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Here's why power banks aren't as big as they seem to be

Here's why power banks aren't as big as they seem to be
There's no denying that power banks are an awesome invention. They are portable, inexpensive, and store back-up charge for those times when we need to revive a dying phone's battery. And they come in shapes and sizes for every need and customer – some are small and pocket-friendly, carrying about 2,000 to 3,000 milliamp-hours (mAh) of charge, while others are chubbier yet more capacious, storing over 10,000 mAh inside their battery cells. 

But since the average Joe does not have a degree in electrical engineering, the capacity of power banks is often misinterpreted by consumers. Many assume that if a power bank can hold, let's say, 10,000 mAh of charge, it will be able to fully recharge a hypothetical 2,000 mAh smartphone battery exactly five times. This would have been true in a perfect universe, but in the real world, the math involves quite a few more variables.

For one to understand what causes the inevitable difference between a power bank's advertised and effective capacities, they have to be familiar with the device's inner workings. In a nutshell, there are two essential components inside of a power bank – a battery for storing charge and an electrical circuit in control of the charging process. When a power bank is advertised as having a charge capacity of, for example, 10,000 mAh, that means it contains one or several battery cells with a combined charge capacity of 10,000 mAh. Makes sense, doesn't it?

Inside a power bank you'll find one or more lithium battery cells. Precise circuitry controls the charging process. Image via Xiaomi

Inside a power bank you'll find one or more lithium battery cells. Precise circuitry controls the charging process. Image via Xiaomi


Here's where it gets tricky and technical. Battery cells based on lithium-ion chemistry, such as the one inside the bank and your smartphone, have a nominal or average voltage of around 3.7 volts. We said "average" because a battery's voltage does not remain constant – it drops as energy reserve is used and increases as the battery is being recharged. Modern smartphone batteries are considered 100% full at about 4.3 volts and empty as they get drained to around 3 volts, hence the 3.7 V figure above. Unfortunately, this low and varying voltage is of no use to your smartphone.

To get the charging juices flowing, a power bank provides 5 volts of boosted voltage output, not 3.7 volts. Why? Because that's the standard voltage a smartphone's charger (or a computer's USB port) delivers, and that's what the charge control circuitry inside a smartphone expects to receive. These 5 volts are then precisely stabilized by the phone's circuitry and reduced to match the maximum voltage of the phone's battery, which is somewhere around 4.3 volts. This may all seem like overkill, but it is not. All this precise control over the charging process is implemented for safety reasons because a battery may explode if it is fed more voltage than it can handle. Conversely, a battery will not charge to its best potential if it is provided with less voltage than it is designed to receive.

Even a high-quality power bank isn't perfect. The Mi power bank, for example, is up to 93% efficient

Even a high-quality power bank isn't perfect. The Mi power bank, for example, is up to 93% efficient

Still with us? Okay, as here's where we highlight the above charging method's flaws. As we mentioned, the power bank needs to boost the voltage of its internal battery to 5 volts, and a special voltage boosting circuit is used for the purpose. However, this circuit is where some of the energy is lost, and the losses may exceed 10%, depending on the circuit's efficiency. Further losses occur inside the smartphone itself, within the circuitry that regulates the voltage fed to its battery. 

Another factor that has to be taken into account is the rate at which the power bank is discharged – drawing more current decreases efficiency. In most cases, there's nothing you can do about this as most power banks are designed to provide 2 amps of output current. However, some models offer both 1-amp and 2-amp outputs. Using the 1-amp output would charge your smartphone slowly, but will provide a few percent more charge.

We must also add that batteries hate extreme temperatures. Your power bank will lose charge if it is stored in a chilly place or on your car's dashboard under the scorching sun. Extreme heat or cold may even cause permanent damage to its battery. 

Even if you treat your power bank well, it will lose charge over time. This is due to a phenomenon called self-discharge – a full battery may be empty in a matter of months even if not used. That's why it is a good idea to discharge and recharge a power bank on a monthly basis.

TL;DR: The actual mAh capacity of a power bank is smaller because the 3.7 V nominal voltage of its battery has to be boosted to 5 V, and this results in about 10% losses. Also, power bank capacity is affected by many factors, including ambient temperature, charging current, the health of the bank and the phone's batteries, and the efficiency of your phone's charge regulating circuitry. To fully charge a smartphone's battery, you should aim for a power bank with a mAh rating of at least 25-30% more than the mAh capacity of your smartphone's battery.

More on smartphone batteries and power banks


9 Comments
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posted on 15 Mar 2015, 18:24

1. bur60 (Posts: 880; Member since: 07 Jul 2014)


I have the mi powerbank. Since I have the Z3 compact I never use it anymore... Might use it when I go for the S6 edge

posted on 15 Mar 2015, 20:24

2. rsiders (Posts: 1124; Member since: 17 Nov 2011)


Thanks for this article. I bought a 10,000mAh battery pack to use with my Lumia 1020 that I had at the time and thought it would charge my phone at least three times and I was surprised to find out that was not the case. This explains a lot as to why the battery pack lost power faster than it could keep recharging my phone.

posted on 15 Mar 2015, 21:44

3. mikeguy1981 (Posts: 88; Member since: 23 Jun 2012)


i dont use external packs for this main reason. just replace the inner battery to a bigger one. i did. i recommend zerolemon batterys. i have a note 4 with a internal 10000mah cell and its great! goes several days before i have to recharge.

posted on 15 Mar 2015, 21:48

4. chocolaking (Posts: 495; Member since: 22 May 2012)


this stupid mi power is gonna lit on fire and burn houses down one day. see you again on the news.

posted on 15 Mar 2015, 22:33 4

5. MaryPoopins (Posts: 223; Member since: 15 Jan 2015)


Let's hope it's your house with you inside it so we don't have to read any of your retarded comments again.

posted on 15 Mar 2015, 23:15

6. TheNeighbor (Posts: 299; Member since: 15 Nov 2013)


I happen to have the Mi 10,400mAh. I personally frown on carrying this extra thing since I just swap to a fresh battery for my Galaxy Note 4, and charge the spent battery on a battery cradle charger. However, I do some work with a Note 8, and after 2 years, the battery can hold up to some 4 hours and therefore that where a power bank comes in handy.

I have read about a Galaxy Tab Active having a removable battery. I really wish this was the case for tablets. Charging a battery with cradle charger connected with a power bank is convenient as well, and I can keep my device virtually wireless and untethered all day. That's why I hate the fact Samsung is starting to go the iPhone way with a fixed battery on the Galaxy S6/S6 Edge.

posted on 16 Mar 2015, 01:19

7. Heisenberg (Posts: 210; Member since: 11 Feb 2015)


Don't hate the player, hate the game. most oems are using fixed battery concept. You just have to accept it

posted on 16 Mar 2015, 02:09 1

8. TheNeighbor (Posts: 299; Member since: 15 Nov 2013)


You mean you rather be more IMPRACTICAL than logical? A majority of the masses are in the don't know/don't care group. At least a removable battery gives us an OPTION and VERSATILITY.

posted on 16 Mar 2015, 06:50

9. sram145 (Posts: 127; Member since: 18 Dec 2014)


"Still with us?" Nah. you lost me long before :P

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