From ac149c0b8fd16fe544f60b3cd6d0d39ab6d60b5d Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Fri, 6 Feb 2026 11:09:22 -0500
Subject: [PATCH 01/14] SITL: refactor SITL::Plane's battery consumption up one
 level

---
 libraries/SITL/SIM_Plane.cpp | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/libraries/SITL/SIM_Plane.cpp b/libraries/SITL/SIM_Plane.cpp
index 6a0322c84a9993..f3b18fb457caec 100644
--- a/libraries/SITL/SIM_Plane.cpp
+++ b/libraries/SITL/SIM_Plane.cpp
@@ -401,7 +401,6 @@ void Plane::calculate_forces(const struct sitl_input &input, Vector3f &rot_accel
     float elevator = filtered_servo_angle(input, 1);
     float rudder   = filtered_servo_angle(input, 3);
     bool launch_triggered = input.servos[6] > 1700;
-    float throttle;
     if (reverse_elevator_rudder) {
         elevator = -elevator;
         rudder = -rudder;
@@ -444,16 +443,7 @@ void Plane::calculate_forces(const struct sitl_input &input, Vector3f &rot_accel
     }
     //printf("Aileron: %.1f elevator: %.1f rudder: %.1f\n", aileron, elevator, rudder);
 
-    if (reverse_thrust) {
-        throttle = filtered_servo_angle(input, 2);
-    } else {
-        throttle = filtered_servo_range(input, 2);
-    }
-    
-    float thrust     = throttle;
-
-    battery_voltage = sitl->batt_voltage - 0.7*throttle;
-    battery_current = (battery_voltage/sitl->batt_voltage)*50.0f*sq(throttle);
+    float thrust = reverse_thrust ? filtered_servo_angle(input, 2) : filtered_servo_range(input, 2);
 
     if (ice_engine) {
         thrust = icengine.update(input);
@@ -526,7 +516,11 @@ void Plane::update(const struct sitl_input &input)
     update_wind(input);
     
     calculate_forces(input, rot_accel);
-    
+
+    float throttle = reverse_thrust ? filtered_servo_angle(input, 2) : filtered_servo_range(input, 2);
+    battery_voltage = sitl->batt_voltage - 0.7*throttle;
+    battery_current = (battery_voltage/sitl->batt_voltage)*50.0f*sq(throttle);
+
     update_dynamics(rot_accel);
 
     /*

From f8ed39605699d3442d8572fa2d5a67f69b01c6bf Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Tue, 3 Feb 2026 09:51:09 -0500
Subject: [PATCH 02/14] SITL: STIL::Aircraft children explicitly manage battery
 temperature

---
 libraries/SITL/SIM_Aircraft.h      | 6 ++++--
 libraries/SITL/SIM_FlightAxis.cpp  | 2 ++
 libraries/SITL/SIM_JSON.cpp        | 6 ++++--
 libraries/SITL/SIM_Multicopter.cpp | 1 +
 libraries/SITL/SIM_Plane.cpp       | 2 ++
 libraries/SITL/SIM_QuadPlane.cpp   | 1 +
 libraries/SITL/SIM_Scrimmage.cpp   | 1 +
 7 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/libraries/SITL/SIM_Aircraft.h b/libraries/SITL/SIM_Aircraft.h
index 13992dfb2b35b9..fb72610f664cd5 100644
--- a/libraries/SITL/SIM_Aircraft.h
+++ b/libraries/SITL/SIM_Aircraft.h
@@ -174,7 +174,7 @@ class Aircraft {
     void set_dronecan_device(DroneCANDevice *_dronecan) { dronecan = _dronecan; }
 #endif
     float get_battery_voltage() const { return battery_voltage; }
-    float get_battery_temperature() const { return battery.get_temperature(); }
+    float get_battery_temperature() const { return battery_temperature; }
 
     float ambient_temperature_degC() const;
 
@@ -222,12 +222,14 @@ class Aircraft {
     float airspeed_pitot;                // m/s, EAS airspeed, as seen by fwd pitot tube
     float battery_voltage;
     float battery_current;
+    float battery_temperature;
     float local_ground_level;            // ground level at local position
     bool lock_step_scheduled;
     bool flightaxis_sync_imus_to_frames; // causes the frame counter to be incremented on each timestep, IMUs will then update at the same rate
     uint32_t last_one_hz_ms;
 
-    // battery model
+    // OPTIONAL battery model
+    // ("OPTIONAL" because a child can ignore it and directly set/get battery_* members.)
     Battery battery;
 
     uint32_t motor_mask;
diff --git a/libraries/SITL/SIM_FlightAxis.cpp b/libraries/SITL/SIM_FlightAxis.cpp
index 28720caec05fbd..f1c8c32ef7dad1 100644
--- a/libraries/SITL/SIM_FlightAxis.cpp
+++ b/libraries/SITL/SIM_FlightAxis.cpp
@@ -656,6 +656,8 @@ void FlightAxis::update(const struct sitl_input &input)
 
     battery_voltage = MAX(state.m_batteryVoltage_VOLTS, 0);
     battery_current = MAX(state.m_batteryCurrentDraw_AMPS, 0);
+    // this aircraft has a constant battery temperature
+    battery_temperature = 273.0f; // kelvin
     rpm[0] = state.m_heliMainRotorRPM;
     rpm[1] = state.m_propRPM;
     motor_mask = 3;
diff --git a/libraries/SITL/SIM_JSON.cpp b/libraries/SITL/SIM_JSON.cpp
index dfa9223e0e01f5..9e6fee92657d31 100644
--- a/libraries/SITL/SIM_JSON.cpp
+++ b/libraries/SITL/SIM_JSON.cpp
@@ -490,11 +490,13 @@ void JSON::recv_fdm(const struct sitl_input &input)
 
     // update battery state
     if ((received_bitmask & BAT_VOLT) != 0) {
-        battery_voltage = state.bat_volt; 
+        battery_voltage = state.bat_volt;
     }
     if ((received_bitmask & BAT_AMP) != 0) {
         battery_current = state.bat_amp; 
     }
+    // this aircraft has a constant battery temperature
+    battery_temperature = 273.0f; // kelvin
 
     double deltat;
     if (state.timestamp_s < last_timestamp_s) {
@@ -615,4 +617,4 @@ void JSON::update(const struct sitl_input &input)
 #endif
 }
 
-#endif  // AP_SIM_JSON_ENABLED
\ No newline at end of file
+#endif  // AP_SIM_JSON_ENABLED
diff --git a/libraries/SITL/SIM_Multicopter.cpp b/libraries/SITL/SIM_Multicopter.cpp
index 30d0cfef7ed62d..7356391a1721c7 100644
--- a/libraries/SITL/SIM_Multicopter.cpp
+++ b/libraries/SITL/SIM_Multicopter.cpp
@@ -74,6 +74,7 @@ void MultiCopter::update(const struct sitl_input &input)
     frame->current_and_voltage(battery_voltage, battery_current);
 
     battery.set_current(battery_current);
+    battery_temperature = battery.get_temperature();
 
     update_dynamics(rot_accel);
     update_external_payload(input);
diff --git a/libraries/SITL/SIM_Plane.cpp b/libraries/SITL/SIM_Plane.cpp
index f3b18fb457caec..ad9fffc61313ea 100644
--- a/libraries/SITL/SIM_Plane.cpp
+++ b/libraries/SITL/SIM_Plane.cpp
@@ -520,6 +520,8 @@ void Plane::update(const struct sitl_input &input)
     float throttle = reverse_thrust ? filtered_servo_angle(input, 2) : filtered_servo_range(input, 2);
     battery_voltage = sitl->batt_voltage - 0.7*throttle;
     battery_current = (battery_voltage/sitl->batt_voltage)*50.0f*sq(throttle);
+    // this aircraft has a constant battery temperature
+    battery_temperature = 273.0f; // kelvin
 
     update_dynamics(rot_accel);
 
diff --git a/libraries/SITL/SIM_QuadPlane.cpp b/libraries/SITL/SIM_QuadPlane.cpp
index 924e1af4be6b42..b7cef453266a93 100644
--- a/libraries/SITL/SIM_QuadPlane.cpp
+++ b/libraries/SITL/SIM_QuadPlane.cpp
@@ -139,6 +139,7 @@ void QuadPlane::update(const struct sitl_input &input)
     frame->current_and_voltage(battery_voltage, battery_current);
 
     battery.set_current(battery_current);
+    battery_temperature = battery.get_temperature();
 
     float throttle;
     if (reverse_thrust) {
diff --git a/libraries/SITL/SIM_Scrimmage.cpp b/libraries/SITL/SIM_Scrimmage.cpp
index 709e3e4afaac95..0aa3ed9d3da8ae 100644
--- a/libraries/SITL/SIM_Scrimmage.cpp
+++ b/libraries/SITL/SIM_Scrimmage.cpp
@@ -117,6 +117,7 @@ void Scrimmage::recv_fdm(const struct sitl_input &input)
 
     battery_voltage = 0;
     battery_current = 0;
+    battery_temperature = 273.0f;
     rpm[0] = 0;
     rpm[1] = 0;
 

From deaf45ed016232b730860103ea27eafb838eba2b Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Thu, 5 Feb 2026 13:53:11 -0500
Subject: [PATCH 03/14] SITL: intermediate refactor of Frame towards removing
 battery

---
 libraries/SITL/SIM_Frame.cpp | 19 ++++++++++++++++---
 libraries/SITL/SIM_Frame.h   |  2 ++
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/libraries/SITL/SIM_Frame.cpp b/libraries/SITL/SIM_Frame.cpp
index b9db8a731086ba..859ea9cad1fcd4 100644
--- a/libraries/SITL/SIM_Frame.cpp
+++ b/libraries/SITL/SIM_Frame.cpp
@@ -728,8 +728,8 @@ void Frame::calculate_forces(const Aircraft &aircraft,
 }
 
 
-// calculate current and voltage
-void Frame::current_and_voltage(float &voltage, float &current)
+// reset battery if its governing params have changed
+void Frame::reset_battery_if_requested(void)
 {
     float param_voltage = AP::sitl()->batt_voltage;
     if (!is_equal(last_param_voltage,param_voltage)) {
@@ -740,10 +740,23 @@ void Frame::current_and_voltage(float &voltage, float &current)
     if (!is_equal(battery->get_capacity(), param_capacity)) {
         battery->init_capacity(param_capacity);
     }
+}
+
+// calculate current and voltage
+void Frame::current_and_voltage(float &voltage, float &current)
+{
+    reset_battery_if_requested();
     voltage = battery->get_voltage();
-    current = 0;
+    current = get_current_amp();
+}
+
+// computes (total) instantaneous current
+float Frame::get_current_amp(void)
+{
+    float current = 0;
     for (uint8_t i=0; i<num_motors; i++) {
         current += motors[i].get_current();
     }
+    return current;
 }
 #endif // AP_SIM_ENABLED
diff --git a/libraries/SITL/SIM_Frame.h b/libraries/SITL/SIM_Frame.h
index 836178518bbb25..f8f722451cd288 100644
--- a/libraries/SITL/SIM_Frame.h
+++ b/libraries/SITL/SIM_Frame.h
@@ -62,7 +62,9 @@ class Frame {
     float terminal_rotation_rate;
     uint8_t motor_offset;
 
+    void reset_battery_if_requested(void);
     // calculate current and voltage
+    float get_current_amp(void);
     void current_and_voltage(float &voltage, float &current);
 
     // get mass in kg

From 9968f1ed354c1e39012e2eb6c36877fe92185121 Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Thu, 5 Feb 2026 13:56:59 -0500
Subject: [PATCH 04/14] SITL: intermediate refactor of Frame, unbundling a
 method into parts

---
 libraries/SITL/SIM_Frame.cpp       | 8 --------
 libraries/SITL/SIM_Frame.h         | 2 --
 libraries/SITL/SIM_Multicopter.cpp | 4 +++-
 libraries/SITL/SIM_QuadPlane.cpp   | 4 +++-
 4 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/libraries/SITL/SIM_Frame.cpp b/libraries/SITL/SIM_Frame.cpp
index 859ea9cad1fcd4..de09023a1d89c7 100644
--- a/libraries/SITL/SIM_Frame.cpp
+++ b/libraries/SITL/SIM_Frame.cpp
@@ -742,14 +742,6 @@ void Frame::reset_battery_if_requested(void)
     }
 }
 
-// calculate current and voltage
-void Frame::current_and_voltage(float &voltage, float &current)
-{
-    reset_battery_if_requested();
-    voltage = battery->get_voltage();
-    current = get_current_amp();
-}
-
 // computes (total) instantaneous current
 float Frame::get_current_amp(void)
 {
diff --git a/libraries/SITL/SIM_Frame.h b/libraries/SITL/SIM_Frame.h
index f8f722451cd288..87ac2713c21233 100644
--- a/libraries/SITL/SIM_Frame.h
+++ b/libraries/SITL/SIM_Frame.h
@@ -63,9 +63,7 @@ class Frame {
     uint8_t motor_offset;
 
     void reset_battery_if_requested(void);
-    // calculate current and voltage
     float get_current_amp(void);
-    void current_and_voltage(float &voltage, float &current);
 
     // get mass in kg
     float get_mass(void) const {
diff --git a/libraries/SITL/SIM_Multicopter.cpp b/libraries/SITL/SIM_Multicopter.cpp
index 7356391a1721c7..76362e866a1286 100644
--- a/libraries/SITL/SIM_Multicopter.cpp
+++ b/libraries/SITL/SIM_Multicopter.cpp
@@ -71,7 +71,9 @@ void MultiCopter::update(const struct sitl_input &input)
     }
 
     // estimate voltage and current
-    frame->current_and_voltage(battery_voltage, battery_current);
+    frame->reset_battery_if_requested();
+    battery_voltage = battery.get_voltage();
+    battery_current = frame->get_current_amp();
 
     battery.set_current(battery_current);
     battery_temperature = battery.get_temperature();
diff --git a/libraries/SITL/SIM_QuadPlane.cpp b/libraries/SITL/SIM_QuadPlane.cpp
index b7cef453266a93..3eec21269dfc91 100644
--- a/libraries/SITL/SIM_QuadPlane.cpp
+++ b/libraries/SITL/SIM_QuadPlane.cpp
@@ -136,7 +136,9 @@ void QuadPlane::update(const struct sitl_input &input)
     }
 
     // estimate voltage and current
-    frame->current_and_voltage(battery_voltage, battery_current);
+    frame->reset_battery_if_requested();
+    battery_voltage = battery.get_voltage();
+    battery_current = frame->get_current_amp();
 
     battery.set_current(battery_current);
     battery_temperature = battery.get_temperature();

From 07b1fa1a09a5848348be5e03a92aca6dd82bfde9 Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Tue, 3 Feb 2026 08:20:36 -0500
Subject: [PATCH 05/14] SITL: SIM::Battery manages its own initial voltage

---
 libraries/SITL/SIM_Battery.cpp | 2 ++
 libraries/SITL/SIM_Battery.h   | 2 ++
 libraries/SITL/SIM_Frame.cpp   | 6 +++---
 libraries/SITL/SIM_Frame.h     | 1 -
 4 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/libraries/SITL/SIM_Battery.cpp b/libraries/SITL/SIM_Battery.cpp
index 7618745cf2796c..de9d804e143bb9 100644
--- a/libraries/SITL/SIM_Battery.cpp
+++ b/libraries/SITL/SIM_Battery.cpp
@@ -119,6 +119,8 @@ void Battery::setup(float _capacity_Ah, float _resistance, float _max_voltage)
     capacity_Ah = _capacity_Ah;
     resistance = _resistance;
     max_voltage = _max_voltage;
+    // Arbitrarily setup battery initially as depleted
+    init_voltage(0.0f);
 }
 
 void Battery::init_voltage(float voltage)
diff --git a/libraries/SITL/SIM_Battery.h b/libraries/SITL/SIM_Battery.h
index 46a686eaaf02a5..2a53bef52db893 100644
--- a/libraries/SITL/SIM_Battery.h
+++ b/libraries/SITL/SIM_Battery.h
@@ -35,6 +35,8 @@ class Battery {
 
     // return battery temperature in Kelvin:
     float get_temperature(void) const { return temperature.kelvin; }
+    // useful for detecting if param-controlled batt voltage has been changed
+    float get_init_voltage(void) const { return voltage_set; }
 
 private:
     float capacity_Ah;
diff --git a/libraries/SITL/SIM_Frame.cpp b/libraries/SITL/SIM_Frame.cpp
index de09023a1d89c7..ad7735568d129c 100644
--- a/libraries/SITL/SIM_Frame.cpp
+++ b/libraries/SITL/SIM_Frame.cpp
@@ -731,10 +731,10 @@ void Frame::calculate_forces(const Aircraft &aircraft,
 // reset battery if its governing params have changed
 void Frame::reset_battery_if_requested(void)
 {
-    float param_voltage = AP::sitl()->batt_voltage;
-    if (!is_equal(last_param_voltage,param_voltage)) {
+    // Treat a param-change as a forced re-init of battery
+    const float param_voltage = AP::sitl()->batt_voltage;
+    if (!is_equal(battery->get_init_voltage(), param_voltage)) {
         battery->init_voltage(param_voltage);
-        last_param_voltage = param_voltage;
     }
     const float param_capacity = AP::sitl()->batt_capacity_ah;
     if (!is_equal(battery->get_capacity(), param_capacity)) {
diff --git a/libraries/SITL/SIM_Frame.h b/libraries/SITL/SIM_Frame.h
index 87ac2713c21233..e64f3513aaf3f9 100644
--- a/libraries/SITL/SIM_Frame.h
+++ b/libraries/SITL/SIM_Frame.h
@@ -158,7 +158,6 @@ class Frame {
     // exposed area times coefficient of drag
     float areaCd;
     float mass;
-    float last_param_voltage;
 #if AP_SIM_ENABLED
     Battery *battery;
 #endif

From 7aec6ed3904d2e0ead1d123b6865371fbae9930b Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Thu, 5 Feb 2026 14:07:18 -0500
Subject: [PATCH 06/14] SITL: make Aircraft responsible for managing battery
 resets

---
 libraries/SITL/SIM_Aircraft.cpp    | 16 ++++++++++++++++
 libraries/SITL/SIM_Aircraft.h      |  2 ++
 libraries/SITL/SIM_Frame.cpp       | 15 ---------------
 libraries/SITL/SIM_Frame.h         |  1 -
 libraries/SITL/SIM_Multicopter.cpp |  3 +--
 libraries/SITL/SIM_QuadPlane.cpp   |  3 +--
 6 files changed, 20 insertions(+), 20 deletions(-)

diff --git a/libraries/SITL/SIM_Aircraft.cpp b/libraries/SITL/SIM_Aircraft.cpp
index 28be7e2baeebb0..a4ed13cad8f495 100644
--- a/libraries/SITL/SIM_Aircraft.cpp
+++ b/libraries/SITL/SIM_Aircraft.cpp
@@ -1415,6 +1415,22 @@ bool Aircraft::set_pose(uint8_t instance, const Location &loc, const Quaternion
     return true;
 }
 
+/*
+  Detect if the sim params controlling the battery have changed (e.g. by user) & respond
+*/
+void Aircraft::reinitialize_battery_if_param_has_changed(void)
+{
+    // Treat a param-change as a forced re-init of battery
+    const float param_voltage = sitl->batt_voltage;
+    if (!is_equal(battery.get_init_voltage(), param_voltage)) {
+        battery.init_voltage(param_voltage);
+    }
+    const float param_capacity = sitl->batt_capacity_ah;
+    if (!is_equal(battery.get_capacity(), param_capacity)) {
+        battery.init_capacity(param_capacity);
+    }
+}
+
 /*
   wrapper for scripting access
  */
diff --git a/libraries/SITL/SIM_Aircraft.h b/libraries/SITL/SIM_Aircraft.h
index fb72610f664cd5..7b1a012995fe8c 100644
--- a/libraries/SITL/SIM_Aircraft.h
+++ b/libraries/SITL/SIM_Aircraft.h
@@ -361,6 +361,8 @@ class Aircraft {
     // update EAS speeds
     void update_eas_airspeed();
 
+    void reinitialize_battery_if_param_has_changed(void);
+
     // clamp support
     class Clamp {
     public:
diff --git a/libraries/SITL/SIM_Frame.cpp b/libraries/SITL/SIM_Frame.cpp
index ad7735568d129c..ddc58465784ac7 100644
--- a/libraries/SITL/SIM_Frame.cpp
+++ b/libraries/SITL/SIM_Frame.cpp
@@ -727,21 +727,6 @@ void Frame::calculate_forces(const Aircraft &aircraft,
     body_accel = thrust/aircraft.gross_mass();
 }
 
-
-// reset battery if its governing params have changed
-void Frame::reset_battery_if_requested(void)
-{
-    // Treat a param-change as a forced re-init of battery
-    const float param_voltage = AP::sitl()->batt_voltage;
-    if (!is_equal(battery->get_init_voltage(), param_voltage)) {
-        battery->init_voltage(param_voltage);
-    }
-    const float param_capacity = AP::sitl()->batt_capacity_ah;
-    if (!is_equal(battery->get_capacity(), param_capacity)) {
-        battery->init_capacity(param_capacity);
-    }
-}
-
 // computes (total) instantaneous current
 float Frame::get_current_amp(void)
 {
diff --git a/libraries/SITL/SIM_Frame.h b/libraries/SITL/SIM_Frame.h
index e64f3513aaf3f9..a8b007bcd76d2c 100644
--- a/libraries/SITL/SIM_Frame.h
+++ b/libraries/SITL/SIM_Frame.h
@@ -62,7 +62,6 @@ class Frame {
     float terminal_rotation_rate;
     uint8_t motor_offset;
 
-    void reset_battery_if_requested(void);
     float get_current_amp(void);
 
     // get mass in kg
diff --git a/libraries/SITL/SIM_Multicopter.cpp b/libraries/SITL/SIM_Multicopter.cpp
index 76362e866a1286..1d8c729fb3d75e 100644
--- a/libraries/SITL/SIM_Multicopter.cpp
+++ b/libraries/SITL/SIM_Multicopter.cpp
@@ -70,8 +70,7 @@ void MultiCopter::update(const struct sitl_input &input)
         accel_body.zero();
     }
 
-    // estimate voltage and current
-    frame->reset_battery_if_requested();
+    reinitialize_battery_if_param_has_changed();
     battery_voltage = battery.get_voltage();
     battery_current = frame->get_current_amp();
 
diff --git a/libraries/SITL/SIM_QuadPlane.cpp b/libraries/SITL/SIM_QuadPlane.cpp
index 3eec21269dfc91..b7e892459ab4f8 100644
--- a/libraries/SITL/SIM_QuadPlane.cpp
+++ b/libraries/SITL/SIM_QuadPlane.cpp
@@ -135,8 +135,7 @@ void QuadPlane::update(const struct sitl_input &input)
         quad_accel_body.rotate(ROTATION_PITCH_270);
     }
 
-    // estimate voltage and current
-    frame->reset_battery_if_requested();
+    reinitialize_battery_if_param_has_changed();
     battery_voltage = battery.get_voltage();
     battery_current = frame->get_current_amp();
 

From f2ca3b3594616d431222d6280ec040503884a02b Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Thu, 5 Feb 2026 14:11:02 -0500
Subject: [PATCH 07/14] SITL: Frame uses aircraft's battery voltage

---
 libraries/SITL/SIM_Frame.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libraries/SITL/SIM_Frame.cpp b/libraries/SITL/SIM_Frame.cpp
index ddc58465784ac7..279378fbe41584 100644
--- a/libraries/SITL/SIM_Frame.cpp
+++ b/libraries/SITL/SIM_Frame.cpp
@@ -682,7 +682,8 @@ void Frame::calculate_forces(const Aircraft &aircraft,
     const auto *_sitl = AP::sitl();
     for (uint8_t i=0; i<num_motors; i++) {
         Vector3f mtorque, mthrust;
-        motors[i].calculate_forces(input, motor_offset, mtorque, mthrust, vel_air_bf, gyro, air_density, battery->get_voltage(), use_drag);
+        motors[i].calculate_forces(input, motor_offset, mtorque, mthrust, vel_air_bf,
+                                   gyro, air_density, aircraft.get_battery_voltage(), use_drag);
         torque += mtorque;
         thrust += mthrust;
         // simulate motor rpm

From 8a205b79e87df0e1f629d263250b00456b1fc52a Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Thu, 5 Feb 2026 14:25:43 -0500
Subject: [PATCH 08/14] SITL: completed refactor of Frame, now does not use
 SIM::Battery

---
 libraries/SITL/SIM_Frame.cpp       |  5 +----
 libraries/SITL/SIM_Frame.h         | 13 ++++++++-----
 libraries/SITL/SIM_Multicopter.cpp |  5 ++++-
 libraries/SITL/SIM_QuadPlane.cpp   |  5 ++++-
 4 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/libraries/SITL/SIM_Frame.cpp b/libraries/SITL/SIM_Frame.cpp
index 279378fbe41584..02db5af0835aa6 100644
--- a/libraries/SITL/SIM_Frame.cpp
+++ b/libraries/SITL/SIM_Frame.cpp
@@ -577,10 +577,9 @@ void Frame::parse_vector3(AP_JSON::value val, const char* label, Vector3f &param
 /*
   initialise the frame
  */
-void Frame::init(const char *frame_str, Battery *_battery)
+void Frame::init(const char *frame_str)
 {
     model = default_model;
-    battery = _battery;
 
     const char *colon = strchr(frame_str, ':');
     size_t slen = strlen(frame_str);
@@ -620,8 +619,6 @@ void Frame::init(const char *frame_str, Battery *_battery)
     // power_factor is ratio of power consumed per newton of thrust
     float power_factor = hover_power / hover_thrust;
 
-    battery->setup(model.battCapacityAh, model.refBatRes, model.maxVoltage);
-
     if (uint8_t(model.num_motors) != num_motors) {
         ::printf("Warning model expected %u motors and got %u\n", uint8_t(model.num_motors), num_motors);
     }
diff --git a/libraries/SITL/SIM_Frame.h b/libraries/SITL/SIM_Frame.h
index a8b007bcd76d2c..9172ce64952aee 100644
--- a/libraries/SITL/SIM_Frame.h
+++ b/libraries/SITL/SIM_Frame.h
@@ -49,7 +49,7 @@ class Frame {
     static Frame *create_frame(const char *name);
     
     // initialise frame
-    void init(const char *frame_str, Battery *_battery);
+    void init(const char *frame_str);
 
     // calculate rotational and linear accelerations
     void calculate_forces(const Aircraft &aircraft,
@@ -73,6 +73,10 @@ class Frame {
     void set_mass(float new_mass) {
         mass = new_mass;
     }
+
+    float get_model_batt_max_voltage(void) const { return model.maxVoltage; }
+    float get_model_batt_capacity_ah(void) const { return model.battCapacityAh; }
+    float get_model_batt_resistance_ohm(void) const { return model.refBatRes; }
     
 private:
     /*
@@ -95,7 +99,6 @@ class Frame {
         float refCurrent = 29.3; // Amps
         float refAlt = 593; // altitude AMSL
         float refTempC = 25; // temperature C
-        float refBatRes = 0.01; // BAT.Res
 
         // full pack voltage
         float maxVoltage = 4.2*3;
@@ -103,6 +106,9 @@ class Frame {
         // battery capacity in Ah. Use zero for unlimited
         float battCapacityAh = 0.0;
 
+        // battery resistance reference value in Ohms
+        float refBatRes = 0.01;
+
         // CTUN.ThO at hover at refAlt
         float hoverThrOut = 0.39;
 
@@ -157,9 +163,6 @@ class Frame {
     // exposed area times coefficient of drag
     float areaCd;
     float mass;
-#if AP_SIM_ENABLED
-    Battery *battery;
-#endif
 
     // json parsing helpers
     void parse_float(AP_JSON::value val, const char* label, float &param);
diff --git a/libraries/SITL/SIM_Multicopter.cpp b/libraries/SITL/SIM_Multicopter.cpp
index 1d8c729fb3d75e..9bbe03c7b39774 100644
--- a/libraries/SITL/SIM_Multicopter.cpp
+++ b/libraries/SITL/SIM_Multicopter.cpp
@@ -32,7 +32,10 @@ MultiCopter::MultiCopter(const char *frame_str) :
         exit(1);
     }
 
-    frame->init(frame_str, &battery);
+    frame->init(frame_str);
+    battery.setup(frame->get_model_batt_capacity_ah(),
+                  frame->get_model_batt_resistance_ohm(),
+                  frame->get_model_batt_max_voltage());
 
     mass = frame->get_mass();
     frame_height = 0.1;
diff --git a/libraries/SITL/SIM_QuadPlane.cpp b/libraries/SITL/SIM_QuadPlane.cpp
index b7e892459ab4f8..7148f7554c60ac 100644
--- a/libraries/SITL/SIM_QuadPlane.cpp
+++ b/libraries/SITL/SIM_QuadPlane.cpp
@@ -101,7 +101,10 @@ QuadPlane::QuadPlane(const char *frame_str) :
     frame->motor_offset = motor_offset;
 
     // we use zero terminal velocity to let the plane model handle the drag
-    frame->init(frame_str, &battery);
+    frame->init(frame_str);
+    battery.setup(frame->get_model_batt_capacity_ah(),
+                  frame->get_model_batt_resistance_ohm(),
+                  frame->get_model_batt_max_voltage());
 
     // increase mass for plane components
     mass = frame->get_mass() * 1.5;

From ca8259275801cd77e53b08d05d5948bccbc05c11 Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Thu, 5 Feb 2026 19:26:11 -0500
Subject: [PATCH 09/14] SITL: put battery maybe-reset logic into SITL::Battery

---
 libraries/SITL/SIM_Aircraft.cpp    | 16 ----------------
 libraries/SITL/SIM_Aircraft.h      |  2 --
 libraries/SITL/SIM_Battery.cpp     | 22 ++++++++++++++++++++--
 libraries/SITL/SIM_Battery.h       |  7 ++++---
 libraries/SITL/SIM_Multicopter.cpp |  2 +-
 libraries/SITL/SIM_QuadPlane.cpp   |  2 +-
 6 files changed, 26 insertions(+), 25 deletions(-)

diff --git a/libraries/SITL/SIM_Aircraft.cpp b/libraries/SITL/SIM_Aircraft.cpp
index a4ed13cad8f495..28be7e2baeebb0 100644
--- a/libraries/SITL/SIM_Aircraft.cpp
+++ b/libraries/SITL/SIM_Aircraft.cpp
@@ -1415,22 +1415,6 @@ bool Aircraft::set_pose(uint8_t instance, const Location &loc, const Quaternion
     return true;
 }
 
-/*
-  Detect if the sim params controlling the battery have changed (e.g. by user) & respond
-*/
-void Aircraft::reinitialize_battery_if_param_has_changed(void)
-{
-    // Treat a param-change as a forced re-init of battery
-    const float param_voltage = sitl->batt_voltage;
-    if (!is_equal(battery.get_init_voltage(), param_voltage)) {
-        battery.init_voltage(param_voltage);
-    }
-    const float param_capacity = sitl->batt_capacity_ah;
-    if (!is_equal(battery.get_capacity(), param_capacity)) {
-        battery.init_capacity(param_capacity);
-    }
-}
-
 /*
   wrapper for scripting access
  */
diff --git a/libraries/SITL/SIM_Aircraft.h b/libraries/SITL/SIM_Aircraft.h
index 7b1a012995fe8c..fb72610f664cd5 100644
--- a/libraries/SITL/SIM_Aircraft.h
+++ b/libraries/SITL/SIM_Aircraft.h
@@ -361,8 +361,6 @@ class Aircraft {
     // update EAS speeds
     void update_eas_airspeed();
 
-    void reinitialize_battery_if_param_has_changed(void);
-
     // clamp support
     class Clamp {
     public:
diff --git a/libraries/SITL/SIM_Battery.cpp b/libraries/SITL/SIM_Battery.cpp
index de9d804e143bb9..65b7b22815bc31 100644
--- a/libraries/SITL/SIM_Battery.cpp
+++ b/libraries/SITL/SIM_Battery.cpp
@@ -17,6 +17,7 @@
 */
 
 #include "SIM_Battery.h"
+#include <AP_Math/AP_Math.h>
 
 using namespace SITL;
 
@@ -119,8 +120,25 @@ void Battery::setup(float _capacity_Ah, float _resistance, float _max_voltage)
     capacity_Ah = _capacity_Ah;
     resistance = _resistance;
     max_voltage = _max_voltage;
-    // Arbitrarily setup battery initially as depleted
-    init_voltage(0.0f);
+
+    voltage_set = max_voltage;
+    voltage_filter.reset(voltage_set);
+    set_initial_SoC(voltage_set);
+}
+
+void Battery::maybe_reset(float desired_voltage, float desired_capacity_Ah)
+{
+    const bool reset_not_needed = (is_equal(voltage_set, desired_voltage)
+                                   && is_equal(capacity_Ah, desired_capacity_Ah));
+    if (reset_not_needed) {
+        return;
+    }
+
+    capacity_Ah = desired_capacity_Ah;
+    // a negative desired voltage is unexpected, but not problematic
+    voltage_set = MIN(desired_voltage, max_voltage);
+    voltage_filter.reset(voltage_set);
+    set_initial_SoC(voltage_set);
 }
 
 void Battery::init_voltage(float voltage)
diff --git a/libraries/SITL/SIM_Battery.h b/libraries/SITL/SIM_Battery.h
index 2a53bef52db893..3336530b6f16c9 100644
--- a/libraries/SITL/SIM_Battery.h
+++ b/libraries/SITL/SIM_Battery.h
@@ -26,17 +26,18 @@ class Battery {
 public:
     void setup(float _capacity_Ah, float _resistance, float _max_voltage);
 
+    // Implements the (re)setting behavior described by SIM_BATT_* parameters
+    // (reminder: if the desired values are different than previous choices, reset)
+    void maybe_reset(float desired_voltage, float desired_capacity_Ah);
+
     void init_voltage(float voltage);
     void init_capacity(float capacity);
 
     void set_current(float current_amps);
     float get_voltage(void) const;
-    float get_capacity(void) const { return capacity_Ah; }
 
     // return battery temperature in Kelvin:
     float get_temperature(void) const { return temperature.kelvin; }
-    // useful for detecting if param-controlled batt voltage has been changed
-    float get_init_voltage(void) const { return voltage_set; }
 
 private:
     float capacity_Ah;
diff --git a/libraries/SITL/SIM_Multicopter.cpp b/libraries/SITL/SIM_Multicopter.cpp
index 9bbe03c7b39774..53e2b571ce3a53 100644
--- a/libraries/SITL/SIM_Multicopter.cpp
+++ b/libraries/SITL/SIM_Multicopter.cpp
@@ -73,7 +73,7 @@ void MultiCopter::update(const struct sitl_input &input)
         accel_body.zero();
     }
 
-    reinitialize_battery_if_param_has_changed();
+    battery.maybe_reset(sitl->batt_voltage, sitl->batt_capacity_ah);
     battery_voltage = battery.get_voltage();
     battery_current = frame->get_current_amp();
 
diff --git a/libraries/SITL/SIM_QuadPlane.cpp b/libraries/SITL/SIM_QuadPlane.cpp
index 7148f7554c60ac..be16cc6c1cee4c 100644
--- a/libraries/SITL/SIM_QuadPlane.cpp
+++ b/libraries/SITL/SIM_QuadPlane.cpp
@@ -138,7 +138,7 @@ void QuadPlane::update(const struct sitl_input &input)
         quad_accel_body.rotate(ROTATION_PITCH_270);
     }
 
-    reinitialize_battery_if_param_has_changed();
+    battery.maybe_reset(sitl->batt_voltage, sitl->batt_capacity_ah);
     battery_voltage = battery.get_voltage();
     battery_current = frame->get_current_amp();
 

From a40fb757cf0328edd7aa349f27b9d7053adb4bc9 Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Thu, 5 Feb 2026 14:58:38 -0500
Subject: [PATCH 10/14] SITL: [minor] remove an unused string, mark it unused
 in ctor

---
 libraries/SITL/SIM_Aircraft.cpp | 3 +--
 libraries/SITL/SIM_Aircraft.h   | 3 +--
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/libraries/SITL/SIM_Aircraft.cpp b/libraries/SITL/SIM_Aircraft.cpp
index 28be7e2baeebb0..73a38bf5d69e59 100644
--- a/libraries/SITL/SIM_Aircraft.cpp
+++ b/libraries/SITL/SIM_Aircraft.cpp
@@ -50,8 +50,7 @@ Aircraft *Aircraft::instances[MAX_SIM_INSTANCES];
   parent class for all simulator types
  */
 
-Aircraft::Aircraft(const char *frame_str) :
-    frame(frame_str)
+Aircraft::Aircraft(const char *unused_frame_str)
 {
     // make the SIM_* variables available to simulator backends
     sitl = AP::sitl();
diff --git a/libraries/SITL/SIM_Aircraft.h b/libraries/SITL/SIM_Aircraft.h
index fb72610f664cd5..4272299ec74ff2 100644
--- a/libraries/SITL/SIM_Aircraft.h
+++ b/libraries/SITL/SIM_Aircraft.h
@@ -53,7 +53,7 @@ namespace SITL {
  */
 class Aircraft {
 public:
-    Aircraft(const char *frame_str);
+    Aircraft(const char *unused_frame_str);
 
     // called directly after constructor:
     virtual void set_start_location(const Location &start_loc, const float start_yaw);
@@ -276,7 +276,6 @@ class Aircraft {
     uint32_t last_frame_count;
     uint8_t instance;
     const char *autotest_dir;
-    const char *frame;
     bool use_time_sync = true;
     float last_speedup = -1.0f;
     const char *config_ = "";

From 21dc1efb03a59d0043a5e398d57ea7bdbd9c4cf7 Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Thu, 5 Feb 2026 18:13:10 -0500
Subject: [PATCH 11/14] SITL: improve SITL::Battery in multiple ways

---
 libraries/SITL/SIM_Battery.cpp     | 95 +++++++++++++++---------------
 libraries/SITL/SIM_Battery.h       | 27 +++++----
 libraries/SITL/SIM_Multicopter.cpp |  2 +-
 libraries/SITL/SIM_QuadPlane.cpp   |  2 +-
 libraries/SITL/SITL.cpp            |  4 +-
 5 files changed, 68 insertions(+), 62 deletions(-)

diff --git a/libraries/SITL/SIM_Battery.cpp b/libraries/SITL/SIM_Battery.cpp
index 65b7b22815bc31..f592b50847f72f 100644
--- a/libraries/SITL/SIM_Battery.cpp
+++ b/libraries/SITL/SIM_Battery.cpp
@@ -17,6 +17,7 @@
 */
 
 #include "SIM_Battery.h"
+#include <float.h>
 #include <AP_Math/AP_Math.h>
 
 using namespace SITL;
@@ -72,9 +73,14 @@ static const struct {
 /*
   use table to get resting voltage from remaining capacity
  */
-float Battery::get_resting_voltage(float charge_pct) const
+float Battery::get_resting_voltage(void) const
 {
+    if (capacity_is_unlimited()) {
+        return max_voltage;
+    }
+    float charge_pct = 100 * remaining_Ah / capacity_Ah;
     const float max_cell_voltage = soc_table[0].volt_per_cell;
+    const float min_cell_voltage = soc_table[ARRAY_SIZE(soc_table) - 1].volt_per_cell;
     for (uint8_t i=1; i<ARRAY_SIZE(soc_table); i++) {
         if (charge_pct >= soc_table[i].soc_pct) {
             // linear interpolation between table rows
@@ -86,15 +92,21 @@ float Battery::get_resting_voltage(float charge_pct) const
             return (cell_volt / max_cell_voltage) * max_voltage;
         }
     }
-    // off the bottom of the table, return a small non-zero to prevent math errors
-    return 0.001;
+    // off the bottom of the table
+    return min_cell_voltage;
 }
 
 /*
-  use table to set initial state of charge from voltage
+  return remaining Amp-hours (aka "charge", "state of charge") corresponding to a voltage
+
+  this is const for readability: it has no "side effects"
  */
-void Battery::set_initial_SoC(float voltage)
+float Battery::compute_remaining_ah(float voltage) const
 {
+    if (capacity_is_unlimited()) {
+        return FLT_MAX;
+    }
+
     const float max_cell_voltage = soc_table[0].volt_per_cell;
     float cell_volt = (voltage / max_voltage) * max_cell_voltage;
 
@@ -106,24 +118,28 @@ void Battery::set_initial_SoC(float voltage)
             float soc1 = soc_table[i].soc_pct;
             float soc2 = soc_table[i-1].soc_pct;
             float soc = soc1 + (dv1 / dv2) * (soc2 - soc1);
-            remaining_Ah = capacity_Ah * soc * 0.01;
-            return;
+            return capacity_Ah * soc * 0.01;
         }
     }
-
     // off the bottom of the table
-    remaining_Ah = 0;
+    return 0.0f;
 }
 
-void Battery::setup(float _capacity_Ah, float _resistance, float _max_voltage)
+void Battery::set_remaining_ah(void)
+{
+    remaining_Ah = compute_remaining_ah(voltage_set);
+}
+
+// Reminder: capacity <= 0 means **unlimited**
+void Battery::setup(float _capacity_Ah, float _resistance_ohm, float _max_voltage)
 {
     capacity_Ah = _capacity_Ah;
-    resistance = _resistance;
+    resistance_ohm = _resistance_ohm;
     max_voltage = _max_voltage;
 
     voltage_set = max_voltage;
     voltage_filter.reset(voltage_set);
-    set_initial_SoC(voltage_set);
+    set_remaining_ah();
 }
 
 void Battery::maybe_reset(float desired_voltage, float desired_capacity_Ah)
@@ -138,23 +154,10 @@ void Battery::maybe_reset(float desired_voltage, float desired_capacity_Ah)
     // a negative desired voltage is unexpected, but not problematic
     voltage_set = MIN(desired_voltage, max_voltage);
     voltage_filter.reset(voltage_set);
-    set_initial_SoC(voltage_set);
+    set_remaining_ah();
 }
 
-void Battery::init_voltage(float voltage)
-{
-    voltage_filter.reset(voltage);
-    voltage_set = voltage;
-    set_initial_SoC(voltage);
-}
-
-void Battery::init_capacity(float capacity)
-{
-    capacity_Ah = capacity;
-    set_initial_SoC(voltage_set);
-}
-
-void Battery::set_current(float current)
+void Battery::consume_energy(float current_amps)
 {
     uint64_t now = AP_HAL::micros64();
     float dt = (now - last_us) * 1.0e-6;
@@ -163,31 +166,29 @@ void Battery::set_current(float current)
         dt = 0;
     }
     last_us = now;
-    float delta_Ah = current * dt / 3600;
+    float delta_Ah = current_amps * dt / 3600;
     remaining_Ah -= delta_Ah;
     remaining_Ah = MAX(0, remaining_Ah);
 
-    float voltage_delta = current * resistance;
-    float voltage;
-    if (!is_positive(capacity_Ah)) {
-        voltage = voltage_set;
-    } else {
-        voltage = get_resting_voltage(100 * remaining_Ah / capacity_Ah) - voltage_delta;
-    }
-
-    voltage_filter.apply(voltage, dt);
+    float voltage_delta = current_amps * resistance_ohm;
+    float sagged_voltage = get_resting_voltage() - voltage_delta;
+    voltage_filter.apply(sagged_voltage, dt);
 
-    {
-        const uint64_t temperature_dt = now - temperature.last_update_micros;
-        temperature.last_update_micros = now;
-        // 1 amp*1 second == 0.1 degrees of energy.  Did those units hurt?
-        temperature.kelvin += 0.1 * current * temperature_dt * 0.000001;
-        // decay temperature at some %second towards ambient
-        temperature.kelvin -= (temperature.kelvin - 273) * 0.10 * temperature_dt * 0.000001;
-    }
+    update_temperature(current_amps, now);
 }
 
-float Battery::get_voltage(void) const
+void Battery::update_temperature(float current_amps, uint64_t now)
 {
-    return voltage_filter.get();
+    const float dt = (now - temperature.last_update_micros) * 1.0e-6;
+    temperature.last_update_micros = now;
+
+    // temperature growth model
+
+    // thermal_capacity value chosen to match previous steady-state behavior at 28amps
+    // (reminder: thermal_capacity = mass * specific_heat)
+    const float inverse_of_thermal_capacity = 0.36f;  // use inverse so we can multiply, not divide
+    temperature.kelvin += (current_amps * current_amps) * resistance_ohm * dt * inverse_of_thermal_capacity;
+
+    // temperature decay model: first-order
+    temperature.kelvin -= (temperature.kelvin - (ambient_temperature + 273.15f)) * 0.10f * dt;
 }
diff --git a/libraries/SITL/SIM_Battery.h b/libraries/SITL/SIM_Battery.h
index 3336530b6f16c9..bf0a46a9412d34 100644
--- a/libraries/SITL/SIM_Battery.h
+++ b/libraries/SITL/SIM_Battery.h
@@ -22,40 +22,45 @@
 
 namespace SITL {
 
+constexpr float ambient_temperature = 0.0f;  // degC
+
 class Battery {
 public:
-    void setup(float _capacity_Ah, float _resistance, float _max_voltage);
+    // Note: capacity<=0 means **unlimited** capacity.
+    void setup(float _capacity_Ah, float _resistance_ohm, float _max_voltage);
 
     // Implements the (re)setting behavior described by SIM_BATT_* parameters
     // (reminder: if the desired values are different than previous choices, reset)
     void maybe_reset(float desired_voltage, float desired_capacity_Ah);
 
-    void init_voltage(float voltage);
-    void init_capacity(float capacity);
+    // Call this periodically to "step" the battery forward in time
+    void consume_energy(float current_amps);
 
-    void set_current(float current_amps);
-    float get_voltage(void) const;
+    float get_voltage(void) const { return voltage_filter.get(); }
 
     // return battery temperature in Kelvin:
     float get_temperature(void) const { return temperature.kelvin; }
 
 private:
-    float capacity_Ah;
-    float resistance;
+    float capacity_Ah; // set <= 0 for unlmited capacity
+    float resistance_ohm;
     float max_voltage;
     float voltage_set;
-    float remaining_Ah;
+    float remaining_Ah; // if capacity is unlimited, set this to FLT_MAX
     uint64_t last_us;
 
     struct {
-        float kelvin = 273;
+        float kelvin = ambient_temperature + 273.15f;
         uint64_t last_update_micros;
     } temperature;
+    void update_temperature(float current_amps, uint64_t now);
 
     // 10Hz filter for battery voltage
     LowPassFilterFloat voltage_filter{10};
 
-    float get_resting_voltage(float charge_pct) const;
-    void set_initial_SoC(float voltage);
+    float get_resting_voltage(void) const;
+    float compute_remaining_ah(float voltage) const; // const shows this has no side effects
+    void set_remaining_ah(void);
+    bool capacity_is_unlimited(void) const { return !(is_positive(capacity_Ah)); } // for readability
 };
 }
diff --git a/libraries/SITL/SIM_Multicopter.cpp b/libraries/SITL/SIM_Multicopter.cpp
index 53e2b571ce3a53..ce50f8b9d0b794 100644
--- a/libraries/SITL/SIM_Multicopter.cpp
+++ b/libraries/SITL/SIM_Multicopter.cpp
@@ -77,7 +77,7 @@ void MultiCopter::update(const struct sitl_input &input)
     battery_voltage = battery.get_voltage();
     battery_current = frame->get_current_amp();
 
-    battery.set_current(battery_current);
+    battery.consume_energy(battery_current);
     battery_temperature = battery.get_temperature();
 
     update_dynamics(rot_accel);
diff --git a/libraries/SITL/SIM_QuadPlane.cpp b/libraries/SITL/SIM_QuadPlane.cpp
index be16cc6c1cee4c..8ebbd710240169 100644
--- a/libraries/SITL/SIM_QuadPlane.cpp
+++ b/libraries/SITL/SIM_QuadPlane.cpp
@@ -142,7 +142,7 @@ void QuadPlane::update(const struct sitl_input &input)
     battery_voltage = battery.get_voltage();
     battery_current = frame->get_current_amp();
 
-    battery.set_current(battery_current);
+    battery.consume_energy(battery_current);
     battery_temperature = battery.get_temperature();
 
     float throttle;
diff --git a/libraries/SITL/SITL.cpp b/libraries/SITL/SITL.cpp
index b12b1c872784cf..84f640cff87f96 100644
--- a/libraries/SITL/SITL.cpp
+++ b/libraries/SITL/SITL.cpp
@@ -117,13 +117,13 @@ const AP_Param::GroupInfo SIM::var_info[] = {
     AP_GROUPINFO("SONAR_ROT",     17, SIM,  sonar_rot, Rotation::ROTATION_PITCH_270),
     // @Param: BATT_VOLTAGE
     // @DisplayName: Simulated battery voltage
-    // @Description: Simulated battery voltage. Constant voltage when SIM_BATT_CAP_AH is 0, otherwise changing this parameter will re-initialize the state of charge of the battery based on this voltage versus the battery's maximum voltage (default is max voltage).
+    // @Description: Simulated battery voltage. This parameter's default value is the battery's maximum voltage. A value higher than the max will be treated identically as exactly that max value. When capacity is unlimited (see SIM_BATT_CAP_AH), this is the (constant) resting voltage. Otherwise, changing this value correspondingly (re)sets the battery's charge percentage (by comparing the new value against the max voltage).
     // @Units: V
     // @User: Advanced
     AP_GROUPINFO("BATT_VOLTAGE",  19, SIM,  batt_voltage,  12.6f),
     // @Param: BATT_CAP_AH
     // @DisplayName: Simulated battery capacity
-    // @Description: Simulated battery capacity. Set to 0 for unlimited capacity. Changing this parameter will re-initialize the state of charge of the battery.
+    // @Description: Simulated battery capacity. Set to 0 for unlimited capacity. Changing this value (re)sets the battery's charge percentage (based on SIM_BATT_VOLTAGE).
     // @Units: Ah
     // @User: Advanced
     AP_GROUPINFO("BATT_CAP_AH",   20, SIM,  batt_capacity_ah,  0),

From f1f12b1a5b94ea0b7f3e137f1918bea14da2eafc Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Fri, 6 Feb 2026 16:54:58 -0500
Subject: [PATCH 12/14] SITL: fix EvaluateBatteryModel example

---
 .../examples/EvaluateBatteryModel/EvaluateBatteryModel.cpp     | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/libraries/SITL/examples/EvaluateBatteryModel/EvaluateBatteryModel.cpp b/libraries/SITL/examples/EvaluateBatteryModel/EvaluateBatteryModel.cpp
index 1e131f758a7943..bcbe8e2fac3968 100644
--- a/libraries/SITL/examples/EvaluateBatteryModel/EvaluateBatteryModel.cpp
+++ b/libraries/SITL/examples/EvaluateBatteryModel/EvaluateBatteryModel.cpp
@@ -62,7 +62,6 @@ void setup(void)
 
     // setup battery model
     battery.setup(amp_hour_capacity, resistance, max_voltage);
-    battery.init_voltage(max_voltage);
 
     uint64_t time = 0;
     hal.scheduler->stop_clock(time);
@@ -70,7 +69,7 @@ void setup(void)
 
     ::printf("time, voltage\n");
     while (battery.get_voltage() >= min_voltage) {
-        battery.set_current(current);
+        battery.consume_energy(current);
 
         ::printf("%0.2f, %0.2f\n", time * 1.0e-6, battery.get_voltage());
 

From 6bde246bfc3c5659fa61936dcc5b5f30b540a51e Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Fri, 6 Feb 2026 11:28:39 -0500
Subject: [PATCH 13/14] SITL: transition off SITL_State battery model

The sub-specific model is put into SITL::Submarine.
The now-unused general model is removed.
---
 libraries/AP_HAL_SITL/SITL_State.cpp        | 24 +++++++++++-
 libraries/AP_HAL_SITL/SITL_State.h          |  1 +
 libraries/AP_HAL_SITL/SITL_State_common.cpp | 43 ---------------------
 libraries/AP_HAL_SITL/SITL_State_common.h   |  2 -
 libraries/SITL/SIM_Aircraft.h               |  1 +
 libraries/SITL/SIM_Submarine.cpp            | 15 +++++++
 libraries/SITL/SIM_Submarine.h              |  2 +
 7 files changed, 42 insertions(+), 46 deletions(-)

diff --git a/libraries/AP_HAL_SITL/SITL_State.cpp b/libraries/AP_HAL_SITL/SITL_State.cpp
index 08e78f3739013e..50ff1e03d9a5c6 100644
--- a/libraries/AP_HAL_SITL/SITL_State.cpp
+++ b/libraries/AP_HAL_SITL/SITL_State.cpp
@@ -213,6 +213,7 @@ void SITL_State::_fdm_input_local(void)
 
     // construct servos structure for FDM
     _simulator_servos(input);
+    _set_voltage_and_current_pins(input);
 
 #if AP_SIM_JSON_MASTER_ENABLED
     // read servo inputs from ride along flight controllers
@@ -368,8 +369,29 @@ void SITL_State::_simulator_servos(struct sitl_input &input)
         }
     }
     _sitl->throttle = throttle;
+}
+
+void SITL_State::_set_voltage_and_current_pins(struct sitl_input &input)
+{
+    float voltage = 0;
+    float current = 0;
+
+    if (_sitl->state.battery_voltage > 0) {
+        // FDM provides voltage and current
+        voltage = _sitl->state.battery_voltage;
+        current = _sitl->state.battery_current;
+    } else {
+        // internal sitl model provides voltage and current
+        voltage = sitl_model->get_battery_voltage();
+        current = sitl_model->get_battery_current();
+    }
 
-    update_voltage_current(input, throttle);
+    // assume 3DR power brick
+    voltage_pin_voltage = voltage / 10.1f;
+    current_pin_voltage = current / 17.0f;
+    // fake battery2 as just a 25% gain on the first one
+    voltage2_pin_voltage = voltage_pin_voltage * 0.25f;
+    current2_pin_voltage = current_pin_voltage * 0.25f;
 }
 
 void SITL_State::init(int argc, char * const argv[])
diff --git a/libraries/AP_HAL_SITL/SITL_State.h b/libraries/AP_HAL_SITL/SITL_State.h
index 97ef75236d8d60..97ae2acfc916d2 100644
--- a/libraries/AP_HAL_SITL/SITL_State.h
+++ b/libraries/AP_HAL_SITL/SITL_State.h
@@ -70,6 +70,7 @@ class HALSITL::SITL_State : public SITL_State_Common {
     void _fdm_input_local(void);
     void _output_to_flightgear(void);
     void _simulator_servos(struct sitl_input &input);
+    void _set_voltage_and_current_pins(struct sitl_input &input);
     void _fdm_input_step(void);
 
     void wait_clock(uint64_t wait_time_usec);
diff --git a/libraries/AP_HAL_SITL/SITL_State_common.cpp b/libraries/AP_HAL_SITL/SITL_State_common.cpp
index acb74cbe3e74c3..4c17d10d7bde7a 100644
--- a/libraries/AP_HAL_SITL/SITL_State_common.cpp
+++ b/libraries/AP_HAL_SITL/SITL_State_common.cpp
@@ -432,47 +432,4 @@ void SITL_State_Common::sim_update(void)
 /*
   update voltage and current pins
  */
-void SITL_State_Common::update_voltage_current(struct sitl_input &input, float throttle)
-{
-    float voltage = 0;
-    float current = 0;
-    
-    if (_sitl != nullptr) {
-        if (_sitl->state.battery_voltage <= 0) {
-            if (_vehicle == ArduSub) {
-                voltage = _sitl->batt_voltage;
-                for (uint8_t i=0; i<6; i++) {
-                    float pwm = input.servos[i];
-                    //printf("i: %d, pwm: %.2f\n", i, pwm);
-                    float fraction = fabsf((pwm - 1500) / 500.0f);
-
-                    voltage -= fraction * 0.5f;
-
-                    float draw = fraction * 15;
-                    current += draw;
-                }
-            } else {
-                // simulate simple battery setup
-                // lose 0.7V at full throttle
-                voltage = _sitl->batt_voltage - 0.7f * throttle;
-
-                // assume 50A at full throttle
-                current = 50.0f * throttle;
-            }
-        } else {
-            // FDM provides voltage and current
-            voltage = _sitl->state.battery_voltage;
-            current = _sitl->state.battery_current;
-        }
-    }
-
-    // assume 3DR power brick
-    voltage_pin_voltage = (voltage / 10.1f);
-    current_pin_voltage = current/17.0f;
-    // fake battery2 as just a 25% gain on the first one
-    voltage2_pin_voltage = voltage_pin_voltage * 0.25f;
-    current2_pin_voltage = current_pin_voltage * 0.25f;
-}
-
 #endif // HAL_BOARD_SITL
-
diff --git a/libraries/AP_HAL_SITL/SITL_State_common.h b/libraries/AP_HAL_SITL/SITL_State_common.h
index 82f9488f95e624..91e407b9bf996c 100644
--- a/libraries/AP_HAL_SITL/SITL_State_common.h
+++ b/libraries/AP_HAL_SITL/SITL_State_common.h
@@ -218,8 +218,6 @@ class HALSITL::SITL_State_Common {
     SITL::Aircraft *sitl_model;
 
     SITL::SIM *_sitl;
-
-    void update_voltage_current(struct sitl_input &input, float throttle);
 };
 
 #endif // CONFIG_HAL_BOARD == HAL_BOARD_SITL
diff --git a/libraries/SITL/SIM_Aircraft.h b/libraries/SITL/SIM_Aircraft.h
index 4272299ec74ff2..fb3f355d1f88ef 100644
--- a/libraries/SITL/SIM_Aircraft.h
+++ b/libraries/SITL/SIM_Aircraft.h
@@ -175,6 +175,7 @@ class Aircraft {
 #endif
     float get_battery_voltage() const { return battery_voltage; }
     float get_battery_temperature() const { return battery_temperature; }
+    float get_battery_current() const { return battery_current; }
 
     float ambient_temperature_degC() const;
 
diff --git a/libraries/SITL/SIM_Submarine.cpp b/libraries/SITL/SIM_Submarine.cpp
index dcfca1dcd3d329..fa9691fcfcd996 100644
--- a/libraries/SITL/SIM_Submarine.cpp
+++ b/libraries/SITL/SIM_Submarine.cpp
@@ -121,6 +121,20 @@ void Submarine::calculate_forces(const struct sitl_input &input, Vector3f &rot_a
 }
 
 
+void Submarine::update_battery(const struct sitl_input &input)
+{
+    battery_voltage = sitl->batt_voltage;
+    battery_current = 0;
+    for (uint8_t i=0; i<6; i++) {
+        float pwm = input.servos[i];
+        //printf("i: %d, pwm: %.2f\n", i, pwm);
+        float fraction = fabsf((pwm - 1500) / 500.0f);
+        battery_voltage -= fraction * 0.5f;
+        float draw = fraction * 15;
+        battery_current += draw;
+    }
+}
+
 /**
  * @brief Calculate the torque induced by buoyancy foam
  *
@@ -239,6 +253,7 @@ void Submarine::update(const struct sitl_input &input)
     Vector3f rot_accel;
 
     calculate_forces(input, rot_accel, accel_body);
+    update_battery(input);
 
     update_dynamics(rot_accel);
     update_external_payload(input);
diff --git a/libraries/SITL/SIM_Submarine.h b/libraries/SITL/SIM_Submarine.h
index ea0e311760b216..fd38af5ec9be64 100644
--- a/libraries/SITL/SIM_Submarine.h
+++ b/libraries/SITL/SIM_Submarine.h
@@ -105,6 +105,8 @@ class Submarine : public Aircraft {
     void calculate_angular_drag_torque(const Vector3f &angular_velocity, const Vector3f &drag_coefficient, Vector3f &torque) const;
     // calculate torque induced by buoyancy foams
     void calculate_buoyancy_torque(Vector3f &torque);
+    // compute and set battery usage
+    void update_battery(const struct sitl_input &input);
 
     Frame *frame;
     Thruster* thrusters;

From dcdf242500ec448e72650d998db8837815206987 Mon Sep 17 00:00:00 2001
From: Hunter McClelland <hunter.mcclelland@gmail.com>
Date: Fri, 6 Feb 2026 10:09:41 -0500
Subject: [PATCH 14/14] SITL (heli): use battery via simple (arbitrary)
 consumption model

---
 libraries/SITL/SIM_Helicopter.cpp | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/libraries/SITL/SIM_Helicopter.cpp b/libraries/SITL/SIM_Helicopter.cpp
index 754a1a38bbd1ad..425dd3913a275a 100644
--- a/libraries/SITL/SIM_Helicopter.cpp
+++ b/libraries/SITL/SIM_Helicopter.cpp
@@ -96,7 +96,8 @@ void Helicopter::update(const struct sitl_input &input)
     float eng_torque = 0;
     float lateral_x_thrust = 0;
     float lateral_y_thrust = 0;
-
+    // for a very simple battery depletion model
+    constexpr float amps_per_newton_thrust = 0.5f;
 
     if (_time_delay == 0) {
         for (uint8_t i = 0; i < 6; i++) {
@@ -153,6 +154,8 @@ void Helicopter::update(const struct sitl_input &input)
         float tail_rotor_torque = (21.6f * 2.96f * yaw_cmd - 2.96f * gyro.z) * sq(rpm[0]/nominal_rpm);
         float tail_rotor_thrust =  -1.0f * tail_rotor_torque * izz / tr_dist;  //right pedal produces left body accel
 
+        battery_current = amps_per_newton_thrust * (thrust + tail_rotor_thrust);
+
         // rotational acceleration, in rad/s/s, in body frame
         rot_accel.x = _tpp_angle.x * Lb1s + Lv * velocity_air_bf.y;
         rot_accel.y = _tpp_angle.y * Ma1s + Mu * velocity_air_bf.x;
@@ -213,6 +216,8 @@ void Helicopter::update(const struct sitl_input &input)
         float vertical_thrust = Zcol * coll * sq(rpm[0]/nominal_rpm) + velocity_air_bf.z * Zw;
         accel_body = Vector3f(lateral_x_thrust, lateral_y_thrust, vertical_thrust);
 
+        battery_current = amps_per_newton_thrust * (lateral_x_thrust + lateral_y_thrust + vertical_thrust);
+
         break;
     }
 
@@ -266,6 +271,7 @@ void Helicopter::update(const struct sitl_input &input)
         lateral_x_thrust = -1.0f * GRAVITY_MSS * (_tpp_angle_1.y + _tpp_angle_2.y) + Xu * velocity_air_bf.x;
         accel_body = Vector3f(lateral_x_thrust, lateral_y_thrust, -(thrust_1 + thrust_2) / mass + velocity_air_bf.z * Zw);
 
+        battery_current = amps_per_newton_thrust * (lateral_x_thrust + lateral_y_thrust + thrust_1 + thrust_2);
 
         break;
     }
@@ -315,10 +321,16 @@ void Helicopter::update(const struct sitl_input &input)
         lateral_x_thrust = (right_thruster_force + left_thruster_force) / mass - GRAVITY_MSS * _tpp_angle.y + Xu * velocity_air_bf.x;
         accel_body = Vector3f(lateral_x_thrust, lateral_y_thrust, -thrust / mass + velocity_air_bf.z * Zw);
 
+        battery_current = amps_per_newton_thrust * (lateral_x_thrust + lateral_y_thrust + thrust);
+
         break;
     }
     }
 
+    battery.maybe_reset(sitl->batt_voltage, sitl->batt_capacity_ah);
+    battery.consume_energy(battery_current);
+    battery_voltage = battery.get_voltage();
+    battery_temperature = battery.get_temperature();
 
     update_dynamics(rot_accel);
     update_external_payload(input);