[SOFT-527] Update NTC thermistor curve (#428)

It now obeys the experimental curve rather than the theoretical one (commented out). See https://www.desmos.com/calculator/fqoelc5mkj.

projects/solar/src/sense_temperature.c

@@ -34,10 +34,16 @@ static void prv_sense_callback(void *context) {
     case NTC_THERMISTOR:
       // NTC: T = 1 / ( ( ln(0.56 * V / (3.3 - V)) / 3428 ) + 1/298.15 ) in KELVIN. For Celsius,
       // subtract 273.15
-      converted_reading =
-          (1 /
-           ((ln(0.56 * converted_reading / (3.3 - converted_reading)) / 3428.0) + 1.0 / 298.15)) -
-          273.15;
+      // NEW NTC CURVE (BASED ON EXPERIMENTAL DATA): T = (-14.195)*V^3 + (37.9434)*V^2 - 64.8244V +
+      // 121.362 (already in celsius)
+      // converted_reading =
+      //     (1 /
+      //      ((ln(0.56 * converted_reading / (3.3 - converted_reading)) / 3428.0) + 1.0 / 298.15))
+      //      -
+      //     273.15;
+      converted_reading = (-14.195) * pow(converted_reading, 3) +
+                          (37.9434) * pow(converted_reading, 2) - 64.8244 * converted_reading +
+                          121.362;
 
       // Convert from Celsius to deciCelsius
       converted_reading = converted_reading * 10;

projects/solar/test/test_sense_temperature.c

@@ -1,12 +1,11 @@
-#include "sense_temperature.h"
-
 #include <stdbool.h>
 
 #include "adc.h"
 #include "data_store.h"
 #include "gpio.h"
 #include "log.h"
 #include "sense.h"
+#include "sense_temperature.h"
 #include "test_helpers.h"
 #include "unity.h"
 
@@ -15,7 +14,7 @@
 // The two test variables below are calculated from inputting 0x800 into the relative conversion
 // formulae (see sense_temperature.c)
 #define TEST_RTD_CONVERTED_READING (-541)
-#define TEST_NTC_CONVERTED_READING (272)
+#define TEST_NTC_CONVERTED_READING (258)
 
 static SenseCallback s_sense_callbacks[MAX_THERMISTORS];
 static void *s_sense_callback_contexts[MAX_THERMISTORS];