Most power bank manufacturers lie about how many times their power can charge certain devices.

Or do they? Well, it turns out that you can actually calculate this yourself without being deceived by other power bank manufacturers. 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.

You’re probably wondering how you’re meant to create these numbers?

The Mathematical Formula

As we all know maths isn’t the most fun subject, however, you may disagree with me. Although you may not like the subject, it’s important to understand how manufacturers are able to calculate how many times their power banks will be able to charge certain devices such as the iPhones or Samsung devices.

Here’s the secret formula:

1. 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.

Note: you cannot use this formula if you are charging your device with a USB-C PD port in a charging port. This is because the output changes according to the phone’s input needs. It’s not set like other power banks. However, you can use 5V as a reference.

Examples Using the Formula

Math teacher

If this confuses you, you probably want a few examples to go with this.

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.

(20,000x3.7 / 5) x 0.85 / 2,716 mAh = 4.6 recharges

For the 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

At this point, you may understand everything or you may not.

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.

Specific Recharge Times

Here are some specific recharge times if you haven’t got time to be doing the math. I’ll be using 2 popular capacities, 10,000 mAh and 20,000 mAh.

So a 10,000 mAh power bank fully precharged can fully charge your iPhone:

iPhone 7:  (10,000×3.7 / 5) x 0.85 / 1960 mAh = 3.2 recharges

iPhone 8: (10,000×3.7 / 5) x 0.85 / 1,821 mAh = 3.4 recharges

iPhone X: (10,000×3.7 / 5) x 0.85 / 2716 mAh = 2.3 recharges

Galaxy S8:  (10,000×3.7 / 5) x 0.85 / 3000 mAh = 2 recharges

Galaxy S9:  (10,000×3.7 / 5) x 0.85 / 3000 mAh = 2 recharges

So a 20,000 mAh power bank fully precharged can fully charge your iPhone:

iPhone 7:  (20,000×3.7 / 5) x 0.85 / 1960 mAh = 6.4 recharges

iPhone 8: (20,000×3.7 / 5) x 0.85 / 1,821 mAh = 6.8 recharges

iPhone X: (20,000×3.7 / 5) x 0.85 / 2716 mAh = 4.6 recharges

Galaxy S8:  (20,000×3.7 / 5) x 0.85 / 3000 mAh = 4 recharges

Galaxy S9:  (20,000×3.7 / 5) x 0.85 / 3000 mAh = 4 recharges

Note: These examples also work for devices such as tablets and cameras.

As you can see with the 20,000 mAh power bank, it charges the devices twice as fast as the 10,000 mAh power bank. If you have a new device that uses a large battery, you’re going to need a power bank with a large battery capacity.

Factors that Affect Recharges

Before you find out how many times your device can be charged, you need to know the factors that affect recharges so you can recharge your device as much as possible.

1. Using Your Device – If you are using your device for entertainment, e.g. you’re watching a movie on Netflix. It’s going to appear as if your device never actually charged.

Excessive use of your device, while it’s connected to the charger, is going to suck the power out your device. You’re effectively removing the charge on the battery as quickly as you’re charging it.

2. Not Using Your Device – Another reason why your recharge time will be different may also be because of how little you’re using it. For example, you’ve left your phone on airplane mode while it’s been charging.

If this is your approach when charging your device, you’re going to get a lot more recharges out of the power bank. This is because the device isn’t using much battery power while it’s being charged.

3. Age of the Battery – When using lithium polymer batteries, you need to know that the capacity of the battery also reduces with age.

For example, a 10,000 mAh power bank you bought today may only have a capacity of 8500 mAh in a years time.

You can slow this process down by storing your power bank in the correct place. A battery that’s stored at high temperatures (such as 30 degrees) is going to deteriorate a lot faster than a power bank stored at a cooler temperature.

This doesn’t mean that you should store your battery in the freezer! Just keep it’s in a cool dry environment or keep it directly away from the sun. This is going to prolong the capacity of your power bank.

4. Efficiency of Your Battery – You also need to know that your device may have lost a few capacities over time. If your device is fairly old, (let’s say over 1 year) this would be the case.

When you charge your device to 100%, you may think that your device actually has 100% charge. In reality, your device is actually charged to 80% or 90% but it’ll appear as if it’s charged to 100%. Again, this is due to the condition and the age of the battery.