AP_BattMonitor: get Solo battery voltage from sum of cell voltages

Minimizes the number of transactions on the bus, and reduces the amount of noise we have to consider

libraries/AP_BattMonitor/AP_BattMonitor_SMBus_Solo.cpp

@@ -11,7 +11,6 @@ extern const AP_HAL::HAL& hal;
 #include <AP_HAL/AP_HAL.h>
 
 #define BATTMONITOR_SMBUS_SOLO_TEMP                 0x08    // temperature register
-#define BATTMONITOR_SMBUS_SOLO_VOLTAGE              0x09    // voltage register
 #define BATTMONITOR_SMBUS_SOLO_REMAINING_CAPACITY   0x0f    // predicted remaining battery capacity in milliamps
 #define BATTMONITOR_SMBUS_SOLO_FULL_CHARGE_CAPACITY 0x10    // full capacity register
 #define BATTMONITOR_SMBUS_SOLO_MANUFACTURE_DATA     0x23    /// manufacturer data
@@ -23,6 +22,7 @@ extern const AP_HAL::HAL& hal;
 
 /*
  * Other potentially useful registers, listed here for future use
+ * #define BATTMONITOR_SMBUS_SOLO_VOLTAGE           0x09    // voltage register
  * #define BATTMONITOR_SMBUS_SOLO_BATTERY_STATUS    0x16    // battery status register including alarms
  * #define BATTMONITOR_SMBUS_SOLO_DESIGN_CAPACITY   0x18    // design capacity register
  * #define BATTMONITOR_SMBUS_SOLO_DESIGN_VOLTAGE    0x19    // design voltage register
@@ -55,9 +55,20 @@ void AP_BattMonitor_SMBus_Solo::timer()
     uint8_t buff[8];
     uint32_t tnow = AP_HAL::micros();
 
-    // read voltage
-    if (read_word(BATTMONITOR_SMBUS_SOLO_VOLTAGE, data)) {
-        _state.voltage = (float)data / 1000.0f;
+
+    // read cell voltages
+    if (read_block(BATTMONITOR_SMBUS_SOLO_CELL_VOLTAGE, buff, 8, false)) {
+        float pack_voltage_mv = 0.0f;
+        for (uint8_t i = 0; i < BATTMONITOR_SMBUS_SOLO_NUM_CELLS; i++) {
+            uint16_t cell = buff[(i * 2) + 1] << 8 | buff[i * 2];
+            _state.cell_voltages.cells[i] = cell;
+            pack_voltage_mv += (float)cell;
+        }
+        // accumulate the pack voltage out of the total of the cells
+        // because the Solo's I2C bus is so noisy, it's worth not spending the
+        // time and bus bandwidth to request the pack voltage as a seperate
+        // transaction
+        _state.voltage = pack_voltage_mv * 1e-3;
         _state.last_time_micros = tnow;
         _state.healthy = true;
     }
@@ -116,13 +127,6 @@ void AP_BattMonitor_SMBus_Solo::timer()
          _state.temperature_time = AP_HAL::millis();
          _state.temperature = ((float)(data - 2731)) * 0.1f;
     }
-
-    // read cell voltages
-    if (read_block(BATTMONITOR_SMBUS_SOLO_CELL_VOLTAGE, buff, 8, false)) {
-        for (uint8_t i = 0; i < BATTMONITOR_SMBUS_SOLO_NUM_CELLS; i++) {
-            _state.cell_voltages.cells[i] = buff[(i * 2) + 1] << 8 | buff[i * 2];
-        }
-    }
 }
 
 // read word from register