How to find out the real capacity of a Power Bank?

What is a Power bank?

A Power bank is an assembly of rechargeable batteries that can be used on-the-go to charge smartphones, laptops, tablets and many other devices.

Some of the common alternative terms used for a Power bank are Portable charger, Battery Bank, External Battery pack, Portable battery and so on.

The Common Assumption

Let's say you own an iPhone 12 that has a battery capacity of 2815mAh. You bought Zyron’s Travelite 10000mAh power bank hoping that it would charge your phone around 3.5 times (10,000mAh/2,815mAh = 3.55 times). You were surprised to see that the power bank hardly charges your phone 2.5-3 times.

This is because you are, yet, unfamiliar with the terms “Real Capacity / True Capacity / Rated Capacity” of a Power bank.

Let us go in depth using an iPhone 12 with 2815mAh battery capacity as an example. The guide below will explain how the real capacity of a portable charger is different from the real capacity.

Difference in Voltage

The rechargeable batteries inside a Portable Charger are mostly Li-ion or Li-polymer. The capacities of these batteries are measured using nominal 3.7V whereas the output required to charge your iPhone 12 is measured at 5V. Zyron’s Travelite has one 10000mAh Li-Polymer battery at nominal 3.7V. Therefore, when the Power bank converts the nominal 3.7V using a PCB board to the supplied 5V, the capacity is reduced as follows:

5V Capacity = 3.7V * 10000mAh / 5V = 7400mAh

This explains how the voltage conversion is important when you are buying a Power bank based on its capacity.

But that’s not it!

Power Losses during Charging

We have learned in our science class, if you were paying attention ;), that some amount of energy is lost during the process of energy transfer. Therefore, the process of conversion from 3.7V to 5V through the circuit leads to power losses. Also, when the power is transferred from the portable battery to your device, more losses occur during the process.

Overall, these losses are between 2-10% for good quality PCB boards and as high as 20% for low quality PCB boards.

The efficiency of a power bank depends on several factor like the quality of the battery of the connected device, type of cable and whether you are using the phone while charging (which results in higher losses).

The Actual Capacity of a Portable Charger

After taking into account the difference in voltage and the power losses, the true capacity of a battery bank can be easily derived using the principle below:

Actual Capacity = (3.7V * Power Bank Capacity * Efficiency) / 5V

If you are using Zyron’s Travelite or Camp-Pro power bank, then assuming a 95% charging efficiency under normal conditions, their actual capacities will be as follows:

Travelite = (3.7V * 10000mAh * 0.95) / 5V = 7030mAh
Camp-Pro = (3.7V * 30000mAh * 0.95) / 5V = 21090mAh

Number of Charges

So if you are using an iPhone 12 with a 2815mAh capacity, then you can calculate the number of charges as below:

Travelite 10000mAh = 7030mAh/2815mAh ~ 2.5 Times
Camp-Pro 30000mAh = 21090mAh/2815mAh ~7.5 Times

Conclusion

We have learned how the Real capacity of a Power bank is ascertained but keep in mind that these determinations are based on certain assumptions under ideal conditions for a brand new battery charger. It is inevitable that the batteries will start reducing in their efficiency and capacity as they grow old. This will result in a further reduced capacity. This concept applies to all batteries in general like our cars.
Remember, we shouldn’t use the device while it is charging as it consumes more energy. This results in higher losses and reduces the number of charges.