battery_status: clearly define and handle zero remaining flight time

msg/battery_status.msg

@@ -8,7 +8,7 @@ float32 current_average_a	# Battery current average in amperes, -1 if unknown
 float32 discharged_mah		# Discharged amount in mAh, -1 if unknown
 float32 remaining			# From 1 to 0, -1 if unknown
 float32 scale				# Power scaling factor, >= 1, or -1 if unknown
-float32 time_remaining_s	# predicted time in seconds remaining until battery is empty under previous averaged load, -1 if unknown
+float32 time_remaining_s	# predicted time in seconds remaining until battery is empty under previous averaged load, NAN if unknown
 float32 temperature			# temperature of the battery. NaN if unknown
 int32 cell_count			# Number of cells
 

src/lib/battery/battery.cpp

@@ -249,7 +249,7 @@ void Battery::computeScale()
 
 float Battery::computeRemainingTime(float current_a)
 {
-	float time_remaining_s = -1.f;
+	float time_remaining_s{NAN};
 
 	// Only estimate remaining time with useful in flight current measurements
 	if (_current_filter_a.getState() > 1.f) {

src/modules/mavlink/streams/BATTERY_STATUS.hpp

@@ -76,8 +76,8 @@ private:
 				bat_msg.energy_consumed = -1;
 				bat_msg.current_battery = (battery_status.connected) ? battery_status.current_filtered_a * 100 : -1;
 				bat_msg.battery_remaining = (battery_status.connected) ? ceilf(battery_status.remaining * 100.f) : -1;
-				// MAVLink extension: 0 is unsupported, in uORB it's -1
-				bat_msg.time_remaining = (battery_status.connected && (battery_status.time_remaining_s >= 0.f)) ?
+				// MAVLink extension: 0 is unsupported, in uORB it's NAN
+				bat_msg.time_remaining = (battery_status.connected && (PX4_ISFINITE(battery_status.time_remaining_s))) ?
 							 math::max((int)battery_status.time_remaining_s, 1) : 0;
 
 				switch (battery_status.warning) {