Mobile phones have become yet another appendage of our bodies. We use them for everything and, inevitably, the battery does not make it to the end of the day.

You don’t agree? Well, just stop and think. Who wakes you up in the morning? Where do you check your agenda, the shopping list, your mail and WhatsApp messages? Who keeps track of your sports achievements, if you have any? It tells you the weather forecast. It tells you where you are and how to get where you want to go…. And much more.

All this comes at a price, increased battery consumption. And we don’t want to be negative but it is always flat when you need it most. A real drama. Just as well that there are external batteries to help you out of a fix.

Sounds familiar, doesn’t it? But, did you know that it is a good idea to have an external battery for your mobile phone?

We recommend you take a good look at the capacity you really need, depending on the model of phone you have and whether you need to cover one or more charge cycles. Think about what you require of your phone. Look for a balance between capacity and portability.

And how to know if the external battery allows you to make a full charge or even several? It depends on a variety of factors, which are described below.

The first thing you need to know is your smartphone’s capacity.

The information is usually stamped on the battery itself with the mAh symbol. If the telephone impedes your view, do a Google search – that almost always works.

Secondly: make sure that the capacity (rated in mAh) of the device that you are going to buy is the at least the same as that of the mobile battery, that is if you want to charge it almost fully.

And this is where a common mistake is made. In order to find out how often you can charge your mobile with the external battery, you divide its capacity by the device’s capacity, without taking into account its real capacity. For example, if your smartphone has a battery of 2500 mAh and the external battery, a capacity of 5000 mAh. Divide it and there you are! 2 full charges. Wrong! Do not expect two full charges because the charge is never 100% efficient.

The main mistake is to apply the formula below.

Number of charges = external battery /device battery capacity

As a rule, an external battery does not have a 100% performance.

Method for measuring real capacity:

To make a real and closer estimate of the number of times you can charge your phone battery, you need to keep in mind the conversion rate. In other words, the percentage that is lost in the voltage change between the external battery and the device (mobile, tablet, eBook…). This depends on each manufacturer and brand. To be clear, the batteries work at 3.7V, but most devices work at 5V. During this conversion, part of the mAh offered by the battery is lost.

The condition or health of the device’s battery has an impact too. Your battery’s capacity is gradually reduced over time. The greater the conversion rate, the fewer the mAh lost. Keep in mind the fact that a good quality battery has a conversion rate higher than 80%.

Not to mention the fact that using the mobile phone while it is charging (WhatsApp, Wi-Fi, background applications, etc.) reduces the number of possible charges.

Taking into account all these factors, the formula is as follows:

Real capacity of the power bank x conversion rate/phone battery capacity

You are probably thinking this is all very complicated. Don’t worry. There is another method. There is a quicker and simpler way of calculating it. Just keep in mind the external battery’s overall performance. We know that most of the power banks on the market are capable of transferring a load of 70% of their capacity.

With this the following formula is obtained:

No. charges= (battery capacity x performance)/device capacity

For example, if the external battery is 3200 mAh and the battery capacity of the iPhone 6 is 1810 mAh, we get: 3200 x 0.7 = 2240 mAh / 1810= 1.2 charges

Choose the system that best suits you but remember that these methods are based on estimates and the number of charges may vary. Here we have based our estimates on ideal conditions.

We also recommend

If what you want is to charge your phone several times or charge other kinds of devices like a tablet, eBook or iPad, you will need high capacity external batteries, of around 8000-16000 mAh, which also have two outputs for different amperages.

To sum up

No need to divide the external battery by the device’s capacity, because then you just get frustrated. A power bank never has 100% performance. That’s why the number of charges will be fewer. Keep this fact in mind whenever you choose the battery. Here are the ones currently available.