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How many times in your life have you lived through the dreaded scenario of watching your phone battery percentage slowly (or indeed not so slowly) decrease into oblivion, while not having any method of charging it back up available to you?

I have been in this situation many times. So many in fact, that I have started to carry around my phone charger wherever I went. Now, this is only helpful of course, if there is a wall socket in your vicinity. More often than not however, I wanted to charge my smartphone on the train, bus or somewhere else, where the luxury of wall sockets wasn’t available.

Now, you might not be familiar with this problem (though, I doubt it), if your smartphone lasts you the whole day of use. I haven’t had that since my good old Sony Ericsson V630.

These past few years however, I’m struggling to get through the day without plugging in my phone. Obviously this depends on the smartphone itself and your typicall use.

That’s where power banks come in and save the day.

I have been using these two power banks for quite a while now.

There are a few factors you need to consider when purchasing your new smartphone saviour:

Capacity

It means how much charge (energy) a power bank can store. They come in all sorts of capacity sizes from smaller ones, which hold around 2,000mAh of charge, to 30,000mAh behemoths. The smallest ones are used to just top up your battery, they normally won’t be able to charge your smartphone from 0 to 100 percent. Meanwhile, the largest power banks can charge your phone fully several times before dying themselves. More powerful power banks do exist, they can contain up to 120,000mAh, but they are really big and heavy and used for activities like camping, where you would want to power refrigerators, heaters, etc. Therefore they do not fall into the scope of this article.

Most often you will see these capacities:

2,200 or 2,600mAh

3,500mAh

5,000mAh

10,000mAh

16,000mAh

20,000mAh

26,000 to 30,000mAh

If you find that your phone dies on you every so often, or that you need just a 20 or 30% boost a day, a battery bank up to 5,000mAh will be perfect for you. They’re small and light and they charge quickly. These two from Xiaomi and Anker are super small, look good and work well.

For those of you who need even more juice, a 10,000 or 16,000mAh battery bank might be a better option. It will charge your smartphone fully a few times, so you could, for example, use it for the whole week and then charge it on the weekend. There are a few good brands to look at, I will name some below, but for these capacities, I again suggest the Xiaomi 10,000mAh, Xiaomi 16,000mAh and the Anker 10,000mAh, as well as the Anker 13,000mAh power banks.

If you are a power user and you demand a lot from your phone or you are planning a camping trip or something similar, you should pick up a 20,000mAh battery bank or an even larger one. I use my Aukey power bank every day (Galaxy S6 battery life is a joke) at the university, I plug my phone in and slip it inside my bag during a lecture; or just keep both devices on the table. Power banks of this capacity will charge your phone from dead to fully charged around 6 times at least (depends on your phone of course). My picks for power banks of this capacity: Xiaomi 20,000mAh, Anker 20,000mAh and a bit pricier Aukey 20,000mAh or even Aukey 30,000mAh.

Dimensions

When it comes to dimensions, you have to look at the best capacity to dimensions ratio.

Dimensions are correlated with the capacity of battery banks. The smallest ones can come in the size of a credit card and can be easily stored in a wallet. These are really handy since you can carry them with you all the time and use them for giving your smartphone an extra push until the end of the day.

Power banks with capacities from 5,000mAh to 16,000mAh are still quite compact (at least those from Anker and Xiaomi and some others) and you can easily find a spot for them in your purse or smaller bag.

The most powerful ones though, they can get large and heavy. However, it is not unbearable, especially because you are probably going to be carrying it in a backpack or a somewhat larger bag.

Inputs & Outputs

Power banks are made with different combinations of ports. When you decide to buy one, you need to look at the output as well as the input ports.

The number of output ports means how many devices you can charge simultaneously with your battery bank. Usually, the smaller capacity PBs (below 10,000mAh) only have one output USB port. This is a bummer, if you have a friend in need, whose phone is also dying constantly. Fortunately, the bigger capacity banks have two or rarely even three USB outputs. Sharing is caring, they say.

All USB ports are not created equal, though. Depending on the current that the PB can supply, you will get a faster/slower charge. Nowadays, power banks have at least one USB output rated at 2.1 amps or stronger. Of course it depends on your smartphone’s battery, what is the max current that the battery can be charged at.

Smartphones with Qualcomm Snapdragon processors use a technology called Quick Charge (version 2.0 or 3.0) which allows them to charge really fast. If you have a smartphone that supports Quick Charge, you need to look at power banks which support it as well to get the fastest charge possible.

You have probably heard of USB Type C before, haven’t you? USB-C is the better, faster, stronger generation of USB standards. It allows for faster charging of your phone and the battery bank itself, so if you own a phone with a USB-C port for charging, you should consider a battery bank with the same port.

Power banks also need to be charged when they are empty. If you have a smaller capacity one (like 5,000mAh), you can charge it in a couple of hours (depending on the bank’s max input and the charger you use). But if you own a big power bank (20,000mAh or more), it will take more than one night of charging to charge it fully. My Aukey 20,000mAh needs a good 10 hours.

Features

If you have seen battery banks before, you know, they most often have a flashlight built in. Whether this is useful depends on the user. When I went camping I used it quite a bit inside the tent at night.

There are also some battery banks that come in a more rugged form (like this one from Sandberg). They usually offer some level of water (some are 100% waterproof) and dust resistance, as well as better protection from bumps and drops. Normally they even have a stronger flashlight.

I don’t think I have ever seen a power bank (maybe once) that didn’t have some sort of LED indicators to show you how much charge is left in them. Most often there are four LEDs of the same color, each representing around 25% of charge left. Some battery banks have an indicator built into the power button, which then lights with different colors, depending on the charge that is left. The Aukey 20,000mAh which you saw in the photos has a white light for above 60%, a green light above 20% and a red one for below 20% of juice left.

Efficiency

Now we have come to the most controversial part of power banks, at least in my opinion.

People are wrongly assuming that the method to calculate the number of full charges (from 0% to 100%) they can get from the power bank to their phone is:

Cp = power bank’s capacity Cs = smartphone’s battery capacity Number of full charges = Cp / Cs

ERRR! Wrong!

If we lived in an ideal world, a 20,000mAh power bank could charge the 2,000mAh battery in my smartphone from dead to fully charged 10 times! Let me assure you, it doesn’t.

There are a few technical reasons for this. There will be a very short TL;DR (too long; didn’t read) paragraph at the bottom of this section.

First, the actual batteries that are inside a power bank have an average voltage of around 3.7V. However, when you connect your smartphone to a power bank, your smartphone, or the charge control circuitry rather, expects 5 volts (that is what the standard smartphone charger or computer USB port will provide), which it then reduces to around 4.3V, the maximum voltage of the phone’s battery. Which means, the power bank must boost the voltage to supply those 5 volts. All of this happens because batteries may explode if you feed them with too much voltage and on the other hand will not charge to their best potential, if they don’t get enough. In the end, the boosting and regulating voltage is where around 10% of energy is lost.

Second, the rate at which you charge your smartphone from the battery bank can decrease efficiency. Which means that if you use the 2.1A port (of course you do, it’s faster) to charge your phone, you will actually lose a few percent of energy compared to charging through the 1A port.

Thirdly, extreme temperatures (hot or cold) do not bode well for the energy stored in a power bank.

The last reason is self-discharge – batteries lose charge over time. A full power bank can be empty in a few months.

TL;DR

Power banks are not 100% efficient because of the technology that is used for them to function properly and safely. It is getting better.

Brands

If you have managed to read through this entire article, kudos to you! I thought I should share my recommendations for which brands to purchase battery banks from as well as my experience with a non-reputable brand. Let’s start with that.

In my need for an energy pack for my extremely inefficient smartphone battery I went to a Müller store where I found a section with power banks. There were a few German brands, none of which I knew, but at least I thought, great, Germans are great engineers, the battery bank is going to be good. I bought the 6,000mAh oval beauty you saw in the photos above. Big mistake.

My phone had a 3,000mAh battery (I think I got a defected unit, since the battery life should have been great, but was in fact terrible). Keeping the laws of physics and our non-ideal world in mind, I expected to get around 1 and a half full charges from the purchased battery bank. How many do you think I got? Not even one.

The best this little power bank could muster was getting my phone from 20% battery to 80%. Appalling! Not to mention I could practically watch it lose charge throughout the day on its own! If I charged it during the night and then wanted to use it in the afternoon, it would only have 50% left (2 LEDs lit).

Learn from my mistakes and you will hopefully not have the same experience as me.

OK, so, which brands do I believe are worth your money?

Anker

Xiaomi

Aukey

EasyAcc

EC Technology

RAVPower

Vinsic

Coocheer

I will try to update the list whenever I get my hands on some other power banks that work well.

I also suggest you pickup some short cables to go with your new power bank. Although a micro USB cable usually comes with the PB itself, there is no love for the iPhone users.

With this new knowledge you can browse for your next (or first) power bank by visiting this link to EnaA online store and tell them I sent you.