A battery monitor refuses arming

What ArduPilot says:

• Battery N …

Before arming, ArduPilot asks every battery monitor that is switched on whether the battery is fit to fly. The number is the monitor (1 uses the BATT_ parameters, 2 the BATT2_ ones) and the words after it are the first test that failed, in this order: “unhealthy”, “below minimum arming voltage”, “below minimum arming capacity”, “low voltage failsafe”, “low capacity failsafe”, “critical voltage failsafe”, “critical capacity failsafe”, “capacity failsafe critical >= low”, “voltage failsafe critical >= low”. Only the first is shown; once it is fixed the next may appear.

What to do, most likely first

1. “unhealthy”: the monitor gives no reading, or BATT_MONITOR was changed and the board not rebooted.

   FixReboot after any change of BATT_MONITOR. If it stays, the type does not match the sensor fitted, or an analog sensor's BATT_VOLT_PIN and BATT_CURR_PIN are not this board's pins.

2. “below minimum arming voltage” or “capacity”: the battery reads under BATT_ARM_VOLT, or has less than BATT_ARM_MAH left.

   FixConnect a charged battery: on USB alone the voltage reads near zero. If a full battery still fails, compare the reading with a multimeter.

3. “low” or “critical” “voltage failsafe” or “capacity failsafe”: the battery is already under a failsafe threshold (BATT_LOW_VOLT, BATT_CRT_VOLT, BATT_LOW_MAH, BATT_CRT_MAH).

   FixCharge or change the battery. If a full one fails too, the thresholds are for a battery with more cells, or the voltage calibration is off.

4. “critical >= low”: a critical threshold is not below its low one.

   FixSet BATT_CRT_VOLT lower than BATT_LOW_VOLT, and BATT_CRT_MAH lower than BATT_LOW_MAH.

Parameters to look at

Where to do it:Setup & Tuning GuideSetup & Tuning GuideFailsafe Explainer