The Fnirsi FNB58 is a versatile, portable USB tester that can carry out a wide array of voltage, current, and energy measurements, in addition to specialized diagnostic tasks related to most Fast Charging protocols. Let’s try it out!
The FNB58 USB meter.


Featuring a variety of input and output ports (namely USB C, USB A, and micro USB), the FNB58 comes equipped with an 2-inch LCD screen to display measurements and offers several buttons for navigation through its menus. Additionally, it includes a micro-USB port for connecting to a PC and using Fnirsi’s software, and a Bluetooth option that enables use with an Android phone app.

What can you do with it?

The FNB58 can be used for an array of tasks related to measuring and testing USB devices. It enables users to measure power supply voltages and currents as well as assess the current consumption of various USB loads, including phones and laptops. There are also many features focused on various Fast Charging protocols.

In the past, if you wished to measure the current consumption of a USB device while in use, it required cutting a USB cable in half and using both a voltmeter and an ammeter. Then you had to painstakingly enter data in a spreadsheet if you wanted to deduce power or battery capacity. Now, with USB 3 and USB C devices becoming increasingly prevalent, cutting cables is becoming less and less practical. That's where tools like the FNB58 come into play.

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Features of the FNB58

The FNB58 has a 2.0-inch TFT LCD display with good viewing angles. Connection to a PC is possible via a micro-USB port, and there's also the option of utilizing a Bluetooth connection for wireless interfacing.

This device enables voltage, current, capacity, power, and time statistics. It is capable of recording both low-speed (2 to 100 samples per second) and high-speed ripple current and voltage waveforms (up to 4 MSps), providing comprehensive data capture.

The measurements are made internally using a 16-bit DAC. Notably, it features an impressive 10µV/µA/µW maximum resolution. This is great but as we know, resolution is not the same as accuracy! There will probably be noise and the last digits will not all be meaningful.

The instrument has triggers for various fast-charge protocols and is equipped with a dedicated USB-PD negotiation chip. A distinct PD switch serves as a critical safety feature, enabling or disabling the PD (and other protocols) negotiation directly by the FNB58.
FNB58 side view
Some of the input/output connectors + the PD safety switch.
When the switch is off, the device connected to the output will conduct negotiations with the source. When the switch is turned on, the FNB58 can initiate PD modes itself.

However, care is needed if you are using the FNB58 to measure current consumption of a 5V-only load, while powering it with a PD charger. In that case, unintentionally triggering a PD operation with the FNB58 would make the charger output a higher voltage and cause damage to the load.

So, Fnirsi recommends keeping the switch off, only turning it on for intentional PD operations with the FNB58.

The FNB58 supports a variety of Quick Charge protocols, including QC2.0, QC3.0, Huawei FCP, SCP, Samsung AFC, PD2.0, PD3.0, VOOC/WARP, SuperVOOC 1.0/Super VOOC 2.0, and MTK-PE, with automatic detection to ease functionality. It also identifies E-marker USB cables or One-plus DASH cables.

Measuring ranges

The FNB58, being a USB tester, has more restricted ranges than multimeters:
Voltage: 4 V – 28 V (can go down to 0 V when powered externally via the PC connection)
Current: 0 A – 7 A
Power: 0 W – 120 W
Capacity: 0 – 9999.99Ah
Energy: 0 – 9999.99Wh
Cable resistance: 0 – 9999.9Ω

Navigating the menus of the FNB58

The user interface features a three-way joystick (left-right-middle) and a separate back button for navigation in the menus. Fnirsi has densely packed these menus, making navigation a bit of an initial learning curve. An additional note: a long press on the BACK button will turn off the screen backlight at any page.
FNB58 side view
Buttons and PC connection.
Much like many instruments of this type, there's a "rolling menu" of four main pages, accessible one after the other using the left/right joystick. The four main pages are:
  • Compact view
  • Detailed view
  • Waveforms page
  • Applications page
Since these four main pages have many submenus, it might get a tad confusing. Here's a representation of the menus, along with some details about possible actions on certain pages.

1. Compact view: press the middle button to run/pause the measurement.
2. Detailed view: press the middle button to access the menu related to recording.
3. Waveforms page, featuring three sub-menus:
a. VBUS: displays high-frequency ripple voltage on the power line.
b. V/A: presents voltage and current waveforms in a rolling display format.
c. D+/D-: indicates the state of the data lines.
In these modes, the vertical scale is automatically set and is not manually adjustable. However, the horizontal scale can be modified from 2.5µs/div to 200µs/div in VBUS mode and from 0.1s/div to 5s/div in other modes.
Here are the possible actions:
  • Short press the middle button to run/stop the measurement.
  • Long press the middle button to switch between modes.
  • Long press the left button to decrease time/div.
  • Long press the right button to increase time/div.
4. Application page, subdivided into four submenus:
a. Fast Charge: Features automatic detection as well as various triggers.
b. Energy Statistics: Includes battery capacity calculation and offline records.
c. Toolbox: Houses numerous additional submenus, such as:
  •   Cable resistance measurement
  •   PD listener
  •   PD converter
  •   USB-C electronic label (e-marker detection)
  •   Read DASH cable
  •   Simulated DASH
  •   Analog Apple 2.4A
d. Settings: General, Recording, Trigger, System, and About.

Case Study 1: Evaluating the Performance of USB Chargers

Not all chargers are created equally. Here, I'm testing an older Apple charger that claims to deliver 2.4 A at 5 V—and indeed, it does. Conversely, another charger falls flat; it's labeled to deliver 1 A, but this seems to be an overstatement. When attempting to draw 1 A, the voltage plummets, dropping well below the broadly accepted lower threshold of 4.75 V. I had to add another cable to power the FNB58 through the micro-USB PC interface to snap a photo; otherwise, the USB meter would shut down.
apple charger.jpg
Apple 2.4 A charger under load
cheap charger.jpg
Cheap 1 A charger... Ooops!

Case Study 2: Testing the Resistance of USB Cables

It's generally acknowledged that lower resistance in a cable is preferable. However, using a multimeter to measure an USB cable's resistance isn't as straightforward as one might think. Even the poorest quality cables tend to have quite low resistance values in Ohms, typically around 1 Ω or 2 Ω at most. Thus, the contact resistance of your DMM probes becomes significant in your measurement. 

The FNB58 can measure the resistance of USB cables, but the way to do it is not obvious. You might assume that simply plugging each end of the cable into the corresponding ports on the tester would suffice, but that's not the case. The FNB58 measures the cable's voltage drop under load, but you have to provide your own load.

First, you need to calibrate the meter by plugging it directly, without the cable you want to test, between a power source (in this case, a phone charger) and a load. Ideally, the load should be constant. Fnirsi recommends between 0.5 A and 1 A. You could use a dedicated DC load module or appropriately sized resistors. Another option is to plug in a device that will draw some current, such as a phone recharging its battery, for example.

Here’s how the setup should look:
Calibration of the "Resistance Measurement" mode.
A short press on the middle button will store the voltage at the input connector. Then, you can change the wiring and add the cable you want to test between the power source and the meter, like so:
cable 142.jpg
Adding a cable between the charger and the FNB58.
Once you plug everything back together, the meter reboots, and if you navigate to the “cable resistance measurement” menu again, the meter will display the resistance introduced by adding the cable. In that case, 142 mΩ.

You can see that this one is pretty good too:
cable 44.jpg
A good USB cable.
This one is excellent:
cable milliohm.jpg
Nice low resistance here.
And this brand new USB-C cable is terrible:
cable 1ohm.jpg
Too much resistance in this cable.
No wonder my phone was charging so slowly! 1 Ω of resistance means that even at 1 A of current, which is small by today's standards, there will be a loss of 1 volt in the cable; the phone will only see 4 V instead of 5 V, which will be too low for the battery charging circuitry. The phone will detect this and will be forced to reduce the current it draws, thus taking much longer to recharge its battery.

This phenomenon can be a challenge when measuring cables with substantial resistance values, particularly when using a phone as a load. If the cable is of such poor quality that the phone must reduce its charging current, as described above, the source voltage will fluctuate, rendering the FNB58 unable to provide a meaningful result. Consequently, Fnirsi recommends employing a constant current load for this purpose.

PC Software for the FNB58

The manufacturer offers some software on its website. The main window of the PC app appears as follows:
FNB58 PC app.png
Fnirsi's PC software for the FNB58.
Real-time measured values, as well as voltage and current curves, are visible. The interface does have a few peculiarities that may require a bit of acclimation. I found that navigating through the curves was notably laborious.

I didn’t find there would be a significant need for PC software, except for making prolonged measurements and logging (for instance, to characterize batteries). You can use the PC app to set up such a measurement and then start it. Subsequently, the curves can be saved in a Fnirsi proprietary binary format, with the .CFN extension.

This is somewhat disappointing, as having the ability to save to CSV would be crucial for additional data processing. Fortunately, a GitHub user, didim99, has written some Python scripts that convert CFN files to CSV. Excellent!

It’s also worth highlighting another project by GitHub user baryluk. This project enables a computer to directly extract data in real time from the FNB58, bypassing the need to use the Windows software to record initially and save afterward.

Android App

The FNB58 can be purchased with a Bluetooth option. If you have this version, you can use the Android app, available as a .apk package on the Fnirsi website. To install it, you’ll need to download the ZIP file, unzip it, tap on the .apk file, and authorize the installation by selecting “Allow from this source” on your phone. The app supports multiple languages; tap on the upper right menu to switch from Chinese to your preferred language. You'll then be brought to this screen:
android app.jpg
Fnirsi's Android app for the FNB58.
I gave it a try, and although it functions, I wasn't convinced that it’s notably more user-friendly than either the standalone unit or the PC software.

Final Thoughts

Overall, the FNB58 stands out as a pretty comprehensive instrument, enabling you to make a myriad of valuable measurements, whether at home or in the lab. It sports a sleek and rugged design and boasts virtually all the functionalities you might anticipate from a USB tester of its kind. You’ll be able to effortlessly diagnose issues with chargers, cables, and power banks, as well as harness your creativity, using it for fairly precise DC measurements—assuming you stay within its limits (0 to 28 V, 0 to 7 A). This is achievable with the assistance of a few USB adapters (like USB to screw terminals), which you can either make yourself or purchase easily. Have fun!
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