Battery Failsafe

The battery failsafe can be configured to automatically switch the vehicle into RTL, SmartRTL or Land mode if the vehicle battery voltage drops below a specified voltage for 10 seconds or the estimated remaining capacity has dropped below a configurable threshold.

Note

This failsafe requires the vehicle have a working Power Module.

When the failsafe will trigger

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

  • voltage drops below the voltage held in the BATT_LOW_VOLT or FS_BATT_VOLTAGE parameter for more than 10 seconds. The default voltage is 10.5 volts. If set to zero the voltage based trigger will be disabled.
  • remaining capacity falls below the BATT_LOW_MAH or FS_BATT_MAH. 20% of the battery’s full capacity is a good choice (i.e. “1000” for a 5000mAh battery). If set to zero the capacity based trigger will be disabled (i.e. only voltage will be used)

What will happen

When the failsafe is triggered:

  • Buzzer will play a loud low-battery alarm
  • LEDs will flash yellow
  • “Low Battery!” will be displayed on the ground stations HUD (if telemetry is connected)

Then 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
  • The user configurable action held in the BATT_FS_LOW_ACT (Copter-3.6 or higher) or FS_BATT_ENABLE (Copter-3.5 or lower) parameter:
  • 0 : None - no action will be taken
  • 1 : Land - the vehicle will switch to Land mode
  • 2 : RTL - the vehicle will switch to Return-to-Launch mode if possible but if not possible (because of no good position estimate) the vehicle will Land (Recommended)
  • 3 : SmartRTL or RTL - the vehicle will switch to SmartRTL mode if possible, if not possible (because SmartRTL’s position buffer is full) the vehicle will switch to RTL. If RTL is also not possible (because of no good position estimate) the vehicle will switch to Land
  • 4 : SmartRTL or Land - the vehicle will switch to SmartRTL mode if possible, if not possible (because SmartRTL’s position buffer is full) the vehicle will switch to Land
  • 5 : Terminate - the vehicle will disarm the motors. This is a dangerous option that should not be used in most cases

As with all failsafes, the user can re-take control of the vehicle by changing the flight mode switch to another mode. The battery failsafe will not trigger again unless the two-layer failsafe is setup (see below)

Note

Even if the failsafe action is set to “None” (i.e. BATT_FS_LOW_ACT = 0) the buzzer will buzz and the LEDs will flash yellow. To completely disable the battery failsafe set BATT_LOW_VOLT and BATT_LOW_MAH to zero.

Note

Once the battery failsafe has triggered, it cannot be reset until the flight controller is rebooted.

Setting up using Mission Planner

  • On the INITIAL SETUP | Mandatory Hardware | Failsafe page:
    • If the Battery section parmeters are greyed out follow instructions to setup the Power Module
    • 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.
    • Select the desired behavior (Land, RTL, SmartRTL, etc) from the drop-down list
../_images/Failsafe_Battery_Setup.png

Two-Layer failsafe

Copter-3.6 (and higher) includes a two-layer battery failsafe. This allows setting up a follow-up action if the battery voltage or remaining capacity falls below an even lower threshold.

Advanced Settings

  • BATT_FS_VOLTSRC allows configuring whether the raw battery voltage or a sag corrected voltage is used
  • BATT_LOW_TIMER can configure how long the voltage must be below the threshold for the failsafe to trigger