AP_BattMonitor: added BATT_VOLT_TIMEO

this is useful for aircraft with very low C batteries, to prevent a low
voltage failsafe on takeoff. For long endurance aircraft low C batteries
are often used, and the voltage sags a lot on takeoff, but it fine for
the rest of the flight

libraries/AP_BattMonitor/AP_BattMonitor.cpp

@@ -143,6 +143,15 @@ const AP_Param::GroupInfo AP_BattMonitor::var_info[] = {
 
 #endif // AP_BATT_MONITOR_MAX_INSTANCES > 1
 
+    // @Param: _VOLT_TIMER
+    // @DisplayName: Low voltage timeout
+    // @Description: This is the timeout in seconds before a low voltage event will be triggered. For aircraft with low C batteries it may be necessary to raise this in order to cope with low voltage on long takeoffs. A value of zero disables low voltage errors.
+    // @Units: Seconds
+    // @Increment: 1
+    // @Range: 0 120
+    // @User: Advanced
+    AP_GROUPINFO("_VOLT_TIMER", 21, AP_BattMonitor, _volt_timeout, AP_BATT_LOW_VOLT_TIMEOUT_DEFAULT),
+        
     AP_GROUPEND
 };
 
@@ -307,7 +316,7 @@ bool AP_BattMonitor::exhausted(uint8_t instance, float low_voltage, float min_ca
         // this is the first time our voltage has dropped below minimum so start timer
         if (state[instance].low_voltage_start_ms == 0) {
             state[instance].low_voltage_start_ms = AP_HAL::millis();
-        } else if (AP_HAL::millis() - state[instance].low_voltage_start_ms > AP_BATT_LOW_VOLT_TIMEOUT_MS) {
+        } else if (_volt_timeout > 0 && AP_HAL::millis() - state[instance].low_voltage_start_ms > uint32_t(_volt_timeout.get())*1000U) {
             return true;
         }
     } else {

libraries/AP_BattMonitor/AP_BattMonitor.h

@@ -12,7 +12,7 @@
 #define AP_BATT_PRIMARY_INSTANCE            0
 
 #define AP_BATT_CAPACITY_DEFAULT            3300
-#define AP_BATT_LOW_VOLT_TIMEOUT_MS         10000   // low voltage of 10 seconds will cause battery_exhausted to return true
+#define AP_BATT_LOW_VOLT_TIMEOUT_DEFAULT    10   // low voltage of 10 seconds will cause battery_exhausted to return true
 #define AP_BATT_MAX_WATT_DEFAULT            0
 #define AP_BATT_SERIAL_NUMBER_DEFAULT       -1
 
@@ -150,6 +150,8 @@ protected:
     AP_Int16    _watt_max[AP_BATT_MONITOR_MAX_INSTANCES];           /// max battery power allowed. Reduce max throttle to reduce current to satisfy this limit
     AP_Int32    _serial_numbers[AP_BATT_MONITOR_MAX_INSTANCES];     /// battery serial number, automatically filled in on SMBus batteries
 
+    AP_Int8     _volt_timeout;
+
 private:
     BattMonitor_State state[AP_BATT_MONITOR_MAX_INSTANCES];
     AP_BattMonitor_Backend *drivers[AP_BATT_MONITOR_MAX_INSTANCES];