A Better Low Battery Alarm (Taranis)

by Kurt0326 | March 28, 2016 | (5) Posted in How To

A while back, when I first got my Taranis, I stumbled on to this tip on YouTube. Unfortunately I cannot find the video anymore. I shared it on the forum thread “Fr Sky Taranis Chat. Questions/Answers/Share” a couple years back and wanted to share it again.

When flying electric R/C, as we all know or should know, it is important to not over discharge your lypo batteries for risk of fire. Or to protect your investments in your batteries, since over discharging damages lypo batteries and shortens battery life; as Josh Bixler is well known for. In the past, all we could do was set a timer to land, which can be highly unreliable. Now thankfully, we can add low voltage alarms to help. For which I highly recommend to everyone.

The beauty of the Open TX is it gives use complete freedom to virtually control anything one could imagine in you radio. With the advent of the FrSky Telemetry, it also gives us a cheap way to monitor your flight. Utilizing both, this setup will help you manage your throttle in relation to your aircraft lypo voltage right down to each cell while in flight. Because the more throttle you give the more your battery voltage drops, due to the increased draw on the battery.

For this, I use a FrSky Lypo Voltage Sensor in conjunction with a telemetry enabled FrSky smart port receiver.

This monitors one 1-6 cell battery. If you have more than one battery, you will need one sensor for each. I have not tried more than one at a time. So I’m not sure it will work.

NOTE: This same setup can be done without this sensor. All you really need is a telemetry reading from your flight battery. If you only get the total battery voltage and not individual cell voltage, then use "Cells" or "VFAS" and not "Cell" and set your battery limits as you like. If you do not know what to use, go back to your telemetry screen and see what readings you get. I like using the individual cell voltage readings because the reading is more accurate since each cell voltage can vary.

NOTE: If you are not getting a telemetry reading using Open TX 2.1, you have to tell the radio to search for new sensors. 

WARNING: Uninstall props or set a "Throttle Hold" in your radio before this step.

If you are not getting a telemetry reading:

  1. If you are using a flight controller, make sure your telemetry settings are enabled.
  2. Plug in your aircraft and arm it to fly if needed.
  3. Go to screen #12 (Telemetry Screen).
  4. Find "Discover new sensors" and click on it.

Still having problems? Try this video.

That should do the trick. Don't forget to unplug battery and remove throttle hold or reinstall props if need.

CAUTION: I do recommend calabrating you battery voltage with your telemetry reading if you can. That will vary with the setup you are using so I will not be going through those steps in this artical

NOTE: I am not currently using Open TX 2.1 so screen shots may vary.

Please use this as a template and alter it in any way you like to suit your needs.

First we setup the alarm switch using the LOGICAL SWITCHES:

I am using the last 7 lines in the logic switches to keep it separate from the rest of my setup that controls my flight. It’s good to separate different setting groups. So you don’t get confused and mess things up later.

  • Lines 26 and 27 will separate your throttle in to three sections “0-32%, 33-66% and 67-100%”.
  • Lines 28 and 29 set single cell voltage limits of the throttle sections set in lines 26 and 27. That’s L28 for 33-66% and L29 for 66-100%
  • Line 30 sets the lowest voltage limit you decide in the 0-32% throttle section.
  • Line 31 ties switches 28 and 29 in to one switch (L31).
  • Line 32 ties switch 30 and 31 to make one universal switch for low voltage. This is what we will use as our alarm switch.

Feel free to add or remove as many throttle sections as you which, just do not forget to make a voltage limit for each section and tie them to one universal switch.  Or feel free to setup two alarms. Maybe  one for "Low Battery" and one for "Critical Battery" (battery damage/fire possible).

WARNING: Uninstall props or set a "Throttle Hold" in your radio before this step.

If you can not calibrate the telemetry reading, you can:  

  1. Discharge a flight battery to a level you deem to be low.
  2. Use a calibrated volt meter and take a reading straight from the battery plug.
  3. Plug in battery to your aircraft and get a voltage reading from telemetry and write it down.
  4. Compare readings and reset low voltage +/- the difference between the voltage actual voltage and telemetry voltage.

Next we will setup the alarms using SPECIAL FUNCTIONS:

So pick two lines I happen to use SF 8 and 9.

So we will set if L32 is true a horn will sound every 10 seconds and a warning description every 20 seconds. Once again, use what sounds you like. Or create your own unique sound, distinct from others on the field.

So that is the basics for a varying low voltage battery alarm. I hope this helped you in some way. If you use this or some other variations, please rate and leave a comment. I would love to hear the differences. If you think this is the stupidest idea or anything between, please rate and leave a comment. I won’t be offended. Thanks for reading!


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A Better Low Battery Alarm (Taranis)