The battery is below the level set as the minimum to arm

The battery monitor's voltage is lower than BATT_ARM_VOLT, or (“arming capacity”) the charge left, BATT_CAPACITY minus the mAh used since power-on, is lower than BATT_ARM_MAH. Both limits are yours to set, and 0 means no limit. The voltage is the reading at this instant, with no averaging and no delay.

What to do, most likely first

1. Only USB is connected, or the flight battery is not plugged in: the monitor reads close to 0 V.

   CheckThe battery voltage on the ground station's main screen.

   FixConnect the flight battery (props off on the bench).

2. The battery is not fully charged.

   CheckMeasure it with a charger or a cell checker: a full LiPo is 4.2 V per cell, 16.8 V for 4S and 25.2 V for 6S.

   FixCharge it. Stopping a take-off on a part-used battery is what this check is for.

3. BATT_ARM_VOLT was set for a battery with more cells than the one fitted.

   CheckDivide BATT_ARM_VOLT by your cell count: more than 4.2 V per cell can never pass.

   FixSet BATT_ARM_VOLT for your own battery: above BATT_LOW_VOLT, below the full voltage.

4. The voltage reading is wrong: the sensor's scale, BATT_VOLT_MULT, does not match the board.

   CheckCompare the voltage the ground station shows with a multimeter on the battery leads.

   FixCalibrate the voltage in the ground station's battery monitor screen (it adjusts BATT_VOLT_MULT) until the two agree.

5. “arming capacity”: this battery has already flown since power-on, or BATT_CAPACITY is smaller than BATT_ARM_MAH.

   FixFit a charged battery and set BATT_CAPACITY to its real capacity in mAh.

The count of used mAh starts from zero at every power-on (a smart battery excepted), so after a reboot the capacity check takes any battery for a full one. For a part-used battery only the voltage check protects you.

Parameters to look at

Where to do it:Setup & Tuning GuideSetup & Tuning Guide