From 7e0c178c6cbd18da3ceee9ff615dc3203a1ec708 Mon Sep 17 00:00:00 2001
From: Peter Barker <pbarker@barker.dropbear.id.au>
Date: Mon, 4 Sep 2023 11:59:43 +1000
Subject: [PATCH] SITL: add temperature to battery simulation

---
 libraries/SITL/SIM_Aircraft.h  | 1 +
 libraries/SITL/SIM_Battery.cpp | 9 +++++++++
 libraries/SITL/SIM_Battery.h   | 8 ++++++++
 3 files changed, 18 insertions(+)

diff --git a/libraries/SITL/SIM_Aircraft.h b/libraries/SITL/SIM_Aircraft.h
index 0a4d576893..645f78c7fc 100644
--- a/libraries/SITL/SIM_Aircraft.h
+++ b/libraries/SITL/SIM_Aircraft.h
@@ -151,6 +151,7 @@ public:
     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(); }
 
 protected:
     SIM *sitl;
diff --git a/libraries/SITL/SIM_Battery.cpp b/libraries/SITL/SIM_Battery.cpp
index ba8a17b263..d5a7219c36 100644
--- a/libraries/SITL/SIM_Battery.cpp
+++ b/libraries/SITL/SIM_Battery.cpp
@@ -150,6 +150,15 @@ void Battery::set_current(float current)
     }
 
     voltage_filter.apply(voltage);
+
+    {
+        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;
+    }
 }
 
 float Battery::get_voltage(void) const
diff --git a/libraries/SITL/SIM_Battery.h b/libraries/SITL/SIM_Battery.h
index 14c72d08df..af592c00eb 100644
--- a/libraries/SITL/SIM_Battery.h
+++ b/libraries/SITL/SIM_Battery.h
@@ -34,6 +34,9 @@ public:
     void set_current(float current_amps);
     float get_voltage(void) const;
 
+    // return battery temperature in Kelvin:
+    float get_temperature(void) const { return temperature.kelvin; }
+
 private:
     float capacity_Ah;
     float resistance;
@@ -42,6 +45,11 @@ private:
     float remaining_Ah;
     uint64_t last_us;
 
+    struct {
+        float kelvin = 273;
+        uint64_t last_update_micros;
+    } temperature;
+
     // 10Hz filter for battery voltage
     LowPassFilterFloat voltage_filter{10};