Battery failsafe: land now

What ArduPilot says:

• ERR FAILSAFE_BATT (Subsys 6)

A battery crossed a failsafe level and the copter is doing what that level's parameter says: BATT_FS_LOW_ACT for low, BATT_FS_CRT_ACT for critical (0 warn only, 1 Land, 2 RTL, 3 SmartRTL or RTL, 4 SmartRTL or Land, 5 stop the motors, 6 mission landing or RTL, 7 Brake or Land). The line “Battery 1 is low 14.20V used 3100 mAh” beside it names the level. A level is reached by voltage (below BATT_LOW_VOLT or BATT_CRT_VOLT for BATT_LOW_TIMER seconds, 10 by default) or by capacity left (BATT_LOW_MAH, BATT_CRT_MAH). Whatever the mode, get it on the ground. The failsafe stays on until reboot.

What to do, most likely first

1. The battery is used up: the flight was longer, heavier or windier than the pack allows.

   CheckIn the log, BAT.Volt and BAT.CurrTot at the moment of the message against the thresholds.

   FixLand earlier. Set BATT_LOW_MAH to the capacity you want left when you turn back (about a quarter of the pack) and a flight timer on the transmitter.

2. The voltage sagged under load on a cold, old or undersized pack, though the capacity was not used up.

   CheckIn the log BAT.Volt dips when BAT.Curr peaks and recovers after landing.

   FixUse a pack that can give the current, keep it warm before flight, and set BATT_FS_VOLTSRC to 1 so the failsafe uses the voltage with the sag taken out. Keep the capacity thresholds as the second guard.

3. The voltage or current sensor is not calibrated, so ArduPilot reads less than the battery has.

   CheckCompare the voltage the ground station shows with a meter on the battery.

   FixCalibrate the battery monitor: BATT_VOLT_MULT against a voltmeter, BATT_AMP_PERVLT against the charge the charger puts back.

4. The critical action never ran: the low-level action that ran first outranks it.

   CheckRank, highest first: 5, 1 (Land), 2 (RTL), 4, 3, 0. Actions 6 and 7 rank below all of them, so after any low-level event, even with action 0, a critical action of 6 or 7 is skipped. Compare BATT_FS_LOW_ACT with BATT_FS_CRT_ACT.

   FixGive the critical level an action that outranks the low one: Land (1) is the usual choice.

5. The battery monitor stopped reporting for 5 s (“Battery 1 is missing, last: …”): the failsafe is announced, no action is taken.

   FixCheck the power module's cable, or the ESC telemetry or CAN link the monitor reads from.

Parameters to look at

Where to do it:Failsafe ExplainerSetup & Tuning GuideSetup & Tuning Guide