What is real power bank capacity? Why does power bank have lower mAh?

Hi,
Why do power banks have lower capacity than is written?

When I charge my phone with 10000 mAh power bank, it should give 2 fully charges, but it gives only 1 full charge and 1 almost full charge. The battery capacity of my phone is 4500 mAh.

What is the real power bank capacity? How to find out actual power bank capacity?

Power banks have 2 types of capacity:

• Nominal capacity (written capacity)
• Real capacity (actual capacity)

Real power bank capacity is lower than its nominal capacity, because there are multiple factors that cause some energy losses. Depending on power bank, a real capacity is usually around 80% - 60%.

Factors that lower power bank capacity:

• Voltage conversion losses - Power banks often store energy at 3.7 volts. When you start charging your device, the 3.7 voltage is converted to output voltage, that is usually 5 volts or more.
• Heat - Caused by voltage conversion and electricity flow. Higher temperature results in higher electical resistance, which causes higher energy loss.

Simply said, power bank can store a certain amount of energy, but amount of energy that power bank can finally deliver is lower.

Real power bank capacity can be quite high when power bank is new. But later, real capacity is decreasing, depending on how intensive do you use your power bank.