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The Secrets of Power Bank Capacity

Table of Contents

Most people think a 10000mAh power bank can charge 4 times for a 2500mAh mobile phone.

However, you will find the power bank is empty before “the expected condition.”

Today, let us explore why it happens.

Understand Real Capacity Of Power Bank

Usually, the power banks are made with 3.7V lithium-ion battery cells. And the printed capacity is based on 3.7V as well.

Nevertheless, 3.7V power cannot move when charging. Because devices like mobile phones are also with 3.7V battery cells. So in most conditions, the power banks will boost to 5V with chips when setting. 

Power Bank Mobile Phone

Then the supplied capacity needs to be calculated as below.

Supplied Capacity = (3.7V * Advertised Capacity) / 5V

Take 10000mAh as an example. The result is 7400mAh. But it is not the final result. There is another thing to consider – power loss.

Understand Power Loss

The power loss is related to 2 things. 

1. Conversation rate of PCB

When boosting the power from 3.7V to 5V, it is with the PCB. And the power is not 100% converted. 

According to market information, the top conversation rate is 93%, like Xiaomi and Morui. 

On average, it is 80%-85%.

2. Internal resistance of charging cable

We can see the sweet cable resistance is under 0.20Ohm per meter from the testing video, and the bad one is over 0.95Ohm. 

Generally, we can consider the total efficiency rate is 80%. Therefore, the real capacity of a 10000mAh power bank is only 5920mAh. So you can estimate that 3/5 of the theoretical capacity is its actual capacity.

Conclusion

When you like to purchase a power bank, it is better to buy one with a bigger capacity than the device has. But it is not the only thing. Actually, it should be careful about efficiency rating and power conversion too. 

Ryan
Ryan
Hi, I’m Ryan Gao, a father of 2 kids, also the content manager of MoruiPower.com. I’ve been working on consumer electronics for 8 years now. On this site, I like to share with you the knowledge related to power banks, wireless headphones, and portable power stations from a Chinese supplier’s perspective.

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