fix: battery percentage showing unavailable when fully charged

When battery voltage slightly exceeds 4.2V (e.g., 4.25V when fully charged),
the previous code returned NAN, causing HA to display 'unavailable'.

Now only return NAN for truly invalid readings (voltage < 2.75V or NaN).
Voltage above 4.2V will correctly show 100% instead of unavailable.

Author: limengdu

arduino/SeeedHADiscovery/examples/IoTButtonV2_DeepSleep/IoTButtonV2_DeepSleep.ino

@@ -463,11 +463,21 @@ float readBatteryVoltage() {
  * Calculate battery percentage from voltage
  * 从电压计算电池百分比
  * Formula: percentage = ((voltage - 2.75) / (4.2 - 2.75)) * 100
+ * 
+ * Note: When fully charged, voltage may slightly exceed 4.2V (e.g., 4.25V).
+ * We should return 100% instead of NAN in this case.
+ * 注意：充满电时，电压可能略微超过 4.2V（如 4.25V）。
+ * 此时应返回 100% 而不是 NAN。
  */
 float calculateBatteryPercentage(float voltage) {
-    if (isnan(voltage) || voltage < BATTERY_VOLTAGE_MIN || voltage > BATTERY_VOLTAGE_MAX) {
+    // Only return NAN for invalid readings (too low or not a number)
+    // 仅对无效读数返回 NAN（过低或非数字）
+    if (isnan(voltage) || voltage < BATTERY_VOLTAGE_MIN) {
         return NAN;
     }
+    
+    // Calculate percentage and clamp to 0-100 range
+    // 计算百分比并限制在 0-100 范围内
     float percentage = ((voltage - BATTERY_VOLTAGE_MIN) / (BATTERY_VOLTAGE_MAX - BATTERY_VOLTAGE_MIN)) * 100.0f;
     return constrain(percentage, 0.0f, 100.0f);
 }