LiPo Watcher

Everyone does not have sensors to measure capacity in their models. But you would still want to get the most of your flights and not use a timer. Here is a simple solution, LiPo Watcher!

How LiPo Watcher works

LiPo-Watchers monitors your main battery voltage and calculates remaining battery-percentage from that.


Featured Functions

Simple but powerful features:

  • Calculates LiPo cell-voltage from main battery voltage or Rx voltage
  • Converts LiPo cell-voltage to remaining percentage with help of cell-count
  • Voltage is counted as average to prevent short voltage-dipping to affect result
  • User selectable alarm-point
  • Voice announcement when remaining percentage is under set limit
  • Bar color changes when voltage is under alarm-limit

Note: LiPo Watcher works in DC/DS-12/14/16/24 with firmware 4.22 and up

Important! Read Q&A in the bottom!


In short: Copy the (for 14/16) or (for 12/24) file from github here to your transmitters Apps-folder, copy the RCT-Li16.jsn or RCT-Li24.jsn file from Lang-folder to Apps/Lang-folder on your transmitter. Head on to “Applications” -> “User applications” and add the app. Guide on installation of Lua-apps available here if you need some more in-depth guide.

Setting up and usage of LiPo Watcher

After installation open the app and you are presented with easy setup with only a few options:

Settings are easy. Select the sensor giving you main battery voltage and set cell-count according your batteries.

Alarm-settings are also very easy. Alarm is made only if alarm-value is set, if it’s left to zero alarm will not be generated. In the example above alarm will sound once if I have 25% or less left in batteries.

Alarm is announced as “voicefile, percentage left”, in set example above that would be “Warning 25 percent”

Add LiPo Watcher to your main screen

You have an option to set telemetry screen for your main screen. Head on to “Main Menu” -> “Timers/Sensors” -> “Displayed Telemetry” and add the “RCT-LiPo Watcher” as screen. You can also add the LiPo Watcher to clock-display on top of the screen by going to “Main Menu” -> “Timers/Sensors” -> “Main Screen” and select RCT-LiPo Watcher in “Telemetry displayed at clock panel”.

You can choose small or big window, both will give you the percentage left in your batteries:

Big screen has additional info:

  • Shows percentage also as bar graphics
  • Shows calculated cell-voltage

If Model is not powered or there is no sensor screens will show as idle:

Really simple!

Questions And Answers

Q: How accurate is this method?

A: Well, it’s not very accurate. Only available close to exact battery charge measurement is capacity-measurement based on voltage and current, and that needs to be calibrated too. This app should be used as a guide to usage, not in the same exact way we are custom to use capacity-measurement. You need to test it in your model, compare readings to your LiPo-checker and so on. Reason is simple, voltage-based battery charge-level determination is not accurate and pretty much impossible when batteries are under load that constantly changes. From version 1.2 app is following max-voltage during measurement periods. This adds some accuracy.

Q: What if I have HV-LiPo’s?

A: No worries. App will just show 100% until your cell-voltages drops below 4.20V per cell.

Q: How is percentage calculated?

A: It’s calculated with simple division, “Total voltage divided with cell-count”. The total voltage is ran through averaging-filter to prevent short voltage-dips in use to effect the result. This also means a small delay in display but this is ok in use.

Q: What is the relation with cell-voltage and percentage?

A: Full battery, meaning 100% is at 4.20V per cell. 20% left is at 3.735V per cell. The whole table can be read in RCT-LiPo.lua on line 168 and can be modified by user if so desired. LiPo-voltage to percentage is not a “straight line” so a conversion table is needed for calculation.

Q: Why this app, there’s current-sensors out there?

A: Good question! Everyone cannot install a current-sensor to their models. They might be too small for example or user is concerned about the weight. Also keep in mind that this can e used to Rx-Batteries too and usually people do not have current-sensors there. Also for some reason nobody have asked this for a very, very long time but this week there’s been several requests for this. So here it is! (Also read the first Q carefully!)

Q: What if I have LiFe or Li-Ion?

A: Batteries charge-level based on voltage in LiFe is pretty much a disaster waiting to happen. When LiFe is close to empty it drops in voltage very fast. Also LiFe voltage drops very little during use (before the end, that is) so measurement should be calibrated with great care. I have code ready for both LiFe and Li-Ion but I’m not sure LiFe will be released, I believe it could cause more harm than good.

Version history

  • 1.0 Release version
  • 1.1 Added possibility to adjust averaging parameter
  • 1.2 Changed the way calculation is done, changed to max. voltage tracking, tested in DS-16
  • 1.3 Added Rx Voltage, bar color, redefined bar calculation, added clock display, separate versions for 14/16 and 12/24, added German translation


Some rules do of course apply!

  • Never use Lua to control a function that would take the model down if misbehaving!
  • Always, always test the function thoroughly before flying!
  • If you do not know, then don’t!
  • RC-Thoughts or myself is NOT responsible for anything this application does
  • Applications are free to use (MIT-license) and linking to this page is encouraged!