feat: Add ability to fetch battery voltage.

* To be able to use the Zephyr `voltage-divider` driver,
  add a mode for fetching raw voltage from the sensor
 and do state of charge calculation outside of the driver.

app/Kconfig

@@ -383,6 +383,20 @@ config ZMK_BATTERY_REPORTING
     select ZMK_LOW_PRIORITY_WORK_QUEUE
     imply BT_BAS if ZMK_BLE
 
+if ZMK_BATTERY_REPORTING
+
+choice ZMK_BATTERY_REPORTING_FETCH_MODE
+    prompt "Battery Reporting Fetch Mode"
+
+config ZMK_BATTERY_REPORTING_FETCH_MODE_STATE_OF_CHARGE
+    bool "State of charge"
+
+config ZMK_BATTERY_REPORTING_FETCH_MODE_LITHIUM_VOLTAGE
+    bool "Lithium Voltage"
+
+endchoice
+endif
+
 config ZMK_IDLE_TIMEOUT
     int "Milliseconds of inactivity before entering idle state (OLED shutoff, etc)"
     default 30000

app/src/battery.c

@@ -34,11 +34,29 @@ static const struct device *const battery = DEVICE_DT_GET(DT_CHOSEN(zmk_battery)
 static const struct device *battery;
 #endif
 
+#if IS_ENABLED(CONFIG_ZMK_BATTERY_REPORTING_FETCH_MODE_LITHIUM_VOLTAGE)
+static uint8_t lithium_ion_mv_to_pct(int16_t bat_mv) {
+    // Simple linear approximation of a battery based off adafruit's discharge graph:
+    // https://learn.adafruit.com/li-ion-and-lipoly-batteries/voltages
+
+    if (bat_mv >= 4200) {
+        return 100;
+    } else if (bat_mv <= 3450) {
+        return 0;
+    }
+
+    return bat_mv * 2 / 15 - 459;
+}
+
+#endif // IS_ENABLED(CONFIG_ZMK_BATTERY_REPORTING_FETCH_MODE_LITHIUM_VOLTAGE)
+
 static int zmk_battery_update(const struct device *battery) {
     struct sensor_value state_of_charge;
+    int rc;
 
-    int rc = sensor_sample_fetch_chan(battery, SENSOR_CHAN_GAUGE_STATE_OF_CHARGE);
+#if IS_ENABLED(CONFIG_ZMK_BATTERY_REPORTING_FETCH_MODE_STATE_OF_CHARGE)
 
+    rc = sensor_sample_fetch_chan(battery, SENSOR_CHAN_GAUGE_STATE_OF_CHARGE);
     if (rc != 0) {
         LOG_DBG("Failed to fetch battery values: %d", rc);
         return rc;
@@ -50,6 +68,28 @@ static int zmk_battery_update(const struct device *battery) {
         LOG_DBG("Failed to get battery state of charge: %d", rc);
         return rc;
     }
+#elif IS_ENABLED(CONFIG_ZMK_BATTERY_REPORTING_FETCH_MODE_LITHIUM_VOLTAGE)
+    rc = sensor_sample_fetch_chan(battery, SENSOR_CHAN_VOLTAGE);
+    if (rc != 0) {
+        LOG_DBG("Failed to fetch battery values: %d", rc);
+        return rc;
+    }
+
+    struct sensor_value voltage;
+    rc = sensor_channel_get(battery, SENSOR_CHAN_VOLTAGE, &voltage);
+
+    if (rc != 0) {
+        LOG_DBG("Failed to get battery voltage: %d", rc);
+        return rc;
+    }
+
+    uint16_t mv = voltage.val1 * 1000 + (voltage.val2 / 1000);
+    state_of_charge.val1 = lithium_ion_mv_to_pct(mv);
+
+    LOG_DBG("State of change %d from %d mv", state_of_charge.val1, mv);
+#else
+#error "Not a supported reporting fetch mode"
+#endif
 
     if (last_state_of_charge != state_of_charge.val1) {
         last_state_of_charge = state_of_charge.val1;