Added battery charge percent estimation

src/power.h

@@ -1,5 +1,18 @@
 #pragma once
 
+/**
+ * Per @spattinson
+ * MIN_BAT_MILLIVOLTS seems high. Typical 18650 are different chemistry to LiPo, even for LiPos that chart seems a bit off, other
+ * charts put 3690mV at about 30% for a lipo, for 18650 i think 10% remaining iis in the region of 3.2-3.3V. Reference 1st graph
+ * in [this test report](https://lygte-info.dk/review/batteries2012/Samsung%20INR18650-30Q%203000mAh%20%28Pink%29%20UK.html)
+ * looking at the red line - discharge at 0.2A - he gets a capacity of 2900mah, 90% of 2900 = 2610, that point in the graph looks
+ * to be a shade above 3.2V
+ */
+#define MIN_BAT_MILLIVOLTS 3250 // millivolts. 10% per https://blog.ampow.com/lipo-voltage-chart/
+
+#define BAT_MILLIVOLTS_FULL 4100
+#define BAT_MILLIVOLTS_EMPTY 3500
+
 namespace meshtastic
 {
 
@@ -9,6 +22,8 @@ struct PowerStatus {
     bool haveBattery;
     /// Battery voltage in mV, valid if haveBattery is true
     int batteryVoltageMv;
+    /// Battery charge percentage, either read directly or estimated
+    int batteryChargePercent;
     /// Whether USB is connected
     bool usb;
     /// Whether we are charging the battery