How to choose an external battery for a smartphone

Capacity and accompanying dimensions

The most important parameter of an external battery is its capacity, which the manufacturer usually indicates in milliampere-hours (mAh). Roughly speaking, this is the amount of energy that a power bank can store (but not transmit).

Please note: due to the fundamental laws of physics, no external battery can transfer 100% of the stored energy and no smartphone is able to fully absorb the received energy. Part of the resource is always lost during voltage conversion, and some is lost to heat.

On average, the cumulative losses range from 30 to 40%, although some (and, as a rule, more expensive) models of external batteries and smartphones boast about 90% efficiency.

In other words, a powerbank with a declared capacity of 10,000 mAh in practice will transfer approximately 7,500 real mAh, that is, it will fully charge a smartphone with a battery with a capacity of 2,500 mAh, not four times, but only three times.

However, please note that this calculation is very average. It is possible to check the real losses when feeding a specific smartphone from a specific external battery only in practice.

But it's not worth chasing the maximum capacity. The higher it is, the more expensive, heavier and larger the powerbank will be - keep this in mind. If you are only going to charge one smartphone, a 10,000 mAh external battery will most likely be sufficient. But if you plan to support several phones or a laptop or tablet with a powerbank, then it is better to choose a device with 20,000-30,000 mAh.

Connectors and cables

Many external batteries, especially budget models, are sold wirelessly. It is assumed that you will use the USB cable that came with your smartphone to connect. In this case, make sure that the end of this wire (most often the USB ‑ A or USB ‑ C port) matches the type of output connector on the powerbank.

Some external batteries are sold with a plug-in or even built-in cable. In this case, it is important to make sure that the end (most often a micro ‑ USB, Lightning or USB ‑ C port) fits your smartphone. If you encounter port incompatibility issues, you can solve them by purchasing a suitable cable or adapter.

In addition to the type of connectors on the power bank, pay attention to their number.

For example, if you are going to charge two devices at the same time, look for an external battery with two output ports.

In addition, power banks also have an input connector for charging the external battery itself from a computer or mains. Usually this is microUSB, and the corresponding cable is included. Do not confuse this connector with output ports.

External battery output current

The charging speed of the smartphone will depend on many parameters of the powerbank, but the output current is decisive. It should be no less than that provided by the smartphone manufacturer.

Finding the optimal amperage for your smartphone is easy. Take the plug that came with the device and look at the OUTPUT value. It will say something like 5 V - 2 A. You need the number next to A because current is measured in amperes.

You can find out the output current of the powerbank on the manufacturer's website or in the instructions. If the external battery has more than one output connector, then the current strength is sometimes indicated directly on the case next to each port.

If there are two or more connectors, remember that the total output current of the powerbank is limited.

For example, a battery can have two outputs of 2.5 A each, but give a total of no more than 4 A. Accordingly, when two devices are connected at once, each of them can receive no more than 2 A.

If the output current of the external battery is less than that for which the smartphone is designed (for example, 1 A for a powerbank and 1.5 A for a smartphone), then charging will still occur, but noticeably slower.

There is no need to be afraid if the output current of the external battery is higher than that for which the mobile device is designed (for example, 2 A for a powerbank and 1 A for a smartphone). Mobile devices have built-in protections that limit the incoming current to an acceptable value. Simply put, your smartphone won't explode or melt.

For the same reason, your mobile device will not charge faster if you use a plug or power bank with an output current higher than that specified by the manufacturer.

Fast charging support

Let's say you find an external battery with a suitable output current strength, but the included plug will still charge your smartphone faster. What's the matter? Most likely, your smartphone supports some kind of accelerated charging technology like Quick Charge 3.0, Quick Charge 4.0, or a variant with its own name from a specific manufacturer (for example, mCharge from Meizu). Look for a power bank that is friendly with exactly the fast charging technology used in your smartphone.

External battery input current

No matter what capacious external battery you buy, its own energy supply will sooner or later be exhausted. It's nice when the powerbank charges your smartphone quickly, but it's even nicer if it charges itself quickly.

The charging speed of a powerbank is determined by its input current strength, voltage and the presence of one or another technology that provides more rapid replenishment of its own charge. There are now external batteries that can be fully charged in less than 30 minutes.

Please note: the powerbank's own charging speed does not affect the speed at which it charges the smartphone.

But lack of understanding of technology and gaps in knowledge of English can be confusing to the user. For example, you see a power bank that says something like 5 minutes charge. You might think that this miracle device charges your smartphone in 5 minutes, but in reality everything is different.

Let's say the capacity of a particular battery is 10,000 mAh and it is fully charged in 20 minutes. In this case, it will replenish approximately 2,500 mAh in 5 minutes. And this charge is really enough to charge a typical smartphone.

The problem is that the charging time of a smartphone is limited by its own maximum input amperage and the availability of support for one or another fast charging technology.

Simply put, the pretentious phrase 5 minutes charge actually means: "This powerbank needs 5 minutes to get the charge necessary for further full charging of the smartphone." Do not get caught up in marketing noodles and pump your technical literacy.

External battery type

Depending on the batteries used, a distinction is made between lithium-ion and lithium-polymer batteries. The latter are considered to be more stable and compact. But the difference is not so significant as to draw attention to it. Therefore, you can choose any of these types.

Other features

There are a few more nuances that are useful to pay attention to when choosing a power bank.

Body material … Plastic batteries are cheaper and lighter than metal ones. Remember this.
Indicator presence … Some power banks display the remaining battery level on a small built-in display, which is very convenient.
Wireless charger … If your smartphone and powerbank share wireless charging technology, you can connect them to each other without a cable.
The presence of a solar panel … If the battery case has a built-in solar plate, it will be able to charge in clear weather.
Protection against external influences … Portable batteries can be protected from moisture, shock and dust. If you lead an active lifestyle, these properties can come in handy.

This material was first published in June 2017. In July 2020, we updated the text.