Modern gadgets are power hungry. If you want to make it through a long commute or a cross-country flight without having to plug your tablet or gaming device in, you’re going to need an external battery pack to keep the electrons flowing. Read on as we show you how to shop for a pack that will meet your needs and keep your screens glowing.

What’s an External Battery Pack and Why Do I Want One?

Normally when you need more juice for your smartphone, tablet, or other mobile electronic device, you plug the USB charging cable in to your computer or to a wall-wart transformer. You top the device off (or keep using it while it charges in the background) and away you go.

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That’s not always convenient (or even possible) if you’re traveling or otherwise away from home. This is where an external battery pack comes in handy. They range in size from as small as a lipstick tube (good for topping off a small smartphone battery) to as big as a thick paperback book (good for keeping your phone going for days or letting multiple friends juice up their tablets).

Instead of plugging your charging cable into the wall, you instead plug the charging cable into the battery pack and fill up the device’s batteries that way. Not all battery packs are created equal, however, and even if the build quality is good, you can easily end up with an external battery pack that doesn’t fit your application and power needs.

Let’s take a look at our field tests of two great battery packs and how their features relate to our shopping-for-a-battery checklist.

First, Meet the Models

As part of the process for writing this guide, we used two higher-capacity battery packs the RAVPower Deluxe 14,000 mAh Power Bank ($29.99), seen above right, and the Jackery Giant 10,400 mAh Power Bank ($39.95), seen above left.

We’d highly recommend both of them as perfectly serviceable high-capacity external battery packs. Rather than delve into all the features before you have a frame of reference, let’s take a look at the general guidelines you want to keep in mind when pack shopping and how they relate to our model packs.

Estimating Your mAh

Before all else, you need to establish how much juice you need. Both device batteries and the external battery packs that top them off have capacities rated in mAh (milliampere hours). This is the principle measuring stick you’ll use to determine how much you need to invest in your pack.

First, gather up the devices you want to charge off the external battery pack. Let’s say, for the sake of example, you have Samsung’s popular SIII smartphone and a new iPad Air. The SIII has a stock battery with a capacity of 2100 mAh and the iPad Air has a stock battery with a capacity of 11, 560 mAh. Now it’s time for a little number crunching.

You can use the following equation to determine just how beefy of a battery pack you need:

(Total mAh) * (% battery life extension expressed in decimal format) = Pack Size

If you wanted a battery pack that could double the battery life of both your devices, you’d need a pack with a capacity of at least 13,660 mAh:

If you wanted to squeeze 50 percent more life out of them, you’d need a device with at least  a capacity of 6,830 mAh. If you only cared about keeping your iPad going during your flight and you’d have your phone turned off, then you could stick with a battery pack that had around the 11,560 mAh capacity of the iPad to double its life. While both of our test models are well suited for this job, only the extra-big RAVPower with 14,000 mAh would be able to truly power both of our devices with a 100%+ boost.

Just like in every other battery application, there’s a trade off to be had between high and low capacity devices, and that takes the form of weight. The little lipstick-sized battery packs we mentioned a moment ago might only have 2,000 or so mAh in them, but they only weigh a few ounces and easily slip into your pocket or purse. Our 14,000 mAh beefcake that can keep your iPad running over a trans-continental flight? It weighs two pounds or so and won’t be very comfortable in your pocket.

Conversely, if you’re looking to power just your phone, getting one of the monster 10,000+ mAh packs will be overkill. Just for fun we charged our SIII phone exclusively off the massive RAVPower pack to see how many days we could go before the pack ran dry. By the eighth day of the experiment we hadn’t depleted it completely; clearly the pack would be overkill for casual travel use if your only device was a smartphone.

Selecting the Right Amperage

In addition to calculating how much battery capacity you need, there’s also the matter of charging amperage. The bigger and more power-hungry your device, the more important having the proper amperage on the USB charging ports is.

Charging ports on battery packs, like charging ports on wall-warts and computers, can provide electricity at two amperage rates: 1A and 2.1A. All USB devices can use both ports, but if a device can only handle 1A of power then it will automatically limit itself to 1A on a 2.1A port and if a 2.1A device is on a 1A port it will also charge (but at a much slower rate). Both of our test devices feature a 1A and a 2.1A port.

For trickle charging, such as you might do overnight or if you just had the device sitting in your briefcase hooked up to the battery pack, the amperage doesn’t matter as much. Yes the 2.1A will charge the device faster, but if you’re not using it and it’s just topping off the device, the speed of the charge isn’t such a big deal.

Where the amperage becomes critical is when you’re shopping for a battery pack that you intend to use on a battery-hungry device while the device is in use. For example, if you want a battery pack that can keep an iPad Air topped off while you’re playing a graphics-intensive video game or otherwise taxing the system, you’re going to need, no questions asked, a battery pack with a 2.1A charging port. Packs with 1A ports simply won’t be able to keep up; you’ll be burning battery life on the device faster than the battery pack can replace it.

Ports for Your Pals

If you’re shopping for just yourself, it’s OK to spend less and get a device with a single port or a 2.1A and 1A port. Need to provide a steady flow of juice to both your iPad and your traveling companion’s iPad, though? You’d better spend the extra money to get a battery pack with two high draw 2A ports. If you’re planning on setting up a multiplayer gaming huddle at 30,000 feet, you can even find battery packs with 4+ 2.1A ports.

Given that it doesn’t cost much more to get a better pack with an extra port or two, you’ll come off looking like a very prepared spouse or business partner if you have some juice to share with your travel mates.

Extras Worthwhile and Worthless

Because the external battery pack market is pretty heavily saturated, many manufacturers have started including little extras to entice buyers. Our advice is to avoid being swayed by the extras unless the extras offer you high-utility or save you money. For example, if the pack you’re looking at costs an extra dollar and comes with an iPad charging cable, and you were planning on buying one anyway, that’s a good value. If it costs a lot more and comes with 12 adapters for crap you don’t even own, then it’s not such a hot buy.

One of our favorite extra features is the inclusion on many battery packs of an LED flashlight. At first glance it seems pretty gimmicky, but we think it’s quite clever. You use battery packs most often when you’re traveling, and since you’ll likely have the battery pack in hand when you’re rooting around in your bag or luggage looking for cables and whatnot in an unfamiliar setting, that burst of light is more than handy. When our RAVPower external pack has a full charge, for example, the LED flashlight is good for a massive 800+ hours of use.

RELATED: This Portable USB Charger Battery Pack Can Also Jump Start Your Car

Another useful feature,with a much more practical application than a flashlight, is indicator lights. Both of our test models included LED indicators that, when the main button on the pack was tapped, displayed the remaining charge in a simple incremental display (the RAVPower used 4 LEDs and the Jackery used 3). On all but the smallest battery packs, don’t settle for anything but an effective remaining power indicator of some sort.

A Shopping Checklist for Your Convenience

Now that you’ve learned about the features you want in your battery pack, make sure you don’t forget them when you’re shopping. Use the following checklist to ensure you end up with a battery pack that meets your needs.

1. Write down your total mAh needs and multiply that value by the percentage you want to extend your battery life (0.5 for 50%, 1.0 for 100%, 1.5 for 150% and so on).

2. Write down how many devices you want to charge simultaneously; this is the minimum number of ports you need.

3. Note the number of high-draw devices you’ll be using (iPads, Kindle Fires, newer smartphones, all benefit from having a 2.1A charging port). When in doubt, error towards getting at least one 2.1A port.

4. What extras do you want? (Extra cables, on-board LED light, etc.)

With all that listed out, you won’t be lost in the sea of battery packs when you hit up the bursting-at-the-seams category on Amazon looking for the right mix of mAh, ports, and accessories.

Profile Photo for Jason Fitzpatrick Jason Fitzpatrick
Jason Fitzpatrick is the Senior Smart Home Editor at How-To Geek. He has over a decade of experience in publishing and has authored thousands of articles at How-To Geek, Review Geek, LifeSavvy, and Lifehacker. Jason served as Lifehacker's Weekend Editor before he joined How-To Geek.
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