How to know if the Power bank is real capacity as its marked?

Power Bank becomes necessary of Smart phone user. As end user, How do we identify its performance good or bad?

First of all to learn knowledge capacity. Power Bank’s marked capacity is based on internal batteries of nominal capacity, the actual output is less than the nominal capacity.
We take a case to explain and analysis the Capacity.

A friend, he bought a 6000mah Power Bank, so-called conversion rate can reach 90%, but why it’s not able to recharge 3 times of Ipones4s?

As we know, Iphone 4S’capacity is 1460mAh??ccording to the most simple arithmetic, 6000÷1460≈4.11.That is to say, 6000mah battery should be able to charging full of 4 times, but why is a far cry from actual situation? Let's talk about Power Bank conversion efficiency.

To briefly explain the charging principle of Power Bank. The current Power Bank using 18650 Li-ion batteries or polymer batteries, the nominal voltage is 3.7 V, and digital products such as mobile phone standard input voltage is 5V.Therefore, it need to boost to 5V when charging these mobile devices.

Real capacity after boosted voltage

According to energy conservation law: the 6000mAh Power Bank’s capacity is 6000mAH×3.7V=22.2Wh. After transform input voltage from 3.7V to 5V, the total energy is constant, but the capacity has changed into 22Wh÷5V=4400mAH. It likes put 3.7L water into 5L glass. After first conversion, the 6000mAh capacity remain 74%, namely, the 4400mAh is the reality capacity can provide to mobile phone.

Energy loss in charging

In the next step, we using the data cable connect Power Bank and mobile phone to charging. The question is, can be the 4400mAh capacity fully reach the battery in mobile phone? Answer is NO. In the processing of charging, there are many energy losing:

1. Circuit switching loss. As we all know, Power Bank generates heat when it is charging. According to energy conservation law, the heat is from the energy of internal battery.

2. Resistance loss. Power Bank’s battery linked with mobile phone’s battery through circuit board and wire when charging. As we know, all conductors(except superconductor) exit resistance. The main physical characteristics of resistance is changing electric energy into heat energy. Therefore, conductor can be generate heat energy when charging. And generating heat always result in energy dissipation and loss of battery.

3. The voltage loss. Lithium battery discharge voltage is not invariable, the safety of the discharge voltage range is 4.2 V to 2.75 V, the power conversion will have difference. Generally, discharge voltage range between 3.6 to 3.9 V, can release the battery 80% or higher power, and if outside this range, power consumption will be increased.

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4. Power supply loss. When Power Bank works, it not only charging the battery but also charging other components of mobile phone, especially the screen, which is the biggest power consumption.

5. Operation loss of Power Bank. When Power Bank works, it need power to keep operation, the power also provide by its internal battery.

Deduct above loss, then the capacity reach into mobile phone’s battery will be around 85% of 4440mAh, is 3774mAh. This 85% is what they called conversion rate of Powerbank, which is not the overall conversion rate. From the 6000mAh to the actual output 3774mAh, it loss about 37% capacity. Namely, the actual transfer efficiency just around 63%. The following picture can help us know how many times will be 6000mAh Power Bank charging for mobile phone.

6000mAh Power Bank real charging times

Formula: Charging times= Power Bank capacity ×3.7V×conversion rate(85%)÷5V÷mobile phone capacity