# How many times can I recharge a cell phone with a power bank?

Posted by Carlos Sun on

Do you know how many times you can recharge your cell phone with a 10000mAh or a 20000mAh power bank?

A very common mistake is to divide the capacities of both devices. For example, if you have a smartphone with a 2500mAh battery capacity and you have a 10000mAh power bank, you would calculate and then get a result like below:

Number of Charges = 10000 mAh / 2500 mAh = 4 charges

THAT’S TOTALLY WRONG!

Because the capacity indicated on the power bank is referred to the capacity of its internal cell or battery, while the effective output capacity of a power bank at its USB output port is very different from it. Let me further clarify.

A power bank is composed of internal cells or batteries of a certain capacity (mAh) with a nominal voltage (usually 3.7 V), which is called Rated Cell Capacity. With it, we can know the Rated Cell Energy that is calculated by multiplying the Capacity(mAh) by the Nominal Voltage of its internal battery:

Rated Cell Energy (Wh) = [Rated Cell Capacity (mAh) x Nominal Voltage (V)] / 1000

For example, the Baseus Bipow 10000mAh has a capacity of 10000mAh and a Nominal Voltage of 3.7V. So its Rated Cell Energy is:

10000 mAh x 3.7 V = 37000 mWh = 37 Wh

Next, let’s consider the output capacity.

When a power bank charges a device, it needs to raise its voltage and works at a standard voltage of 5V. In an ideal condition,

Ideal Output Capacity (mAh) = Rated Cell Energy / Rated Charging Voltage (V)

Taking the Baseus Bipow 10000mAh power bank as an example, then the result is:

37000 mWh / 5V = 7400 mAh

As you see, the Output Capacity 7400mAh is already lower than the Power Bank Capacity 10000mAh.

Moreover, during the process of charging a device, the electronic circuitry of a power bank consumes energy to raise the nominal voltage of the battery to the voltage of the USB output port (5V rated voltage). In addition, led display and other functions would also consume some of the energy. The amount of energy loss basically depends on the design of the electronic circuit and the quality of the electronic components.

Therefore, the Effective Output Capacity, which is also called Rated Capacity, is very different from the Ideal Output Capacity and can only be measured by a meter in real condition. Usually, the manufacturer must test the factual performance of a power bank before manufacturing and list out its specifications.

If you take a close look at their specifications, they vary among different brands. Some nameless power banks even have no such specifications. It’s highly recommended to acquire those specifications from the seller before making a purchase.

For instance, above is the photo of Baseus Bipow 10000mAh power bank. Here we know that its Rated Capacity is 5800mAh. With it, now we can calculate the Rated Energy of a power bank that is calculated by multiplying the Rated Capacity(mAh) by the Rated Voltage of its output port:

Rated Energy  =  Rated Capacity  x  Rated Voltage

Let’s continue using Baseus Bipow 10000mAh power bank as an example, that is:

5800mAh  x  5V =  29,000 mWh = 29 Wh

Now, we can figure out the Conversion Efficiency of a power bank at its output port, which is the ratio between the Rated Energy and Rated Cell Energy:

Conversion Efficiency  =  Rated Energy  /  Rated Cell Energy  x 100%

So, the Conversion Efficiency of Baseus Bipow 10000mAh power bank is:

29 Wh / 37 Wh * 100%  = 78.4%

Similarly, we can also figure out the Conversion Efficiency of Baseus Bipow 20000mAh power bank and 30000mAh power bank :

60 Wh / 74 Wh * 100%  = 81.1%

90 Wh / 111 Wh * 100%  = 81.1%

It’s reflected in its specification -  Energy Conversion Rate ≥ 75%.

Basically, the conversion efficiency of power banks is around 60% - 85%, then higher, the better. When considering its price, Baseus Bipow Banks are quite competitive.

In short, when you choose a power bank, you should not only look at its capacity and brands but also take a look at its conversion efficiency. The higher, the better. DO NOT buy those power banks without the specification of Rated Capacity and Conversion Efficiency.

Here's another example of a power bank in my hands. Though its capacity reaches 20000mAh, it only has a rated capacity of 5000mAh, and 30% conversion efficiency, much lower than the Baseus Bipow 10000mAh battery.

And let's have a look at this Xiaomi power bank 10000mAh. It has a rated capacity of 5500mAh, and 75.8% conversion efficiency, better than the previous one but a bit lower than the Baseus Bipow 10000mAh battery.

Up to now, is it enough for us to calculate the numbers of charging my cell phone with a power bank? Could I get the result by dividing the Rated Energy by Rated Cellphone Energy?

Unfortunately, NO!

Though we’ve already known the rated energy of the power bank at its output port, some energy loss still happens on the transmission(cable) and the process of storing the energy into your smartphone’s battery. As you’ve ever noticed, when you charge your cellphone, it becomes warm, which means some of the energy turns to heat.

It seems impossible to get the accurate and uniform data as cellphones are different, but we can use an approximate ratio to make a general calculation.

Approximately, 15% of energy becomes heat during charging a cell phone and 85% of Rated Energy turns energy stored into the cells or battery in your phone. Since the nominal voltage of the cells or battery of cellphone is 3.7V, first let's calculate the Cellphone Battery Energy below:

Cellphone Battery Energy  =  Cellphone Battery Capacity  x  Rated Voltage

When your phone battery is 2500mAh, the energy is calculated like below:

2500mAh  x  3.7V =  9,250 mWh = 9.25 Wh

And up to now, we can calculate the number of charges:

Charges =  [ Rated Energy *  85% ] /  Cellphone Battery Energy

So, when you use the Baseus Bipow 10000mAh power bank to recharge your smartphone with a 2500mAh battery, the number or charge is:

[ 29 Wh  x  85% ]  / 9.25 Ah = 2.7

Yes, instead of 4 charges, it's only 2.7. Now, do you know how to calculate the number of charges? Check the specifications of your powerbank and cellphone, and make a try.

To sum up, in order to figure out the number of charges, you should find out the Rated Energy of the power bank in its specification, instead of the capacity. And I would highly suggest that do not only focus on the capacity of the power bank, as a smart buyer, you should also take a look at the Rated Energy and the Conversion Efficiency, only choose those quality products instead of those cheap products at an attractive price while using inferior components and materials to reduce its cost as well as its quality.