How to: Setup FrSky Telemetry for battery voltage readouts and warnings.

How to: Setup FrSky Telemetry for battery voltage readouts and warnings.

Posted by Lee McKenzie on 28th Dec 2015

Telemetry doesn’t have a whole lot of use on racing quads as we don’t run GPS and shouldn’t need to know altitude as all the racing is close proximity and quite close to the ground, however there is data included in telemetry output that is extremely useful if you aren’t flying with an OSD that can indicate how much juice you’ve drained from your high performance battery. You can also setup your Taranis to audibly tell you when you’ve drained your battery to a specific level, reminding you to bring your ship in to land. In the long run, preserving the life and performance of those expensive racing LiPO’s.

A couple of advantages of using telemetry over using a buzzer or low voltage checker is that:

  • You can hear low voltage warnings even if your quad is too far away to hear the buzzer as the warning comes straight from your radio.
  • You will never lose a low voltage cell checker in a crash ever again. Something I used to do quite frequently as I don’t fly with an OSD.

Each pro has its associated Cons too:

  • Another point of failure on your quad. More wiring to break in a crash, another plug to come loose.
  • Adds a tiny bit of weight (1 wire and teeny weeny plug)
  • Since RC5 & 6 are used for SoftSerial on the Naze it means that you cannot use SoftSerial and addressable LED lights at the same time.
  • More work in the long run requiring setup etc.
  • If there is lots of background noise, you can struggle to hear the Taranis. However you can plug headphones into the Taranis.

Setting up the D4R-II or X4R-SB is easy once you know what you’re doing. I spent countless hours sifting through false information on the internet as well as soldering and de-soldering wires etc but managed to problem solve my way to getting Telemetry to work, the software side of things is where all the complexity is so let’s get started.

We will assume you have a D4R or X4R-SB receiver already installed and receiving radio commands with PPM or SBUS.

TIP: Before we start setting up Telemetry on the Naze 32 and D4R-II with Taranis X9D Plus, please make sure your sound files are version 2.1 files and not 2.0 files. It took me a very long time to figure out why the audio reading from my Taranis was around 0.6v different to what was displayed on the screen. As soon as I updated my sound files, the correct voltage was read back to me.

Prerequisites: Taranis X9D or X9D Plus with OpenTX 2.1.6 (older versions will have different menu layouts but should still work), D4R-II or X4R-SB connected to a Naze 32 Rev6 and bound running Betaflight v2.1.6 firmware.

NOTE: This tutorial was put together usingOpenTX 2.1.6 flashed to the Taranis. If you are running the standard FrSky OpenTX firmware you will see completely different menus and options and this tutorial will be inaccurate. I would advise backing up your settings and flashing OpenTX using the OpenTX companion software. (Open TX runs well on the Taranis and flashing is easy to do, however do so at your own risk)


Step 1: You will need to directly connect your raw battery voltage to the Naze 32’s VBAT terminal. Connect the positive and negative wires accordingly to both your battery line or PDB and to the Naze VBAT pads. VBAT on Naze32 Rev6 can handle up to 35v (6s) and has a built in voltage divider. (the D4R-II and X4R-SB only accept readings from 0-3.3v for telemetry, hence the need for a voltage divider which the Naze 32 conveniently has already)

It must be RAW battery voltage and not a regulated or divided voltage. 

There is no reverse polarity protection so please ensure you double check your connections.

  Naze 32 FrSky Telemtry

Step 2:

For D4R-II: Connect the green wire from the Telemetry plug to receiver input 6 on the Naze 32. We don’t need the other 3 wires.

For X4R-SB: Connect the green wire to receiver inputs 5 & 6, you can split the wire and solder to both or bridge the two inputs. We don’t need the other 3 wires.

Excuse my dodgy looking D4R-II receiver.

Step 3: Go into Cleanflight, Configuration tab, enable VBAT (under the Battery Voltage Tab) and set your values the same as the following screenshot (or to whatever values you prefer). And then scroll down to Other Features and enable Telemetry and soft serial. Hit save and reboot. (warning: soft serial may not enable if you have LED Strip enabled and vice versa so just double check these options)


Step 4: Connect a battery and then move to the Setup Tab, double check that a voltage is displaying within the info box to the right. If not, you’ve missed a step somewhere, or you have a bad solder connecting your VBAT wires.

Now is the time to check that this voltage matches the actual voltage of the battery. Disconnect the battery and use a multimeter to test the actual voltage, then reconnect to the quad. Double check the figure in the info box is the same reading. 

If not you may need to adjust the voltage scale setting under Battery Voltage in the configurator until they match. The Naze will automatically detect how many cells the battery is so no need to configure the amount of cells.

Step 5: Move to the Ports tab and you should see two soft serial ports.

For D4R-II - Enable soft serial 1, change the telemetry drop down to FrSky and leave all other settings as is.

For X4R-SB - Enable soft serial 1, change the telemetry drop down to smart port and then check serial RX. Leave all other settings as is.

Hit Save & Reboot

Step 6: Head to the CLI and type set telemetry_inversion = on then hit enter to ensure the signal inversion is turned on. (not sure if this is the default value or not)

Then type save and hit enter to save settings and reboot.

Now you should have Telemetry being sent from your receiver!

Now onto the second half, and this is where you configure the settings in your Taranis to accept and display the Telemetry info.

Please make sure you have installed the latest firmware for your Taranis X9D plus along with OpenTX companion, As well as make sure the SOUND files are updated to the most latest version. If the sound files aren’t updated, the audible voltage readouts won’t match what is shown in the display.

Step 1: Check that the Taranis is picking up the correct Telemetry data by short pressing the menu button to go into the Model selection menu (select your model), then long press the page button, which will take you to the Telemetry menu.

Scroll down to where you see sensors, plug a battery into your quad to power it up, then select Discover New Sensors. At this point you should see around 20 sensors appear including the VFAS and Cels sensors. If you only see SWR, RSSI, A1 & A2 sensors, your Telemetry isn’t configured properly. Either softserial hasn’t been configured properly or the green Telemetry wire is connected to the wrong receiver input.

Now click Stop Discovery.

Change the Voltage Source setting to VFAS

Step 2:

Now you can scroll down to Screen 1 options and select what you would like to display on the telemetry screen. (accessed by going to the home screen and holding down the page button for a few seconds)

VFAS is the one we want for battery voltage monitoring, the rest is up to your personal preferences.

Step 3:

Head to the Special Functions menu, here we will assign a switch to audibly read out the current battery voltage on demand. (assigned to a switch)

Select an empty SF channel and setup the SH switch to Play Value,VFAS. The Taranis will then tell you what the Battery voltage is when you flick the switch up momentarily. You can assign any switch to do this, I just found this the most suitable option for me.

Step 4:

For audible low battery warning we need to head to the Logical switches menu to setup a rule that will enable the switch to activate once the VFAS value gets below a certain point.

To do this select an empty logical switch. In my case it was L1. Input the following values from the following screenshot. I have it setup for 4S, you will need to lower the value for 3S. This rule sets it up to a logical switch to activate as true when the battery voltage drops below the assigned value and will only do so if the duration is for longer than 1 second. The duration may need to be adjusted dependent on how much your battery sags under load.

Step 5: Then the final step is to setup the audio for the Logical Switch rule we have created.

Head back to the Special Functions menu where you will assign the previously setup logical switch (in my case L1) to play a track letting you know when the battery is getting low. In the following photo you will see that the L1 logical switch I created is set to Play the Track batlow when it is activated by the battery voltage dropping below 13.8 for more than a second.

The end…. that was a fairly comprehensive setup guide, but it is designed to clear up a heap of confusion if followed correctly. Once you understand the setup it becomes fairly simple to replicate into other models and aircraft.

Any questions or comments feel free to share in the comments box.......

Until next time.