# How Many Times Can a Power Bank Recharge your Phone?

You can actually calculate this yourself. All you need to know is a tiny bit of maths. With a few calculations, you’ll be on your way to being a power bank expert.

Today I will be explaining the formula that’s used in order to calculate how many times your power bank can charge your device.

You’ll find out the steps taken to calculate out the performance of your power bank plus you’ll be given the best knowledge before buying a new product.

### The Mathematical Formula

Here’s the secret formula:

**The labelled capacity on the power bank x 3.7 / output voltage of the power bank x 0.85 / the battery capacity of the of the drive = total number of charges for the device.**

**Examples Using the Formula**

For example, the iPhone X has a battery size of 2,716mAh which is the largest out of all the iOS devices.

With the formula, you would be able to expect up to 4.6 recharges from 20,000mah power bank. Obviously, older models will have much higher recharges than newer models.

**(20,000×3.7 / 5) x 0.85 / 2,716 mAh = 4.6 recharges**

For the new Galaxy S9 (3,500 mAh) I’ll give another example. I’ll use the same example for a 20,000 mAh power bank with an output of 5v.

**(20,000 x 3.7 / 5) x 0.85 / 3,500 mAh) = 3.5 recharges**

Rule of thumb: Divide the mAh of your power bank by the mAh of your device battery.

If you’re struggling to grasp the concept of the formula, you can always use the more simple concept above, however, it won’t be as accurate as using the formula.

### What do the Numbers Mean

Here’s an explanation of what everything means:

3.7 – The first multiple is the 3.7. There are different levels of voltage that a battery cell can have but the average voltage of a battery cell is 3.7v. Li-ion batteries that feature manganese also average at 3.7 volts. Li-ion batteries that feature cobalt average at about 3.6v.

5 – The next digit that you multiply by is 5. The number 5 is the average voltage output of a typical power bank. Different power banks have different amounts of output.

For example, a power bank that supports fast charge is going to be different to the one that only supports USB. If you already have a power bank, this should be on the packaging in the description. If you are able to find out what the output of your power bank is, you can change the equation and replace the 5 with the output of your power bank.

0.85 – In the equation, the first 3 digits in the brackets are multiplied by the 0.85. All power banks aren’t 100% efficient when giving power to devices, instead of giving 100% efficiency to the device, power banks lose some of their efficiency through heat due to energy transfers.

They also lose some energy through the light.

The average efficiency given directly to your device is roughly 85% due to the wasted voltage which is why you use 0.85 in the equation.

This varies between different chargers so remember it’s an average. If any manufacturers advertise their power banks to be 100% efficiency, this is **NOT** true.

Acknowledments go to *Kk Ezekiel* from Techzillo.com