Amperage, watts, volts and mAh: a handbook to understand more. Power bank voltage

What Are Portable Power Banks?

Let’s get right to it. If there aren’t any outlets to plug into, what do you rely on for power? How do you charge the device you have and plan on continuing to use? The answer to that question is quite simple and is an increasingly popular industry, Portable Power, more specifically, Portable Power Banks.

If you want to stay powered you must be outfitted with a reliable source of reserve power that comes in a compact and easy to carry package. We are going to go over the most frequently asked questions and explain what portable power banks are.

What Are Portable Power Banks?

If you want to make an informed purchase, the best practice is to understand the basic terminology. A portable power bank is a battery which resides in a special case that has a specific circuit that controls power flow.

Much like a bank account where you deposit you hard earned cash and withdraw it later, a power bank allows you to store electrical energy and then use it later to charge your device. Before we get into what types of power banks there are, let’s understand the power that is stored within these units.

What Does mAh Stand for?

While these units come in all shapes and sizes, they also vary in power capacity, much like the variety of smartphones on the market.

The term you most often seen while researching these units is mAh. It’s an abbreviation for “milliampere hour,” and it’s a way to express the electrical capacity of smaller batteries. The A is capitalized because, under the International System of Units, “ampere” is always represented with a capital A. To put it simply, the mAh rating denotes capacity for power flow over time.

Now that we know what a power bank is and what the power that resides in them is, let’s take a look at what types of power banks there are.

What Types of Power Banks are There?

While there are various styles and brands that cater to a user’s specific needs, you can break down the types of portable power banks into three categories.

Universal Power Banks

These come in many sizes and configurations that cater to you device requirements. Some may feature both an AC outlet and USB ports plus USB-C, while others may only have a single USB option.

Solar-Charged Power Banks

These have photovoltaic panels which can charge the internal battery of the pack when placed in sunlight.

Battery Case (for your phone)

These typically attach to the back of your phone and, while convenient, they have narrow device compatibility. Portable power banks for your phone are typically smaller in power (mAh) and will only recharge your phone a couple of times before requiring a recharge of its own. Here are some of the top portable power banks according to Android Central.

How Many Times Can a Power Bank Charge my Device?

Here’s a question we see frequently and that can be answered with a formula as shown below.

(Labeled capacity of the power bank x 3.7 / output voltage of power bank) x 0.85 / battery capacity of device = Total number of recharges.

Now, that you have the formula all let’s try an example. We can use a 40,200 mAh power supply and an iPhone 8 which has a battery capacity of 1821 mAh.

(40,200 mAh x 3.7 / 5) x 0.85 / 1821 mAH = 13.8 recharges

First we have the labeled capacity for the power bank – 40,200 mAh

Second we multiply that by 3.7. This number is the average voltage you will find in a battery cell. We get this number normally by finding the midpoint between a fully charged cell and an empty cell.

Third step is dividing 148,740 (sum of 40,200 x 3.7) by 5. We divide by 5 because this is the average voltage output of a typical power bank.

We continue on to the fourth step which is multiplying 29,748 (sum of 148,740 / 5) by 0.85, which is the average efficiency of a power bank.

Yes, this means you will never get 100% of the total expected output.

Finally we divide 25,285.8 (the sum of 29,748 x 0.85) by the battery capacity of the desired device you are trying to charge, in this case the iPhone 8 is 1,821 mAh.

The final sum comes out to 13.8 and that is how many times you can recharge an iPhone 8 with a 40,200 mAh portable power supply.

There you have it. You now know what a power bank is, what power (mAh) is stored in a power bank, and how to calculate how many recharges your power bank can supply to whatever device you decide to use. If you have any questions feel free to leave a comment below and reach out for additional support. Keep an eye out for future posts!

Amperage, watts, volts and mAh: a handbook to understand more

How many of you in your life have lost at least one smartphone battery charger? raise your hand! And how many of you don’t know about amperage and voltage? Are you by chance waving both hands? Well, let’s make a wave of solidarity!

Losing or breaking a battery charger is a rite of passage, especially for travellers. But replacing it isn’t always that simple, unless you know a little about electricity. The same difficult problem arises with the purchase of a power bank suitable for charging your tablet or smartphone. The impulse purchase can mislead you, leaving you with a battery that completes fewer charge cycles than you would have expected or that takes much longer than expected.

How can we solve these problems?

The answer is a small handbook with some key electrical engineering concepts that will help you in choosing batteries and power banks.


Power is the first term encountered when talking about electricity. When the light goes out in the house it happens because you have exceeded the power threshold set by your contract. But what does power mean?

Very often it is confused with capacity, another concept that we will see shortly. Actually the power, which is measured in Watts (W) or kilowatts (kW), helps to understand how much energy your device consumes. The one indicated on the labels of your appliances and devices is the maximum power: it therefore gives you an approximate idea of consumption, which is always less than that figure.

In mathematical terms it is the product of Voltage and Intensity:

Let’s see what the other two terms of this product correspond to.


Volt (V) is the unit of measurement for voltage, a word that indicates the “strength” of the current needed to operate a device or charge a device. We could compare it to the pressure exerted by water in a pipe.

In order not to damage the battery of your device it is important to keep in mind the maximum voltage indicated by the manufacturer and always use a charger or power bank with the same value. Usually the output voltage for smartphones is 5V.


On the manufacturer’s indications, immediately after the voltage, the Intensity value is indicated: also called Amperage, it is the quantity of energy that is transmitted and measured in Ampere (A). To return to the hydraulic metaphor, it corresponds to the flow of water passing through the pipe, in relation to its diameter.

If precision on voltage is fundamental, with regards to amperage it is not necessary to be so rigorous. The difference in intensity turns into a lower or higher charging speed. Using a higher amperage, but with the same voltage indicated by the manufacturer, there is no risk of damaging the battery. In fact in many cases it is the device that determines and limits the amperage necessary for charging. Charging time changes from device to device.


A key feature for a good power bank is its charging capacity. Put simply: how many charge cycles does it allow you to do? The indication to look for in order to obtain an answer is the data expressed in milliampere-hour (mAh), which indicate the quantity of Amperes or milliamperes transferred in a given period of time.

The higher the mAh number the longer your phone battery will last, and the more charge cycles your portable battery will do.

The more modest power banks can accumulate 1000-2000 mAh, the larger ones up to 22,000-30,000 mAh. Together with the capacity, however, size and weight also increase, which can even reach 300 gr.

Purchase recommendations

So here are our tips in pills:

  • Write down the electrical information of your original battery charger, so at the first loss you will be facilitated in choosing a replacement.
  • The voltage must necessarily be the same, the amperage not necessarily: in this way you will not damage the battery of the smartphone.
  • For a good power bank, check the milliampere-hour, so you will guarantee multiple charge cycles.
  • Where to find this information? Normally on the original charger adapter of your phone.

For a truly appreciated corporate gift, you can find all the best in portable electricity in the Power Lot model – a metal Power Bank with a high charge capacity: the battery starts at 17,600 mAh and reaches up to 24,000 mAh. The USB port has an intensity of 2A. The shell is available in satin silver, satin gold and black versions and can be customised on both sides by silk-screen printing and laser engraving.

Battery capacities explained: Here’s how much charge your power bank really has

Now that we’re apart more often than we’re together, smartphone battery life has taken on new importance. If your smartphone’s battery dies, your ability to communicate with friends and family — and therefore your social life — goes with it, or at least is paused until you can recharge your phone. Pick up a charging case or a power bank and you can ensure your phone battery never dies — and keep your social life on course too.

Of course, charging cases and power banks may not always operate exactly as their specs or marketing would suggest. While a typical power bank or charging case might come with an advertised capacity of 20,000mAh, this doesn’t mean that it can recharge your Motorola Edge Plus or Samsung Galaxy S21 Ultra exactly four times. In practice, its real (i.e., transferrable) capacity is likely to be around two-thirds of this, meaning that it can recharge a smartphone with a 5,000mAh battery only twice (or two and a half times) before needing a charge itself.

In this article, we explain how exactly charging cases and power banks work. Most importantly, we provide a general rule of thumb for estimating how much real power they actually have, so that you have a more realistic idea of what to expect from your power bank or charging case. Hopefully, this should help your phone live a longer, fuller life.

Battery and power bank capacities explained

Power banks are basically portable batteries for your smartphone. If your iPhone or Android is running low on battery power, you can use a power bank to recharge it. As such, they’re great if you travel often or are on the go frequently.

However, a 20,000mAh power bank does not mean you can transfer exactly 20,000mAh to your smartphone in a single charge. In practice, your phone will get less out of your power bank than 20,000mAh. In general, your power bank can transfer around two-thirds (66%) of its own battery power to your smartphone. and there are two main reasons for this.

amperage, watts, volts, handbook

Reason 1: Power banks output at 3.7 volts, while due to USB technical standards, smartphone batteries charge at 5 volts. This creates an imbalance between the output of the power bank and the input of your phone.

For example, if your power bank has a capacity of 20,000mAh, multiplying this by 3.7 will reveal that it has total energy — as measured in mWh — of 74,000mWh. However, it will need to output at 5 volts to charge a smartphone, so dividing 74,000mWh by 5 — to convert back into mAh — will equal a smartphone battery charge of 14,800mAh.

That said, in practice, your 20,000mAh power bank won’t even provide a total smartphone battery charge of 14,800mAh, because there is one more factor that reduces its real total output.

Reason 2: Inefficiency in the charging process also means that 20,000mAh of power bank charge equals noticeably less than 20,000mAh of smartphone battery charge. For instance, electrical resistance in the USB cable can reduce the total amount of energy transferred to your smartphone. Likewise, a certain percentage of the power bank’s energy may be converted into heat during the charging process, which obviously means it isn’t stored as energy in your smartphone ’s battery. On top of this, such environmental factors as the temperature may affect charging efficiency.

In total, inefficiency can be expected to reduce your power bank’s transferrable energy by around 10%. If your 20,000mAh power bank can be assumed to have 14,800mAh of real, transferrable power, inefficiency will mean that it actually has 13,3200mAh of power it can transfer to your phone in total.

As a result, a 20,000mAh power bank can actually charge a 5,000mAh smartphone around 2.66 times before needing a recharge itself. Of course, this figure will vary according to your power bank or phone, so expect more recharges depending on whether you have a bigger power bank and/or a phone with a smaller battery. In general, as we’ve said above, multiplying your battery’s capacity by 3.7, dividing it by 5, and reducing it by 10% will give you a rough estimate.

Charging cases

Much the same goes for charging cases. They also output at 3.7 volts, while, again, smartphone batteries input at 5 volts, due to USB requirements. When combined with the inefficiency factor, they end up actually providing around 66% of their advertised charge.

That said, they may be slightly less inefficient than power banks, since they don’t require the use of cables and can instead plug directly into your phone. On the other hand, due to size constraints, their batteries are generally smaller than those of power banks, so you’re likely to get less juice out of them overall.

Charging Terminology

In the charging tech world there are terms that are used to describe things that may not be so clear to understand. It’s important to know what these words mean because they are heavily related to the quality and the traits that a charging product may have.

Let’s get started on these terms that you should know the meaning of when you read them, hear or see them on articles, reviews or product pages.


An acronym that is short for Milliamp Hour. It’s the measurement of the capacity for a battery. Primarily used to tell the capacity of power banks and smartphones, there are many other devices that use the same kind of battery capacity measurement.

Example: The capacity of the iPhone 6 is 1,810mAh. The capacity of the Anker PowerCore 5000 is 5,000mAh. The PowerCore 5000 can charge your iPhone 6 about 2.5 times. You’d just subtract the capacity of your phone from the power bank you’re using until the power bank reaches a capacity of 0mAh. That’s how you know how much charges you can get from a power bank.

The higher mAh for a battery, the better because it’s the capacity of the battery.


Amps refers to the current or energy rate that is being transferred to the device that is being charged. Amps can refer to the Output or Input charging.

There’s a range of Amp currents for different charging products.

1.0A means that there’s a 1.0 Amp of charging speed. 1.0 is the slowest charging speed and should be avoided with most charging electronics.

Standard Amp charging for chargers are 2.0 Amps and 2.1 Amps. The highest rate of amp charging that you can get from standard charging technology is 2.4 Amps.

The next level of charging is 3.0 Amps. 3 Amps of charging goes into the category of Quick Charge charging capabilities.

Then there’s the epitome of charging Amps which is 4.0A. charging speed of 4 Amps is currently achievable with the One Plus Three. A phone that uses a 4.0A charging output.


Voltage refers to the stability of charging speed for a charger. The base for most recharging electronics are 5 Volts. If there are more Voltage options for a charger, then that means the Amp current for a charger can stay at the same speed for a longer time.

A charger with 5 Volts and a output of 2.0 may be at 2.0A charging speed but will also regress down to 1.0A if the Voltage of the charger decreases at anytime.

amperage, watts, volts, handbook

A charger with Voltage capabilities of 5V/10V/15V and a output of 2.4A, can stay at a Amp current of 2.4A for a longer time and can stabilize at a higher Amp current for a longer period of time.

Just because a charger has a higher Amp current does not mean that it will charge at that speed constantly. Voltage is equally important.


Outputs refers to the charging that is being done to the external device.

Example: The output of the charger is – 5V / 2.0A

If a smartphone is connected to a power bank, then that is Output charging taking place. This means that the smartphone is charging at 5 Volts and 2.0 Amps.


Input refers to the charging that is being done to the charging device itself.

Example: The Input of the charger is – 5V / 2.1A

amperage, watts, volts, handbook

If a power bank is connected to a wall charger via a Micro USB cable or whatever cable is connected into the Input port then that is Input charging taking place. Input for a power bank refers to the rate of recharging for the Power bank or another device that holds power as its sole purpose of holding energy, like a Battery Case.

Quick Charge

Quick Charge charging refers to the technology from Qualcomm. It’s a fast charging technology that is able to charge compatible smartphones at 3.0 Amps.

Notice how we said, “Compatible Smartphones”. Quick Charge works with smartphones that have certain Qualcomm processors. Learn more about Quick Charge technology here.

Smart Charging

Smart Charging refers to the detection charging technology that many tech companies now specialize in. Smart Charging is able to detect the device you have and charge it at the fastest possible charging speed.

IP Water Proof Rating

IP in terms of water proofing refers to rating of how much of a device is water proof.

IP66 – The product is dust tight and can withstand protection against water shot out from a nozzle.

IP68 – The product is dust tight and can withstand protection against complete water exposure.

Know more about enclosure safety rating for products here.

Designing a Power Bank (Part 2/9)

As the popularity and use of smartphones and tablets have grown, the demand for portable and hands on power supplies have also increased. The smartphone and tablets come with a battery which gets discharged in 4 to 5 hours of use. As a solution to this problem, power banks have been introduced in the market for the frequent users. These power banks also come to resort when the user is on a long journey and has no facility to charge up his phone or tablet. A power bank is basically a portable device which can supply power to the gadgets like smartphones and tablets through the USB port. Power bank itself can be charged by USB port and stores charge which later can be used to power up other devices.

In this experiment, a power bank will be designed which can provide a 5V/4A power output. The power bank will be constructed using a 3.7V Li – ion battery and will have a charger circuit built using TP056 IC and power booster circuit at the output. The Li-ion battery will store the charge and then the stored charge in the battery will be used to supply power to the devices. For storing the charge, the Li-ion battery first itself needs to be charged using a charger circuit for which TP056 IC is used. This IC is commonly used for charging the Li-ion batteries. The IC is specially designed to charge a single 3.7 V Li-ion battery and can provide maximum charging current of 1A.

The mobile phones and most of the electronic gadgets need 5V to power up but the Li-ion battery will provide a maximum voltage of 4.2 V. Therefore, a power booster circuit will be needed which can amplify the output power to 5V. For amplifying the power stored in the battery XL6009 regulator IC is used which will boost the DC power from the battery to a regulated 5V DC. The XL6009 provides maximum 4 A current at the output (as per its datasheet). Therefore, the power bank designed in this electronics project will provide an output power of 5V / 4 A.

How the circuit works –

The circuit of the power bank has two building blocks – 1) Battery recharge circuit and 2) Output amplifier circuit. If the output voltage required would have been 3.7 V or 4 V only, the amplifier circuit would not have been required. But, the required output voltage is 5V, that is why amplifier circuit at the output of the device is a must. As per the circuit sections, the device also operates in two stages – 1) Charging of the battery and 2) Taking the output from the battery through amplifier circuit.

1) Charging of the Li-ion battery with TP4056 charger

In this electronics project, a 3.7 V Li-ion battery is used to store the charge which is fully charged when its terminal voltage reaches 4.2 V. As any battery charges, the voltage output across its terminals keeps on increasing. Every battery has a peak terminal voltage value for which the battery is fully charged. So, the charging percentage of battery is also estimated by measuring the terminal voltage. The Li-ion battery needs to be handled carefully as the battery may catch fire due to overcharging. Therefore for charging the Li-ion battery, special ICs like TP4056 IC are used which automatically disconnect the battery from input supply as the battery is fully charged.

The TP4056 is a specially designed IC to charge the 3.7 V Li-ion batteries. This is a linear battery charger controller with constant current and constant voltage. By adding a single programmable resistor the IC can be used to charge a 3.7V Li-ion battery. The charge voltage is fixed at 4.2V and charging current can be set by adding some resistor and capacitor according to the battery to be charged. The IC also provides internal thermal protection and current limitation. There is no need to add extra blocking diode due to internal P-MOSFET which blocks the reverse current.

The TP4056 IC comes in SOP package which makes it ideal for use in portable devices. It also requires less external components, none other than few resistors and capacitors. The IC has 8 pins with the following pin configuration –

The IC needs minimum 4V to 8V voltage for its operation. It can provide maximum 1000mA charging current to the battery and a fixed 4.2 V at the output. The circuit as given in the datasheet of the IC is used to design the charger.

Fig. 4: Circuit Diagram of TP4056 IC based Power Bank Battery Charger

For deciding the value of charging current of the battery a R prog resistor has to be connected at the PROG pin as described the function of the PROG pin in the datasheet.

For 1000mA charging current Rprog can be calculated as follows –

The battery should be connected as per polarity indicated on the IC because the TP4056 IC does not have any reverse polarity protection circuit.

o Battery Charging Indicators

For visual indication of charge termination and charging state of the battery, LEDs can be connected to pin 6 and pin 7 of the IC. When the input supply is provided to the circuit then the Red LED at pin 7 lights up which will indicate the charging state of the battery. When the battery voltage will reach to 4.2V then the battery will draw less current. The charging current when drops down to 1/10 th of the programmed current (1000mA) then charging will be terminated. The Green LED at pin 6 will light up and give a visual indication that the battery is fully charged (as terminal voltage has reached 4.2V).

2) Drawing output from the battery through Voltage amplifier and regulator circuit –

Once the battery is fully charged by the TP4056 charger circuit, it is ready to supply output. The output voltage from Li-ion battery needs a boost converter which will increase the output voltage of the battery to 5V.

A boost converter is used to convert the input DC signal to the higher voltage level. The XL6009 regulator IC is used for boost converter circuit which provides regulated and amplified voltage. This boost converter amplifies the signal to around 1.6 times the input signal from the battery with an efficiency of 94%. The XL6009 is a DC to DC converter which is capable of generating either positive or negative output voltages with the input voltage in the range from 5V to 32V.

The IC has built-In N-Channel Power MOSFET and fixed frequency oscillator which allows providing a stable output over a wide range of input voltages. The IC is specially designed for use in automotive Boost, Inverting converters, Notebook car adapters, and portable electronics equipment. The IC has features like frequency compensation, thermal shutdown, current limitation and soft start. It comes available in the T0263-5L package. The XL6009 will work on input supply voltage of.0.3V to 36V and can provide an output in the range of.0.3V to 60V. The IC has five pins with following pin configuration –

The circuit specified in the datasheet of the IC for typical boost converter application is used in this project.

Note: You can find the XL6009 Boost Converter Circuit under the “Circuit Diagram 2” tab.

At input and output of the regulator, the capacitors (Cin and Cout) are used which reduces the unwanted ripples and noise from the signal.The Cout provides a regulated and smooth DC voltage at the output. A small value of the capacitor 1uF (C4) is also connected in parallel with the high-value capacitor Cout to reduce the ESR (equivalent series resistance) at the output (as high-value capacitors have high ESR).

The inductor connected between pin 3 and 4 plays an important role in the boost converter. The main function of the inductor is to store the current. The higher the value of the inductor, the higher will be the current stored in it but a high-value inductor also has an increased size. So an inductor should be selected which can provide the desired current at the output. In the project, an inductor (L1) of 47 uH and a Schottky diode (D3) are used. The SS34 diode is chosen as this diode has a less forward voltage drop and works fine in high frequency. A list of suitable Schottky diodes for the IC according to the current demand and input voltage can be found in the data sheet of the XL6009 IC. For the convenience, the table is precisely repeated below –

Internally the XL6009 has N-channel power MOSFET with fixed frequency of the oscillator (as in below fig 4). This MOSFET acts as a switching transistor and oscillator which generates a square wave of around 400kHz (as per the datasheet). During the positive half-cycle of the square wave, the inductor stores some energy and generates a magnetic field so the left terminal of the inductor is a positive voltage and right one is at negative. Therefore the anode of the diode is at lower potential and acts like an open circuit.

The base of the MOSFET gets positive voltage and the MOSFET turns ON. So all the current from the supply flows through the inductor to MOSFET and finally to the ground.

During the negative half cycle, the MOSFET turns off. Due to this, the inductor does not get a path to charge up. Now the current at the inductor generates a back emf (as per the Lenz law) which reverses the inductor polarity (as shown in below image). Therefore the diode gets forward biased. Now the charge stored in the inductor starts discharging through the diode and a regulated voltage is obtained at the output.

In this case, the output voltage now depends upon the stored charge in the inductor, the more the stored charge, the more the output voltage is. Therefore if the charging time of inductor is more then the stored charged in the inductor also increases. So there become two sources of voltage as input, one is an inductor and another one is input supply. Therefore there is always an output voltage greater than the input voltage.

For setting the 5V at the output of XL6009, an external resistive voltage divider circuit is used at feedback pin (pin 5) of the regulator IC (as in below image). This feedback pin senses the output voltage and regulates it.

As the internal feedback threshold voltage of XL6009 is 1.25V. This means there is a constant voltage at pin 5 and a constant current will flow through R4 as well as R5. Therefore, sum of resistor drop across R4 and R5 gives V out as

By putting R4 and R5 values in above equation

The output voltage is not exactly 5V because any device which requires 5V not exactly operates on 5V. It needs some higher voltage than the 5V due to some resistive losses and drops in the device. For boosting the input signal to 5V, any boost converter modules available in the market can also be used. Like XL6009 booster board are also available which gives a constant and regulated voltage of 5V at the output.

Testing –

After connecting all the components together, 5V is supplied to the TP4056 IC which starts charging the Li-ion battery. The output voltage of the battery acts as an input to the boost circuit. So, the input voltage at boost circuit/Li-ion Battery voltage, V in = 4.2V when the battery is fully charged. The boost circuit amplifies the input and give output voltage of 5.18 V. Now by connecting different load at the output, different values of load current are observed which are as follow –

The power bank designed in this project can be used to charge any electronic device which needs a regulated 5V and 4A maximum current for its operation. The power bank designed in this project has efficiency around 94% and due use of suitable ICs has Internal overvoltage and thermal protection. The power bank is automatically cut off from the supply as it is fully charged and has the LED indicators for the charging and full charge indications. It should be taken care that the battery should not be discharged by connecting a load, during the charging process of the battery. Simultaneous charging and discharging can reduce battery life and can harm TP4056 IC.

Circuit Diagrams

Questions related to this article?Ask and discuss on and forums.

You must be logged in to post a comment.

Leave a Comment