Powerbanks are portable battery backup devices that are designed to supply electrical power to charge small electrical devices. These include smartphones, wireless Bluetooth speakers, LED lights, etc. There are Powerbanks that are even capable of charging laptops these days.

There are so many misconceptions and myths about power that this author can spend a whole day debunking. But we do not have all day.

How do powerbanks work?

A powerbank is essentially a battery
with a circuit board that is designed to charge other devices. Every good
powerbank should have a circuit board with a chip that can intelligently monitor the battery, ensure safe charging, and modify or cut off the power supply if need be.

Every powerbank that uses wired charging conforms to the USB 5V standard. The batteries inside a powerbank have a standard voltage of 3.7V. When a device is plugged into a powerbank, the current is stepped up from 3.7V to 4.2V – 5V. This stepped-up current is then delivered to the device being charged.

Read: Smartphone chargers

Powerbank capacity

Due to this conversion of battery
voltage from 3.7V to 5V, a good amount of electrical power is lost, mostly as heat. As a result, powerbanks cannot deliver their full advertised capacity.

Powerbank manufacturers advertise the capacity of their batteries when it is stored at 3.7V. When checking a power bank’s capacity, it could be written as 30,000mAh at 3.7V or 18,000mAh at 5V.

Powerbanks (Romoss and Xiaomi)

When 3.7V is converted to between 4.2 and 5V, the amount of current that the powerbank can deliver falls drastically. There is a very simple formula to convert this.

Actual capacity = Advertised capacity (mAh) × 3.7V x 0.8 ÷ 4.2V

For a 20,000mAh powerbank,

  • Actual capacity = 20,000 × 3.7 × 0.8 ÷ 4.2V
  • Actual capacity = 59200 ÷ 4.2
  • Actual capacity = 14,000mAh (approx.)


Actual capacity = Advertised capacity × 3.7V ÷ 5V

For a 20,000mAh power bank,

  • Actual capacity = 20,000 × 3.7 ÷ 5
  • Actual capacity = 74,000 ÷ 5
  • Actual capacity = 14,800mAh

If you cannot remember these formulas, then always remember this. A powerbank can only deliver approximately two-thirds (2/3) of the advertised capacity.

This means that:

  • A 5,000mAh powerbank would deliver approximately (5,000 x 2/3=) 3,400mAh.
  • A 10,000mAh would give approximately (10,000 x 2/3 =) 6,700mAh.
  • A 30,000mAh can give out 20,000mAh approximately.

How much charge can a powerbank give?

Now that we have seen how powerbank capacity is being calculated, how much charge a powerbank gives out, especially to a phone is not an exact science. If a phone is used while being charged, work done could be equal to zero.

If the phone was left alone or even switched off, then the powerbank would accurately charge its battery properly. The age of the powerbank and its charge capacity is also another factor to look at.

Powerbanks tend to lose capacity as they age. A 2-year-old 20,000mAh powerbank may only have 10,000mAh left.


Read: How to charge your phone

Do powerbanks destroy a phone

Yes and no. A good powerbank has no negative effect on your battery. It should come with a good circuit board to ensure safe and quality charging.

A bad powerbank on the other hand would destroy your battery. This is why it is important to buy powerbanks from reputable brands instead of cheap generic brands from roadside vendors.

How to choose a good powerbank

The first step is to always do research. A lot of people are too lazy these days to do research. Read reviews and learn to distinguish between real and paid reviews. The manufacturer’s brand name and reputation matter a lot here as well. Names like Anker, Aukey, Baseus, Romoss, Xiaomi, etc. should come to mind.


Check the power bank’s specs (input,
output, capacity, etc.) as well. If a spec looks too good to be true, it probably is. Someone once showed me a 100,000mAh powerbank. I laughed it off and said it was a scam. The bigger the capacity, the heavier the powerbank. Always do your research before splashing your cash.

Thank you for reading to the end. As always, ensure to check out our links for more information and…