RD USB Meter UM25C


Official specifications: I got it from RD official store

RD makes a couple of electric products, including a few models of usb meters. This meter is a high precision one with Bluetooth build in, it includes USB-C connectors.


I got the meter in a small metal can


It contained the meter and a inlay with a link to the application.




The meter has four buttons one for each corner of the display. There is also two USB-C connectors, one for input and one for output, they are parallel with the usb A connectors.

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On this side is a micro usb connector in parallel with the main A input connector.
On the Bluetooth model there is a sliding switch to turn Bluetooth on/off.

Display and functions


The meter has a couple of screens, here is the voltage, current and summations (mAh, mWh), together with load impedance and load power. There is also a temperature display showing the internal temperature of the processor. The groups (0-9) can be used to save the summations temporary, group 0 will automatic clear when a new recording is started, the other groups must manually be cleared.
To change between screens use NEXT or PREV buttons.


The two top buttons will rotate the display when held down, it can be rotated to all 4 orientations.
The functions of the buttons will rotate with the display.


One of the top buttons (the one with ?) will show a simple help screen when pressed, it will change depending on screen shown.
The other top button (with a z and a moon) will turn the display off/on.

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This screen shows voltage and current, in addition to voltage on data lines and fast charge mode (data lines are often used to select fast charge mode).
With a QC charger at 12V both data lines will be at about 0.6V


This screen is the capacity (mAh) and energy (mWh) and over how long time they are collected. There is also a setting for when to start recording, here it is at 0.1A, but it can be adjusted.


This screen is for measuring the resistance of a cable and connections. It requires a constant current load is connected to the output and then first recording the voltage and current without a cable and next with a cable. With these measurement measured the resistance will be calculated and shown.
If the test current is too high the display will turn off, not the best way to report an error!

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This screen is used for charting the voltage and current or the voltage for the two data pins, the update rate is fixed.


The configuration screen


The voltage display is very precise with no current flowing.


The current display is also very precise and will not change with voltage.


M1: 45.2C, HS1: 51.5C


M1: 46.8C, HS1: 52.2C

The above IR photo was taken after 30minutes with 5A, during that time the readouts did not change..

Android application The Bluetooth interface makes it possible to connect to a Android phone or a PC with Bluetooth interface.

Screenshots Screenshot2s Screenshot3s Screenshot4s

The interface basically shows all screens (Except cable test) at once and makes it possible to send the four button presses to the usb meter and change configuration on the meter.
The chart can be zoomed in/out and scrolled sideways.
With the globe it is possible to select a different languages (The list is long than shown above because it can scroll).


It is possible to save the data to the memory in the phone, the format is an Excel datafile, but always with the same name, i.e. it is not possible to save multiple data sets.

PC application The Bluetooth interface makes it possible to connect to a Android phone or a PC with Bluetooth interface.


Installation is both the application and some libraries from National Instruments and it is, of course, the NI part that takes up all the space.
A supplied font file is not automatic installed. This is the reason for the slightly large letters in all the screen shots.


When started the application shows 0 values in all fields. To start it select the virtual com port that is assigned to the Bluetooth device and press connect.


The display is one page with all information on it. It is possible to send commands and change configuration on the meter.


A right click on the chart makes it possible to export the data, either as image or as data.


Here is the BMP version of the image (I have converted to PNG).


The data export to clipboard with TAB delimiters, this is not nearly as nice as the format used by the Android application.

Tear down


It is very easy to open, there is four screws on each side.


The processor (U3: STM8S005C6) only has a 10 bit ADC, this means there must be an external ADC (U4: marked CAMW) and it looks like there is a chip (U6: marked A801) to multiplex between voltage and current from the shunt (R2: 0.01ohm). The circuit also has a 3.3V regulator (U2: M5333B).
I wonder why the big capacitor, it might be because the processor only saves values when the power goes low (The EEPROM can "only" handle 100000 writes).




On the display side there is space for another chip, but it is obvious not used.




The bottom is the Bluetooth module, including the switch. It has its own voltage regulator (U2: M5333B).
There is also a mux (U6: marked A801) here, it is connected from the data in on the Bluetooth module to the RXD pin.
There is no parts mounted below the display.




This is a very precise usb meter with both capacity and energy summation and can show the most common fast charge protocols. The cable resistance checking is a nice feature and the screen illustrates how to do it very nicely, but turning the screen off to show error is a bad idea.
I am not very impressed with the PC application


For these USB meters I used precise equipment (Keithley: DMM7510, 2280S, Keysight: 34470A).

The tester was supplied by RD for review.

How do I make the test