that a 5000 mAh battery may only supply 4000 mAh of juice at best.


Do You Actually Need One?

So now we know what is going on in with batteries in general we can decide if they are right for me. If I take a 1470mAh Kindle for book reading and an 1420mAh iPhone 4 for I would probably only need the battery for the phone since the Kindle lasts a long time between charges and is not at all vital to keep charged – whenever I run across a usable power outlet every week or two, that will be fine.

Some devices allow you to change the internal battery. Cameras and some cellphones for example. You can maybe even purchase such a replacement battery with a bigger capacity than the manufacturer supplied one making it a real double-win for you. In this instance you may elect to simply take a spare battery for that device only, rather than use a generic external battery, or at least use that idea to supplement your battery needs. For example, I will take two camera batteries because I can get a 1500mAh battery for my Olympus TG-630 for £5 to supplement the Olympus-supplied 925mAh one. Together they ought to last a week or two.

That  iPhone 4 has a 1420mAh capacity. A battery that weighs about the same as the phone (136g) will be about 5000mAh capaity so will supply about 2.8 full charges (5000*0.80 efficiency/1420). That’s pretty useful. A fully-charged iPhone and battery will give me at least 3.8 uses of the phone.

One tip I picked up is that many devices ‘fast-charge’ up to 80% of its full capacity and then slow- or trickle-charges up to 100%. This means that it is more time-efficient to only recharge to 80% each time as the final 20% could take almost as long. Check your own device before setting off and see if this true for it.

And another thing: there is a long-held myth that batteries such as these or the battery in your device must be fully drained before each recharge, and that each recharge must be to 100% each time, or else the battery will develop a ‘memory effect’ such that it doesn’t store as much juice as when you first bought it. This is not true with modern lithium-ion batteries (“li-ion”). Check your manual. It may recommend the occasional such cycle, in which case follow those instructions, but nothing more is necessary.


Shortlist Criteria

My core needs were as following:

I am taking an iPhone 5 and a Kindle The iPhone has a capacity of 1440mAh, accept 1 Watt input and uses an Apple Lightning port. Apple supply a USB-Lightning cable with the phone so that can be used. The Kindle is about 1300mAh, uses micro-USB and accepts 500 ma (milliamps) of input.

My shortlist criteria therefore were:

  • A 5000-10000 mAh battery with a full-size USB output port capable of supplying at least 1 amp. This is a pretty standard requirement.
  • I want it to be as light as possible (of course).
  • One output port. Nothing wrong with 2 but I only need 1 as I have no requirement to charge my phone and Kindle simultaneously.
  • I want it charge as quickly as possible so I want the input rating to be as high as possible (2 amp or more). The vast majority of batteries only take 1 amp so will charge twice as slowly as one that takes 2 amps. This was a big filter that cut down the shortlist to a very short list.
  • Good reputation. I didn’t want a battery direct from the Far East with no trading history, no US- or European-based company to take responsibility. I wanted to be able to rely on the unit working in the backcountry, I wouldn’t have the chance to ring suppliers, return the product, get a replacement, etc. It had to work first time and best way to ensure that is to stay away from the ‘bleeding edge’.
  • Optional: 2 amp or greater output. While an iPhone and Kindle would not need it, many people report an iPhone charges slightly quicker when the charger exceeds the standard 1 amp and I also wanted the battery to work for the tablets we have at home so it had more use outside of the AT hike. This was a minor decision criterion therefore.


Product Shortlist

Mophie Powerstation 4000

Mophie Powerstation 4000

Mophie Powerstation 4000

2.1 amp in and out. 4.4oz / 124g. £70. It provides 32.2 mAh per gram. Great reputation including a personal recommendation from a current AT thru-hiker who has three!


Soulra Boost 4200 charging an iPad

Soulra Boost 4200 in action

Soulra Boost 4200

2.1 amp in and out. 5oz / 142g. £45. It only provides 29.6 mAh per gram, the worst on the shortlist. Cheaper price than the Mophie but heavier. Included mostly so the shortlist wouldn’t be so short.



EasyAcc iChoc 5000

EasyAcc iChoc 5000

EasyAcc iChoc 5000

2.0 amp in, 2.5 out. 4.6oz / 131g (actually measured). £20. 38.2 mAh per gram. Great price, 25% more capacity than the Mophie for just 7g more weight. Not as a “proper” a brand as Mophie but thousands of top reviews on Amazon.


EasyAcc U-bright 9000 Powerbank

EasyAcc 9000 Powerbank

EasyAcc 9000 Powerbank

2.0 amp in, 2.1 and 1 amp out. 6.7oz / 191g (actually measured). Provides a massive 47.1 mAh per gram, easily the best on the shortlist. £20. Really amazing price! This was the largest battery I could find on the market with at least a 2 amp input. Having tested it myself I can say it fully charges from the wall in around 3 hours and 50 minutes which sure beats waiting over 8 hours for most batteries of this size. It also contains a 50 lumen flashlight which is a useful backup option to have in case of emergency.


And that was it. Very few batteries accept 2 amp or more input but you only need to buy one battery and these 3 are worthy shortlist contenders. If you know of any more that meet all the shortlist criteria feel free to leave a comment about it so it helps others out in the future.


Wall Chargers

OK, so as educated buyers we know to buy a battery with more than 1 amp input, preferably 2 amp. But in order to take advantage of that feature we must be sure to supply it with 2 amps of power when re-charging it.

If you already own a USB wall charger (e.g. one that came with your smartphone) then you need to go get a magnifying glass (that may or may not be a joke actually) and read the small print on the back of the charger itself. You want one that supplies at least as much juice as your battery will accept. More is not a problem but won’t be of any benefit.

As an example, any full-size (i.e. not Mini) Apple iPad came with a high quality 2 amp charger. Most, if not all, smartphone chargers however will only be 1 amp so you may need to buy a new wall charger.

You should also consider whether you want one USB output port or two (or more!). One will charge your external battery but if you have two – and they EACH supply enough power then you could potentially hook up both your external battery and one other device at the same time – which could be super useful!

For example, if you have a 3+ amp charger with two USB ports and one can supply 2 amps and one can supply 1amp AT THE SAME TIME then you can recharge your phone and battery simultaneously with no loss of time. Win.

Anker dual USB wall charger

Anker dual USB wall charger

One example product is the Anker dual-USB 3.6 amp 18W wall charger which can supply up to 2.5W in one port and 1.1W in the other, sufficient for almost any battery and smartphone out there. And at 79g it only weighs a little more than a basic charger.

Another example is the slightly heavier (86g) Powergen 17W dual USB charger which provides 3.4 amp and has foldaway prongs which is useful to reduce pack size and prevent sharpish prongs damaging your other gear.


Final Decision

While the Mophie has the better reputation and a 0.1 amp greater input than either of the EasyAcc devices I decided to give the two EasyAcc units the first chance. At around a quarter the price of the Mophie and with a greater capacity per gram it was an easy decision.

The iChoc 5000 has a small built-in micro-usb cable to charge devices like a Kindle that is ‘free’ in terms of weight and pack size and the lightly rubberised outer coating is very good as it feels quite robust and is totally non-slippy.

However, the 9000 unit provides 80% more juice for only 60% more weight and has a useful ‘proper’ flashlight. It charges up in less than 4 hours so even just an hour of charging (say while eating lunch in a cafe) will provide a useful amount of juice. It has a normal metal exterior and feels well-made and robust.

I will take the 9000 unit on the hike because it will need to support a phone, a MiFi device, a Kindle, an MP3 player and a camera. (I even toyed with the idea of choosing a headlamp with a rechargeable-via-USB battery but decided to stick with AAA batteries). A 5000 unit would probably not last beyond a a few days at a time. The 9000 unit provides more flexibility with regard to when I would need to get off the trail for recharging so those extra grams could buy me a few extra consecutive days on trail.

4 thoughts on “Choosing an External Battery

  1. Sparky – I saw your link to this site on the BPL forums. Very nice summary. One factor you didn’t mention, that I’ve found to be very important, is whether the battery will allow pass-through charging. We’ve been carrying the Jackery battery on long hikes, and it does NOT allow pass through charging. That means that when we stop at a grocery store we can not use one power adapter and plug the battery+phone into the wall and leave it there to do its business while we shop, we have to decide which of the two to plug in. I’m looking for a replacement for the Jackery to solve this exact problem, and so far the FluxMob Bolt looks promising (I just learned about it in that same BPL thread).

    1. Hi Amy, thanks for your very useful comment. I didn’t mention pass-through because it’s so rarely available that it restricts your choice a lot. Plus it adds bulk and weight. None of the 3 products I shortlisted above have it, for example. I did recommend a dual-port (or 4-port indeed) wall charger though and this allows you to charge multiple device at once without restricting your choice of battery. Cheers.

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