Battery estimator: Warn on dangerously low levels.

We need to differentiate between a level where the user should act and where we are about to fall out of the sky (emergency). This helps performing more suitable failsafe actions.

src/modules/systemlib/battery.cpp

@@ -51,6 +51,7 @@ Battery::Battery() :
 	_param_r_internal(this, "R_INTERNAL"),
 	_param_low_thr(this, "LOW_THR"),
 	_param_crit_thr(this, "CRIT_THR"),
+	_param_emergency_thr(this, "EMERGEN_THR"),
 	_voltage_filtered_v(-1.0f),
 	_discharged_mah(0.0f),
 	_remaining_voltage(1.0f),
@@ -219,7 +220,10 @@ void
 Battery::determineWarning()
 {
 	// Smallest values must come first
-	if (_remaining < _param_crit_thr.get()) {
+	if (_remaining < _param_emergency_thr.get()) {
+		_warning = battery_status_s::BATTERY_WARNING_EMERGENCY;
+
+	} else if (_remaining < _param_crit_thr.get()) {
 		_warning = battery_status_s::BATTERY_WARNING_CRITICAL;
 
 	} else if (_remaining < _param_low_thr.get()) {