Modified 'battery_getCharge': now it takes as input the battery voltage in mV and returns charge percentage as an integer ranging from 0 to 100. Function body is implemented using fixed point math instead of floats for improved platform compatibility when hardware floating point support is not present.

openrtx/include/battery.h

@@ -20,12 +20,13 @@
 #ifndef BATTERY_H
 #define BATTERY_H
 
+#include <stdint.h>
+
 /**
- * This function uses battery charge tables to convert a battery voltage into a
- * charge percentage.
- * @param vbat: the voltage read from the battery in volt.
- * @return the charge percentage.
+ * This function computes the battery's state of charge given its current voltage.
+ * @param vbat: battery voltage in millivolt.
+ * @return state of charge percentage, from 0% to 100%.
  */
-float battery_getCharge(float vbat);
+uint8_t battery_getCharge(uint16_t vbat);
 
 #endif /* BATTERY_H */

openrtx/src/battery.c

@@ -21,35 +21,59 @@
 #include <hwconfig.h>
 #include <math.h>
 
-/* This array acts as a lookup table for converting Li-Po voltage into
- * charge percentage, elements range from 5% to 95% (included) with 5% steps.
- * Data is taken from (https://blog.ampow.com/lipo-voltage-chart/).
+/*
+ * Minimum and maximum battery voltages expressed in fixed point Q8.8 format.
+ * Obtained by multiplying the values in volt by 256.
  */
-#define V_LUT_STEPS 21
+
 #if defined BAT_LIPO_1S
-float bat_v_min = 3.61f;
-float bat_v_max = 4.15f;
+static const uint16_t bat_v_min = 0x039C;   // 3.61V
+static const uint16_t bat_v_max = 0x0426;   // 4.15V
 #elif defined BAT_LIPO_2S
-float bat_v_min = 7.10f;
-float bat_v_max = 8.10f;
+static const uint16_t bat_v_min = 0x071A;   // 7.10V
+static const uint16_t bat_v_max = 0x0819;   // 8.10V
 #elif defined BAT_LIPO_3S
-float bat_v_min = 10.83;
-float bat_v_max = 12.45;
+static const uint16_t bat_v_min = 0x0AD4;   // 10.83V
+static const uint16_t bat_v_max = 0x0C73;   // 12.45V
 #elif defined BAT_NONE
-float bat_v_min = 0.0;
-float bat_v_max = 0.0;
+static const uint16_t bat_v_min = 0;
+static const uint16_t bat_v_max = 0;
 #else
 #error Please define a battery type into platform/targets/.../hwconfig.h
 #endif
 
-float battery_getCharge(float vbat)
+uint8_t battery_getCharge(uint16_t vbat)
 {
-    #ifndef BAT_NONE
-    // Perform a linear interpolation between minimum and maximum charge values.
-    return (vbat - bat_v_min) / (bat_v_max - bat_v_min);
-    #else
-    // Return full charge if no battery is present.
+    #ifdef BAT_NONE
+    /* Return full charge if no battery is present. */
     (void) vbat;
-    return 1.0f;
+    return 100;
+    #else
+
+    /*
+     * Compute battery percentage by linear interpolation between zero and full
+     * charge voltage values using Q8.8 fixed-point math to avoid using both
+     * floating point and 64 bit variables, for maximum portability.
+     *
+     * Given that battery voltage parameter is an unsigned 16 bit value expressing
+     * the voltage in mV, we first have to convert it to Q8.8 before computing
+     * the charge percentage.
+     *
+     * Comparison between battery percentage computed using fixed point and
+     * floating point routines on a voltage range from 10.83V to 12.45V with
+     * increments of 1mV resulted in an average error of -0.015%, maximum error
+     * of 0.79% and minimum error of -0.78%
+     */
+
+    uint32_t vb = vbat << 16;
+    vb          = vb / 1000;
+    vb          = (vb + 256) >> 8;
+
+    uint32_t diff   = vb - bat_v_min;
+    uint32_t range  = bat_v_max - bat_v_min;
+    uint32_t result = ((diff << 8) / range) * 100;
+    result         += 128;
+    return result >> 8;
+
     #endif
 }