Parameter for virtual battery in SITL (#12299)

* New parameter for virtual battery in SITL

* Update src/modules/simulator/simulator_mavlink.cpp

Co-Authored-By: Beat Küng <beat-kueng@gmx.net>

* Updated description for parameter SIM_BAT_MIN_PCT

src/modules/simulator/simulator_params.c

@@ -51,3 +51,17 @@
  * @group SITL
  */
 PARAM_DEFINE_FLOAT(SIM_BAT_DRAIN, 60);
+
+/**
+ * Simulator Battery minimal percentage. Can be used to alter
+ * the battery level during SITL- or HITL-simulation on the fly.
+ * Particularly useful for testing different low-battery behaviour.
+ *
+ * @min 0
+ * @max 100
+ * @increment 0.1
+ * @unit %
+ *
+ * @group SITL
+ */
+PARAM_DEFINE_FLOAT(SIM_BAT_MIN_PCT, 50.0f);