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How to test the capacity of a power bank

How to test the capacity of a power bank

When it comes to testing the actual capacity of a power bank, there are several different methods, but two of them are the most convenient:

  • using a USB multimeter.
  • using a smartphone app.

For most of us, the story begins this way:

We purchase a power bank, and after using it for a while, we realize that its capacity is considerably less than expected. To make matters worse, we don’t know what the actual capacity is, so we blame the company and maybe even post a negative review. Ouch!

This is how most people learn the hard way that the listed power is actually different from its real capacity. From a technical point of view, the main reason behind this disparity is represented by power losses; however, we cannot overlook the business-savvy nature of manufacturers, who disguise the real capacity to boost their sales. For a more in-depth analysis, check our “Why is there a difference between rated and real battery capacity?” article.

However, this time, we present you with an easy guide on how to test your power bank capacity so that you know exactly how much power it’s outputting.

So how do you test the actual capacity of a power bank?

There are several methods you can use to test the capacity of your power bank. One of them involves cutting the end of a USB cable, insulating its green and white wires, and then attaching a 5 Ohm resistor to the black and red terminals (1 and 4).

When connected to the power bank, a current of 1 Amp or 1,000 mA will be drawn at a nominal voltage of 5V for as many hours as the advertised capacity divided by 1000. The idea is to track time and monitor the voltage with a multimeter until it drops to 3V, and finally, multiply the total number of hours by 1000mA to obtain the real capacity.

Although effective, this method seems complicated for the regular user to try it at home. Thus, we’ll explore other alternatives that are both more simple and accurate, but which also require you to buy a USB multimeter or download an app.

Testing a power bank capacity using a USB multimeter

In case you don’t know what a USB multimeter is, here’s a brief description:

This small and quite handy device, placed between the power source and the device you wish to charge, measures parameters involved in the charging of your gadgets, such as voltage, current, and power readings. It doesn’t require any battery to operate; it’s the wall charger or power bank which feeds it instead.

There are many types of multimeters in terms of ports, with different inputs and outputs to adjust your needs. While some have USB standard ports, others have USB Type-C, but regardless of the kinds of port you need, one thing you must always take into account is the voltage and the current range; otherwise, it could damage and affect your devices.

Our picks:

PortaPow Dual USB Power Monitor with Data Blocker

The PortaPow Dual USB Power Monitor with Data Blocker can measure and display the output voltage (V) and current (A) from any USB socket or micro USB cable, as well as the total current that has been transferred (mAh). It supports measurements between 3.5 to 7V and 0 to 3 A, with an accuracy of +/- 1%.

It can be used for power consumption measurements, to test if your charger (or portable charger) is working normally, or finding out the power adapter that will charge your phone the fastest. The best thing is that it’s not expensive, selling for $14.99 on Amazon.

PortaPow Dual USB Power Monitor with Data Blocker

  • USB 1.0 port that blocks data transfer and charging.
  • USB 2.0 port that allows data transfer and charging.
  • 4.0A (4000mAh), 3-20V Range.
  • Resolution: voltage 0.1V (>10V), 0.01V (<10V), current 10mA.
  • Error: Voltage <± 1%-; Current <± 1%
  • Compatible with Qualcomm Quick Charge 1.0, 2.0 & 3.0.
  • 9 different memory positions to store mAh values.
  • High-quality LCD display to monitor data lines.
  • Built-in extension cable.
  • micro USB input port.
  • 1-year warranty.

Last update on 2020-10-24 / Affiliate links / Images from Amazon Product Advertising API

AT35 USB 3.0 Meter

With the AT35 USB 3.0 Meter, you can monitor the charging process of USB devices in real-time, as it displays many indicators involved such as voltage, current, capacity, energy, power, and temperature, to name a few. It comes with a convenient LCD screen which allows easy reading of multiple data thanks to the use of different colors.

Not only AT35 USB 3.0 Meter works as a USB multimeter, but it also recognizes and indicates the fast charge protocol of the power source connected to it. Thus, for just $14.98, you will get a great piece of equipment that will help you know how well your power bank works and how much outputs.

AT35 USB 3.0 Meter

  • USB 3.0 port that allows data transfer and charging.
  • Measurement and Quick Charge Recognition functions.
  • Voltage measurement range:3.700-30.000V.
  • Current measurement range: 0-4.0000A.
  • Energy accumulation range:0-99999mWh-999.99Wh.
  • Power measurement range:0-120W.
  • Error: Voltage ±(0.1%+2digit); Current ±(0.2%+4 digits).
  • Quick charge recognition mode: QC2.0, QC3.0, Apple 2.4A/2.1A/ 1A/0.5A, Android DCP, Samsung.
  • 0.96-inch IPS high-definition color LCD display.
  • 180-degree rotate screen function.
  • Dimensions: 64mm x 22mm x 12mm.
  • Micro USB switchable load included (1A/2A).
  • 6-month warranty.

Last update on 2020-10-24 / Affiliate links / Images from Amazon Product Advertising API

The first method to test a power bank capacity

This method consists in discharging the power bank using a 1 Amp USB dummy load, which is nothing more than two resistors placed on a regular USB connection. The great thing about this method is its accuracy since it doesn’t take into account the losses caused by the 3.7V/5V voltage conversion that occurs between the power bank and a smartphone.

For this method, you’ll need:

  • A completely charged power bank.
  • A USB multimeter (AT35 USB 3.0 Meter).
  • A USB cable.
  • A USB dummy load of 1A.

Firstly, plug the male port of the AT35 USB 3.0 Meter to the power bank you wish to test. You’ll see 5V but a current reading of 0 Amps as there’s no device drawing current.

Then, connect the dummy load to the female port of the multimeter. Now, you will read current in the multimeter because it’s flowing from the power bank to the dummy load. At the same time, the multimeter will also display the amount of mAh consumed.

Now, you’ll have to wait for several hours until the power bank depletes completely. Then, you will have the final mAh measurement, which is the total power bank capacity. Should you want to convert it to Watt-hours (Wh), simply multiply the mAh by the output voltage, and divide the result by 1000, as it shows the equation.

 Capacity in Watt-hours = final mAh measurement x Output Voltage / 1000

The second method to test a power bank capacity

With this method, you can both get an estimate of the total capacity of a power bank, and at the same time, knowing its output. It involves charging your power bank from 0% to 100%, measuring the current throughout.

Here’s what you’ll need:

Firstly, plug the PortaPow Dual USB Power Monitor to the USB power supply (either a charger or a power bank) through the in-built cable.

Next, using a USB cable, plug the power bank you want to test to the USB 1 port at the bottom of the device. Make sure that the power bank is completely dead before the test. At this point, the display will show the current flow from the power source to the dead power bank, so take note of such reading and begin tracking time.

Then, continue charging for a few hours, and when the power bank is almost at 100%, take note of the last Amp reading. This is why we suggest using a camera to record the process.

Lastly, with the last measure and total charging time,  you can calculate the actual capacity using the following formula:

Capacity = Average current flow (mA) x total charging time (h)

Average current flow = (initial current reading – last current reading)/2

To get a more accurate number, try taking several measurements of the current to calculate the average.

Testing a power bank output using a multimeter app

If you don’t have a USB multimeter or want to spend on one, then you could install a multimeter app on your smartphone. This is a relatively easier way to test how much power your portable charger delivers, as it does not require other connection than the one between your power bank and your smartphone.

Considering that there are different operating systems, we’ve decided on an app for Apple users and another for Android users.

Our pick for Android devices: Ampere by Braintrapp

Ampere by Braintrapp is a useful app to measure the charging/discharging current of the battery. It has a simple interface that shows several metrics such as the charging status battery percentage, maximum current, minimum current, voltage, maximum capacity, temperature, and even the model of your phone and the Android version installed.

However, one of the downsides of this app is that not every smartphone might support it if not equipped with the right measurement chip or the interface. Besides, the developer has stated that the app is “not meant to be mA accurate”, but the readings are good enough to assess the performance of chargers and USB cables on a given device.

Our pick for Apple devices: Ampere by CrioSoft LLC

Likewise, Ampere by CrioSoft LLC is a user-friendly app that measures values to help you check wired and wireless chargers. The charge test consists in providing information on Watts, Amperes, and Voltage. Additionally, the app also displays the thermal state and t battery charge alarm, which will let you know the charging status of your battery level and more.

test power bank capacity

Final Thoughts

Overall, anyone can get the accurate measurement of the current their devices are receiving with the help of a small USB multimeter or apps, allowing you to calculate the actual capacity of your power bank. Knowing how much power your power bank outputs will help you estimate how long you can rely on it, so we highly recommend testing the power bank charging a dumb load, as it’s the most accurate method.

Nevertheless, we believe the options on our list are not the ultimate since there are many multimeters and apps out there. Just remember to pick a multimeter with the right ports for your power bank and smartphone, and to choose an app that is supported by its operating system.