Battery: Declare fault on overtemperature

If the measured temperature exceeds the fixed threshold of 100 degC, a
fault is declared, causing batteryCheck to show a failsafe warning to
the user.

src/lib/battery/battery.cpp

@@ -330,6 +330,10 @@ uint16_t Battery::determineFaults()
 		faults |= (1 << battery_status_s::FAULT_SPIKES);
 	}
 
+	if (PX4_ISFINITE(_temperature_c) && _temperature_c > BAT_TEMP_MAX) {
+		faults |= (1 << battery_status_s::FAULT_OVER_TEMPERATURE);
+	}
+
 	return faults;
 }
 

src/lib/battery/battery.h

@@ -217,4 +217,10 @@ private:
 	static constexpr float OCV_DEFAULT = 4.2f; // [V] Initial per cell estimate of the open circuit voltage
 	static constexpr float R_COVARIANCE = 0.1f; // Initial per cell covariance of the internal resistance
 	static constexpr float OCV_COVARIANCE = 1.5f; // Initial per cell covariance of the open circuit voltage
+
+	// Temperature [degC] above which an overtemperature fault is declared,
+	// leading to a failsafe warning recommending immediate landing. Note
+	// that depending on the setup this may be measured in/close to the
+	// battery (smart battery) or from a separate power monitor module.
+	static constexpr float BAT_TEMP_MAX = 100.0f;
 };