epic: calculate the battery capacity

epicardium/epicardium.h

@@ -62,6 +62,7 @@ typedef _Bool bool;
 #define API_CHARGEIN_CURRENT       0x33
 #define API_SYSTEM_VOLTAGE         0x34
 #define API_THERMISTOR_VOLTAGE     0x35
+#define API_BATTERY_CAPACITY       0x36
 
 #define API_FILE_OPEN              0x40
 #define API_FILE_CLOSE             0x41
@@ -296,6 +297,13 @@ API(API_SYSTEM_VOLTAGE, int epic_read_system_voltage(float *result));
  */
 API(API_THERMISTOR_VOLTAGE, int epic_read_thermistor_voltage(float *result));
 
+/**
+ * Calculate the remaining battery capacity in percent.
+ *
+ * Uses a lookup table of 42 logarithmic voltage values to calculate
+ * the remaining capatity in percent.
+ */
+API(API_BATTERY_CAPACITY, int epic_read_battery_capacity(float *result));
 
 /**
  * UART/Serial Interface

epicardium/modules/pmic.c

@@ -338,3 +338,46 @@ void vPmicTask(void *pvParameters)
 		}
 	}
 }
+
+/* Voltage values for 42 capcity values (logarithmic) */
+const float battery_voltage_to_capacity[] = {
+	3.400, 3.505, 3.575, 3.628, 3.670, 3.705, 3.735, 3.761, 3.785,
+	3.806, 3.825, 3.843, 3.859, 3.874, 3.888, 3.901, 3.913, 3.925,
+	3.936, 3.947, 3.957, 3.966, 3.975, 3.984, 3.993, 4.001, 4.008,
+	4.016, 4.023, 4.030, 4.037, 4.044, 4.050, 4.056, 4.062, 4.068,
+	4.074, 4.079, 4.085, 4.090, 4.095, 4.100
+};
+
+/*
+ * Find the index for the given voltage
+ *
+ * TODO this can be done more efficient
+ */
+int _voltage_to_index(float voltage)
+{
+	const int N = sizeof(battery_voltage_to_capacity) / sizeof(float);
+
+	for (int i = 0; i < N; i++) {
+		if (voltage < battery_voltage_to_capacity[i])
+			return i;
+	}
+
+	return N - 1;
+}
+
+int epic_read_battery_capacity(float *result)
+{
+	float v;
+	int res = epic_read_battery_voltage(&v);
+	if (res < 0) {
+		return res;
+	}
+
+	int idx = _voltage_to_index(v);
+
+	const int N = sizeof(battery_voltage_to_capacity) / sizeof(float);
+
+	*result = idx * 100 / (N - 1);
+
+	return 0;
+}