battery_status: report remaining flight time in seconds

This allows more accurate reporting and is compliant with the
MAVLink interface.

msg/battery_status.msg

@@ -8,6 +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 temperature			# temperature of the battery. NaN if unknown
 int32 cell_count			# Number of cells
 
@@ -18,7 +19,6 @@ uint8 source				# Battery source
 uint8 priority				# Zero based priority is the connection on the Power Controller V1..Vn AKA BrickN-1
 uint16 capacity				# actual capacity of the battery
 uint16 cycle_count			# number of discharge cycles the battery has experienced
-uint16 run_time_to_empty	# predicted remaining battery capacity based on the present rate of discharge in min
 uint16 average_time_to_empty	# predicted remaining battery capacity based on the average rate of discharge in min
 uint16 serial_number		# serial number of the battery pack
 uint16 manufacture_date		# manufacture date, part of serial number of the battery pack. formated as: Day + Month×32 + (Year–1980)×512

src/drivers/batt_smbus/batt_smbus.cpp

@@ -136,7 +136,7 @@ void BATT_SMBUS::RunImpl()
 
 	// Read run time to empty (minutes).
 	ret |= _interface->read_word(BATT_SMBUS_RUN_TIME_TO_EMPTY, result);
-	new_report.run_time_to_empty = result;
+	new_report.time_remaining_s = result * 60;
 
 	// Read average time to empty (minutes).
 	ret |= _interface->read_word(BATT_SMBUS_AVERAGE_TIME_TO_EMPTY, result);

src/drivers/smart_battery/batmon/batmon.cpp

@@ -162,7 +162,7 @@ void Batmon::RunImpl()
 
 	// Read run time to empty (minutes).
 	ret |= _interface->read_word(BATT_SMBUS_RUN_TIME_TO_EMPTY, result);
-	new_report.run_time_to_empty = result;
+	new_report.time_remaining_s = result * 60;
 
 	// Read average time to empty (minutes).
 	ret |= _interface->read_word(BATT_SMBUS_AVERAGE_TIME_TO_EMPTY, result);

src/drivers/uavcan/sensors/battery.cpp

@@ -83,7 +83,7 @@ UavcanBatteryBridge::battery_sub_cb(const uavcan::ReceivedDataStructure<uavcan::
 	// battery.priority = msg.;
 	battery.capacity = msg.full_charge_capacity_wh;
 	// battery.cycle_count = msg.;
-	// battery.run_time_to_empty = msg.;
+	// battery.time_remaining_s = msg.;
 	// battery.average_time_to_empty = msg.;
 	battery.serial_number = msg.model_instance_id;
 	battery.id = msg.getSrcNodeID().get();

src/drivers/uavcan/sensors/cbat.cpp

@@ -79,7 +79,7 @@ UavcanCBATBridge::battery_sub_cb(const uavcan::ReceivedDataStructure<cuav::equip
 	// battery.priority = msg.;
 	battery.capacity = msg.full_charge_capacity * 1000;
 	battery.cycle_count = msg.cycle_count;
-	battery.run_time_to_empty = msg.average_time_to_empty;
+	battery.time_remaining_s = msg.average_time_to_empty * 60;
 	battery.average_time_to_empty = msg.average_time_to_empty;
 	battery.serial_number = msg.serial_number;
 	battery.manufacture_date = msg.manufacture_date;

src/drivers/uavcan_v1/Subscribers/legacy/LegacyBatteryInfo.hpp

@@ -107,7 +107,7 @@ public:
 		 * connected (partly)
 		 * priority
 		 * cycle_count
-		 * run_time_to_empty
+		 * time_remaining_s
 		 * average_time_to_empty
 		 * manufacture_date
 		 * max_error

src/modules/mavlink/streams/BATTERY_STATUS.hpp

@@ -76,7 +76,9 @@ 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;
-				bat_msg.time_remaining = (battery_status.connected) ? battery_status.run_time_to_empty * 60 : 0;
+				// MAVLink extension: 0 is unsupported, in uORB it's -1
+				bat_msg.time_remaining = (battery_status.connected && (battery_status.time_remaining_s >= 0.f)) ?
+							 math::max((int)battery_status.time_remaining_s, 1) : 0;
 
 				switch (battery_status.warning) {
 				case (battery_status_s::BATTERY_WARNING_NONE):