AP_BattMonitor: add new param BATT_WATT_MAX

Description: If battery wattage (voltage * current) exceeds this value then the system will reduce max throttle (THR_MAX and TKOFF_THR_MAX) to satisfy this limit. This helps limit high current to low C rated batteries regardless of battery voltage. The max throttle will slowly grow back to THR_MAX (or TKOFF_THR_MAX) if demanding the current max and under the watt max.

libraries/AP_BattMonitor/AP_BattMonitor.cpp

@@ -57,7 +57,16 @@ const AP_Param::GroupInfo AP_BattMonitor::var_info[] = {
     AP_GROUPINFO("_CAPACITY", 6, AP_BattMonitor, _pack_capacity[0], AP_BATT_CAPACITY_DEFAULT),
 
     // 7 & 8 were used for VOLT2_PIN and VOLT2_MULT
-    // 9..10 left for future expansion
+
+    // @Param: _WATT_MAX
+    // @DisplayName: Maximum allowed power (Watts)
+    // @Description: If battery wattage (voltage * current) exceeds this value then the system will reduce max throttle (THR_MAX, TKOFF_THR_MAX and THR_MIN for reverse thrust) to satisfy this limit. This helps limit high current to low C rated batteries regardless of battery voltage. The max throttle will slowly grow back to THR_MAX (or TKOFF_THR_MAX ) and THR_MIN if demanding the current max and under the watt max. Use 0 to disable.
+    // @Units: Watts
+    // @Increment: 1
+    // @User: Advanced
+    AP_GROUPINFO("_WATT_MAX", 9, AP_BattMonitor, _watt_max[0], AP_BATT_MAX_WATT_DEFAULT),
+
+    // 10 is left for future expansion
 
 #if AP_BATT_MONITOR_MAX_INSTANCES > 1
     // @Param: 2_MONITOR
@@ -109,6 +118,15 @@ const AP_Param::GroupInfo AP_BattMonitor::var_info[] = {
     // @User: Standard
     AP_GROUPINFO("2_CAPACITY", 17, AP_BattMonitor, _pack_capacity[1], AP_BATT_CAPACITY_DEFAULT),
 
+
+    // @Param: 2_WATT_MAX
+    // @DisplayName: Maximum allowed current
+    // @Description: If battery wattage (voltage * current) exceeds this value then the system will reduce max throttle (THR_MAX, TKOFF_THR_MAX and THR_MIN for reverse thrust) to satisfy this limit. This helps limit high current to low C rated batteries regardless of battery voltage. The max throttle will slowly grow back to THR_MAX (or TKOFF_THR_MAX ) and THR_MIN if demanding the current max and under the watt max. Use 0 to disable.
+    // @Units: Amps
+    // @Increment: 1
+    // @User: Advanced
+    AP_GROUPINFO("2_WATT_MAX", 18, AP_BattMonitor, _watt_max[1], AP_BATT_MAX_WATT_DEFAULT),
+
 #endif // AP_BATT_MONITOR_MAX_INSTANCES > 1
 
     AP_GROUPEND
@@ -303,3 +321,22 @@ bool AP_BattMonitor::exhausted(uint8_t instance, float low_voltage, float min_ca
     // if we've gotten this far battery is ok
     return false;
 }
+
+// return true if any battery is pushing too much power
+bool AP_BattMonitor::overpower_detected() const
+{
+    bool result = false;
+    for (int instance = 0; instance < AP_BATT_MONITOR_MAX_INSTANCES; instance++) {
+        result |= overpower_detected(instance);
+    }
+    return result;
+}
+
+bool AP_BattMonitor::overpower_detected(uint8_t instance) const
+{
+    if (instance < AP_BATT_MONITOR_MAX_INSTANCES && _watt_max[instance] > 0) {
+        float power = _BattMonitor_STATE(instance).current_amps * _BattMonitor_STATE(instance).voltage;
+        return _BattMonitor_STATE(instance).healthy && (power > _watt_max[instance]);
+    }
+    return false;
+}

libraries/AP_BattMonitor/AP_BattMonitor.h

@@ -13,6 +13,7 @@
 
 #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_MAX_WATT_DEFAULT            0
 
 // declare backend class
 class AP_BattMonitor_Backend;
@@ -107,6 +108,13 @@ public:
     /// set_monitoring - sets the monitor type (used for example sketch only)
     void set_monitoring(uint8_t instance, uint8_t mon) { _monitoring[instance].set(mon); }
 
+    bool get_watt_max() { return get_watt_max(AP_BATT_PRIMARY_INSTANCE); }
+    bool get_watt_max(uint8_t instance) { return _watt_max[instance]; }
+
+    /// true when (voltage * current) > watt_max
+    bool overpower_detected() const;
+    bool overpower_detected(uint8_t instance) const;
+
     static const struct AP_Param::GroupInfo var_info[];
 
 protected:
@@ -119,6 +127,7 @@ protected:
     AP_Float    _curr_amp_per_volt[AP_BATT_MONITOR_MAX_INSTANCES];  /// voltage on current pin multiplied by this to calculate current in amps
     AP_Float    _curr_amp_offset[AP_BATT_MONITOR_MAX_INSTANCES];    /// offset voltage that is subtracted from current pin before conversion to amps
     AP_Int32    _pack_capacity[AP_BATT_MONITOR_MAX_INSTANCES];      /// battery pack capacity less reserve in mAh
+    AP_Int16    _watt_max[AP_BATT_MONITOR_MAX_INSTANCES];           /// max battery power allowed. Reduce max throttle to reduce current to satisfy this limit
 
 private:
     BattMonitor_State state[AP_BATT_MONITOR_MAX_INSTANCES];