AP_BattMonitor: Allow generic SMBus batteries to not have cell voltages

libraries/AP_BattMonitor/AP_BattMonitor_SMBus.h

@@ -23,9 +23,10 @@ public:
     // virtual destructor to reduce compiler warnings
     virtual ~AP_BattMonitor_SMBus() {}
 
-    // all smart batteries provide current and individual cell voltages
+    bool has_cell_voltages() const override { return _has_cell_voltages; }
+
+    // all smart batteries are expected to provide current
     bool has_current() const override { return true; }
-    bool has_cell_voltages() const override { return true; }
 
 protected:
 
@@ -62,6 +63,8 @@ protected:
     int32_t _serial_number = -1;    // battery serial number
     uint16_t _full_charge_capacity; // full charge capacity, used to stash the value before setting the parameter
 
+    bool _has_cell_voltages;        // smbus backends flag this as true once they have recieved a valid cell voltage report
+
 };
 
 // include specific implementations

libraries/AP_BattMonitor/AP_BattMonitor_SMBus_Maxell.cpp

@@ -64,6 +64,7 @@ void AP_BattMonitor_SMBus_Maxell::timer()
     // read cell voltages
     for (uint8_t i = 0; i < BATTMONITOR_SMBUS_MAXELL_NUM_CELLS; i++) {
         if (read_word(maxell_cell_ids[i], data)) {
+            _has_cell_voltages = true;
             _state.cell_voltages.cells[i] = data;
         } else {
             _state.cell_voltages.cells[i] = UINT16_MAX;

libraries/AP_BattMonitor/AP_BattMonitor_SMBus_Solo.cpp

@@ -50,6 +50,8 @@ void AP_BattMonitor_SMBus_Solo::timer()
             _state.cell_voltages.cells[i] = cell;
             pack_voltage_mv += (float)cell;
         }
+        _has_cell_voltages = true;
+
         // 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