AP_BattMonitor: Maxell cell voltages timeout after 5sec

libraries/AP_BattMonitor/AP_BattMonitor_SMBus_Maxell.cpp

@@ -5,7 +5,6 @@
 #include "AP_BattMonitor_SMBus_Maxell.h"
 #include <utility>
 
-#define BATTMONITOR_SMBUS_MAXELL_NUM_CELLS 6
 uint8_t maxell_cell_ids[] = { 0x3f,  // cell 1
                               0x3e,  // cell 2
                               0x3d,  // cell 3
@@ -60,7 +59,8 @@ void AP_BattMonitor_SMBus_Maxell::timer()
         if (read_word(maxell_cell_ids[i], data)) {
             _has_cell_voltages = true;
             _state.cell_voltages.cells[i] = data;
-        } else {
+            _last_cell_update_ms[i] = tnow;
+        } else if ((tnow - _last_cell_update_ms[i]) > AP_BATTMONITOR_SMBUS_TIMEOUT_MICROS) {
             _state.cell_voltages.cells[i] = UINT16_MAX;
         }
     }

libraries/AP_BattMonitor/AP_BattMonitor_SMBus_Maxell.h

@@ -6,6 +6,8 @@
 #include "AP_BattMonitor_SMBus.h"
 #include <AP_HAL/I2CDevice.h>
 
+#define BATTMONITOR_SMBUS_MAXELL_NUM_CELLS 6
+
 class AP_BattMonitor_SMBus_Maxell : public AP_BattMonitor_SMBus
 {
 public:
@@ -28,4 +30,5 @@ private:
     uint8_t read_block(uint8_t reg, uint8_t* data, bool append_zero) const;
 
     uint8_t _pec_confirmed; // count of the number of times PEC has been confirmed as working
+    uint32_t _last_cell_update_ms[BATTMONITOR_SMBUS_MAXELL_NUM_CELLS];  // system time of last successful read of cell voltage
 };