AP_BattMonitor: add option minimum volt option

Allows aggregate monitors like Sum and ESC to report the minimum voltage
instead of the average voltage. This is useful for systems with multiple
isolated batteries where the lowest voltage is the limiting factor.

SW-97

libraries/AP_BattMonitor/AP_BattMonitor_ESC.cpp

@@ -32,6 +32,7 @@ void AP_BattMonitor_ESC::read(void)
     uint8_t voltage_escs = 0;     // number of ESCs with valid voltage
     uint8_t temperature_escs = 0; // number of ESCs with valid temperature
     float voltage_sum = 0;
+    float voltage_min = 0;
     float current_sum = 0;
     float temperature_sum = 0;
     uint32_t highest_ms = 0;
@@ -50,6 +51,9 @@ void AP_BattMonitor_ESC::read(void)
         }
 
         if (telem.get_voltage(i, voltage)) {
+            if (voltage_escs == 0 || voltage_min > voltage) {
+                voltage_min = voltage;
+            }
             voltage_sum += voltage;
             voltage_escs++;
         }
@@ -69,7 +73,11 @@ void AP_BattMonitor_ESC::read(void)
     }
 
     if (voltage_escs > 0) {
-        _state.voltage = voltage_sum / voltage_escs;
+        if (uint32_t(_params._options.get()) & uint32_t(AP_BattMonitor_Params::Options::Minimum_Voltage)) {
+            _state.voltage = voltage_min;
+        } else {
+            _state.voltage = voltage_sum / voltage_escs;
+        }
         _state.healthy = true;
     } else {
         _state.voltage = 0;

libraries/AP_BattMonitor/AP_BattMonitor_Params.cpp

@@ -143,7 +143,7 @@ const AP_Param::GroupInfo AP_BattMonitor_Params::var_info[] = {
     // @Param: OPTIONS
     // @DisplayName: Battery monitor options
     // @Description: This sets options to change the behaviour of the battery monitor
-    // @Bitmask: 0:Ignore DroneCAN SoC, 1:MPPT reports input voltage and current, 2:MPPT Powered off when disarmed, 3:MPPT Powered on when armed, 4:MPPT Powered off at boot, 5:MPPT Powered on at boot, 6:Send resistance compensated voltage to GCS
+    // @Bitmask: 0:Ignore DroneCAN SoC, 1:MPPT reports input voltage and current, 2:MPPT Powered off when disarmed, 3:MPPT Powered on when armed, 4:MPPT Powered off at boot, 5:MPPT Powered on at boot, 6:Send resistance compensated voltage to GCS, 7:Aggregate monitor reports minimum voltage instead of average, 8:Aggregate monitor reports maximum current instead of average, 9:Aggregate monitor reports maximum temperature instead of average, 10:Aggregate monitor reports minimum temperature instead of average
     // @User: Advanced
     AP_GROUPINFO("OPTIONS", 21, AP_BattMonitor_Params, _options, 0),
 

libraries/AP_BattMonitor/AP_BattMonitor_Params.h

@@ -17,14 +17,16 @@ class AP_BattMonitor_Params {
         BattMonitor_LowVoltageSource_Raw            = 0,
         BattMonitor_LowVoltageSource_SagCompensated = 1
     };
-    enum class Options : uint8_t {
+    enum class Options : uint16_t {
         Ignore_UAVCAN_SoC                   = (1U<<0),  // Ignore UAVCAN State-of-Charge (charge %) supplied value from the device and use the internally calculated one
         MPPT_Use_Input_Value                = (1U<<1),  // MPPT reports voltage and current from Input (usually solar panel) instead of the output
         MPPT_Power_Off_At_Disarm            = (1U<<2),  // MPPT Disabled when vehicle is disarmed, if HW supports it
         MPPT_Power_On_At_Arm                = (1U<<3),  // MPPT Enabled when vehicle is armed, if HW supports it
         MPPT_Power_Off_At_Boot              = (1U<<4),  // MPPT Disabled at startup (aka boot), if HW supports it
         MPPT_Power_On_At_Boot               = (1U<<5),  // MPPT Enabled at startup (aka boot), if HW supports it. If Power_Off_at_Boot is also set, the behavior is Power_Off_at_Boot
         GCS_Resting_Voltage                 = (1U<<6),  // send resistance resting voltage to GCS
+        // AllowSplitAuxInfo                = (1U<<7),  // allow different node to provide aux info for DroneCAN (Reserved for ArduPilot 4.5+)
+        Minimum_Voltage                     = (1U<<8),  // aggregate monitors report minimum voltage rather than average
     };
 
     BattMonitor_LowVoltage_Source failsafe_voltage_source(void) const { return (enum BattMonitor_LowVoltage_Source)_failsafe_voltage_source.get(); }

libraries/AP_BattMonitor/AP_BattMonitor_Sum.cpp

@@ -45,6 +45,7 @@ void
 AP_BattMonitor_Sum::read()
 {
     float voltage_sum = 0;
+    float voltage_min = 0;
     uint8_t voltage_count = 0;
     float current_sum = 0;
     uint8_t current_count = 0;
@@ -65,6 +66,9 @@ AP_BattMonitor_Sum::read()
         if (!_mon.healthy(i)) {
             continue;
         }
+        if (voltage_count == 0 || _mon.voltage(i) < voltage_min) {
+            voltage_min = _mon.voltage(i);
+        }
         voltage_sum += _mon.voltage(i);
         voltage_count++;
         float current;
@@ -77,7 +81,11 @@ AP_BattMonitor_Sum::read()
     const uint32_t dt_us = tnow_us - _state.last_time_micros;
 
     if (voltage_count > 0) {
-        _state.voltage = voltage_sum / voltage_count;
+        if (uint32_t(_params._options.get()) & uint32_t(AP_BattMonitor_Params::Options::Minimum_Voltage)) {
+            _state.voltage = voltage_min;
+        } else {
+            _state.voltage = voltage_sum / voltage_count;
+        }
     }
     if (current_count > 0) {
         _state.current_amps = current_sum;