only redraw battery indicator when needed

src/image.c

  */
 
 #include <stdio.h>
+#include <string.h>
 
 #include "config.h"
 #include "lcd.h"
 
 void draw_battery_indicator(void) {
     static const float batt_warn_limit = 15.0f;
-    static char s[30];
+    static char prev_s[30] = {0};
+    static uint32_t prev_c = 0;
+
+    char s[30] = {0};
     float v = lipo_voltage();
     uint32_t c = RGB_565(0xFF, 0x00, 0x00);
     if (lipo_charging()) {
         }
     }
 
+    // only re-draw battery indicator when it has changed
+    if ((strcmp(s, prev_s) == 0) && (prev_c == c)) {
+        return;
+    }
+    strcpy(prev_s, s);
+    prev_c = c;
+
     static struct text_font font = {
         .fontname = "fixed_10x20",
         .font = NULL,