Battery Failsafe

This article explains how to setup and test the battery failsafe.


The battery failsafe can be set-up to automatically trigger an RTL or LAND when the vehicle battery voltage or estimated remaining power has crossed a configurable threshold.

To use this failsafe the vehicle must have a Power Module or other voltage and (optionally) current monitor.


Copter also supports other failsafes: Radio, GCS, and EKF / DCM Check.

When the failsafe will trigger

If enabled and set-up correctly the battery failsafe will trigger if the main battery:

  • voltage drops below 10.5 volts (configurable) for more than 10 seconds
  • remaining capacity falls below the configurable Reserved MAH

What will happen

When the failsafe is triggered one of the following will happen:

  • Nothing if the vehicle is already disarmed
  • Disarm motors if the vehicle is in Stabilize or Acro mode and the throttle is at zero OR the vehicle is landed
  • Return-to-Launch (RTL) if the FS_BATT_ENABLE param is set to “2” (“RTL”) OR the vehicle is in AUTO mode, has a GPS lock and are at least 2 meters from your home position
  • Land in all other cases

Setting up the battery failsafe

  • Setup the power module as described in the Power Modules section of this wiki including setting the totally battery capacity if using a current monitor
  • On the INITIAL SETUP | Mandatory Hardware | Failsafe page:
    • set the “Low Battery” threshold voltage (i.e. 10.5 volts)
    • set the “Reserved MAH” or leave as “0” if the failsafe should never trigger based on estimated current consumed. “600” would be an appropriate number to cause the vehicle to LAND or RTL when only 20% of a 3000mAH battery remains.
    • select the desired behavior as “Land” or “RTL” (Note: the placement of the drop-down is confusing but this will be corrected in a future version of the mission planner)

Low Battery warning (even with failsafe disabled)

Even if the battery failsafe is not enabled (i.e. FS_BATT_ENABLE = 0) the “Low Battery” voltage threshold and “Reserved MAH” numbers are used to issue a battery warning. This will result in:

  • “Low Battery!” message appearing on the ground stations HUD (if using telemetry)
  • Loud beeping if a buzzer is attached
  • Flashing LEDs

The only way to disable this warning completely is to set the “Low Battery” voltage and “Reserved MAH” values to zero.