I had already tested various voltmeters and USB ammeters like here, here, here or even here. They do their job the right way, but very quickly, we notice the limits of these multimeters fairly quickly: no registration, so no tracking, no indication on resistance, power, limited ports, etc… 
Features of Ruideng UM34C
- Bluetooth 2.1 + EDR
- Switch on/OFF for Bluetooth
- 1.44-inch color display, 5 brightness levels, 4 rotations
- Editable colors
- 3 Male and female USB Port, up to 4a of intensity
- C-Type USB Port
- Android/IOS Application available: 18 languages + file Export + Remote module control
- Voltage measuring range: 4 to 24v
- Resolution of voltage measurement: 0, 01V
- Measuring range of intensity: 0 to 5a
- Resolution of intensity measurement: 0, 001A
- Range of accumulation capacity: 0 to 99999mAh
- Accuracy of voltage measurement: ± (0.2% + 1digit)
- Energy accumulation Range: 0 to 999, 99Wh
- Measuring range of Resistance: 0.8 ω to 9999.9 ω
- Time measurement range: 0 to 99h59 min 59s
- Temperature range:-10 to 100 ℃
- Quick charge recognition Mode: Quick Charge 2.0, QC 3.0, APPLE 2.4 A/2.1 A/1a/0.5 a DC Android DCP SAMSUNG
The UM34C + LD25 RD Test
The unit is pretty well packed in an aluminum box and the tester is protected by foam inside. No manual, to have a manual, it is necessary to scan the QR code present on the end of carton. (By the way, I also point out that the QR code of the LD25 module is located on the fan… It took me a while before I noticed…) I'm talking about 2 different modules, because when I bought it, I had the 2 at the same time, they often go together. The UM34C serves as a tester, intensity meter, voltage, resistance, capacity… While the LD25 module is a "USB intelligent electronic Load", basically in French, I would say that it is an unloader that allows to control precisely the intensity. What is the LD25 for? To see if a cable is good quality or not, as well as to check if your battery has a good output intensity. And I have to say that these 2 tools bring a lot of information about your external battery, the type of port that your external battery uses, your cable, your device, the type of fast charge or not and if so, of what type, etc… 
And this information is accurate of course and reliable (I tested with different tools and everything fits) for someone who has never used these tools, this can be a bit complicated, but with some basic electronics reminders and some Tests here and there, we are quickly mastering the 2 devices and we highly appreciate all the data that can be harvested!
- Is it the device to load that is problematic or is it the external battery that gives the current that is problematic?
- I have 5 USB cables, which one is the best?
- 2 USB ports on my external battery, which is the fastest?
- Does my battery really have the capacity that the firm claims? It seems to me that it does not last long…
- Is the intensity stable?
- What is the state of charge of my battery?
- Etc…
These are examples of questions to which these 2 modules can be answered in an encrypted way! The finish is very good, the UM34C module is protected while letting you see part of the components and you can see that the welds are very well made. The LD25 module is much less protected, you just have 4 plastic feet to elevate the module, otherwise you have access directly to the electronic card. We can see that the welds are also very well made. 
Conclusion
I do not regret at all the purchase of the 2 modules! They have helped me and will help me a lot in diagnosing various problems regarding batteries and USB cables! And I have to say that before I had this kind of module, I was a little scared because I did not know, especially the LD25 that was completely obscure, but in the end, the 2 are very easy to use and bring much more data compared to the simple voltmeters Usb! The ability to harvest and see real-time data as well as to drive the remote module via Bluetooth is also a great advantage! Link: 
https://www.fasttech.com/products/0/10053144/9655580-ruideng-um34c-digital-usb-3-0-multimeter-ld25 