diff --git a/src/utility/Power_Class.hpp b/src/utility/Power_Class.hpp
index 70ec618..291b8f7 100644
--- a/src/utility/Power_Class.hpp
+++ b/src/utility/Power_Class.hpp
@@ -184,6 +184,9 @@ namespace m5
 
     /// get battery current
     /// @return battery current [mA] ( +=charge / -=discharge )
+    /// @attention This reading comes from the hardware of the board: an AXP192, or a
+    /// dedicated current sense IC ( ex. Core2 v1.1 , M5Tab5 , M5PowerHub ).
+    /// Boards without either of them return 0.
     int32_t getBatteryCurrent(void);
 
     /// Get Ext Port voltage
diff --git a/src/utility/power/AXP2101_Class.cpp b/src/utility/power/AXP2101_Class.cpp
index 0710d00..30a072e 100644
--- a/src/utility/power/AXP2101_Class.cpp
+++ b/src/utility/power/AXP2101_Class.cpp
@@ -8,6 +8,7 @@
 #endif
 
 #include <algorithm>
+#include <limits>
 
 #define IS_BIT_SET(val,mask)            (((val)&(mask)) == (mask))
 
@@ -213,14 +214,28 @@ return false;
     return val >> 2;
   }
 
-  float AXP2101_Class::getACINVoltage(void)
+  /// Used where the AXP2101 provides no reading for the requested value.
+  /// Returning NaN instead of 0 lets the caller tell it from a real zero.
+  static constexpr float not_available(void)
   {
-return 0;
+    return std::numeric_limits<float>::quiet_NaN();
+  }
+
+  /// A 14 bit ADC channel sits at the top of its range when there is nothing to measure
+  /// on it, so a reading within this margin of full scale is not treated as a value.
+  /// A channel that does have an input reads far below the margin; measured on a CoreS3,
+  /// the closest was TS at 5563, while VBUS with nothing connected read 16372.
+  static constexpr std::size_t adc_full_scale  = 0x3FFF;
+  static constexpr std::size_t adc_invalid_min = adc_full_scale - 32;
+
+  float AXP2101_Class::getACINVoltage(void)
+  { // The AXP2101 takes its input from VBUS. ( see getVBUSVoltage )
+    return not_available();
   }
 
   float AXP2101_Class::getACINCurrent(void)
-  {
-return 0;
+  { // The AXP2101 takes its input from VBUS.
+    return not_available();
   }
 
   float AXP2101_Class::getVBUSVoltage(void)
@@ -228,20 +243,20 @@ return 0;
     if (isVBUS() == false) { return 0.0f; }
     
     float vBus = readRegister14(0x38);
-    if (vBus >= 16375) { return 0.0f; }
+    if (vBus >= adc_invalid_min) { return 0.0f; }
 
     return vBus / 1000.0f;
   }
 
   float AXP2101_Class::getVBUSCurrent(void)
-  {
-return 0;
+  { // Not provided by the AXP2101. ( see getBatteryChargeCurrent )
+    return not_available();
   }
 
   float AXP2101_Class::getTSVoltage(void)
   {
     float volt = readRegister14(0x36);
-    if (volt >= 16375) { return 0.0f; }
+    if (volt >= adc_invalid_min) { return 0.0f; }
 
     return volt / 2000.0f;
   }
@@ -252,8 +267,8 @@ return 0;
   }
 
   float AXP2101_Class::getBatteryPower(void)
-  {
-return 0;
+  { // Derived from the battery current, which the AXP2101 does not provide.
+    return not_available();
   }
 
   float AXP2101_Class::getBatteryVoltage(void)
@@ -262,18 +277,24 @@ return 0;
   }
 
   float AXP2101_Class::getBatteryChargeCurrent(void)
-  {
-return 0;
+  { // The ADC of the AXP2101 covers VBAT / TS / VBUS / VSYS / TDIE. Current is measured
+    // by a dedicated sense IC where the board provides one ( ex. INA3221 on Core2 v1.1 ,
+    // INA226 on M5Tab5 ) , and Power_Class::getBatteryCurrent() reads that IC.
+    return not_available();
   }
 
   float AXP2101_Class::getBatteryDischargeCurrent(void)
-  {
-return 0;
+  { // Not provided by the AXP2101. ( see getBatteryChargeCurrent )
+    return not_available();
   }
 
   float AXP2101_Class::getAPSVoltage(void)
-  {
-return 0;
+  { // VSYS is the equivalent measurement point on the AXP2101 of the APS rail
+    // of the AXP192.
+    float volt = readRegister14(0x3A);
+    if (volt >= adc_invalid_min) { return 0.0f; }
+
+    return volt / 1000.0f;
   }
 
   bool AXP2101_Class::enableIRQ(std::uint64_t registerEn)
diff --git a/src/utility/power/AXP2101_Class.hpp b/src/utility/power/AXP2101_Class.hpp
index be09f14..faf3736 100644
--- a/src/utility/power/AXP2101_Class.hpp
+++ b/src/utility/power/AXP2101_Class.hpp
@@ -122,15 +122,30 @@ namespace m5
     bool isVBUS(void);
     bool getBatState(void);
 
+    /// The ADC of the AXP2101 covers these measurement points:
+    ///   VBAT , TS , VBUS , VSYS , TDIE
+    /// The getters below that fall outside this set return NaN, so that a caller can
+    /// tell an unavailable reading from a real zero with std::isnan().
+    /// Current is measured by a dedicated sense IC where the board provides one
+    /// ( ex. INA3221 on Core2 v1.1 , INA226 on M5Tab5 ) , and Power_Class reads that IC.
+
     float getBatteryVoltage(void);
+    /// @return NaN. This reading is not provided by the AXP2101.
     float getBatteryDischargeCurrent(void);
+    /// @return NaN. This reading is not provided by the AXP2101.
     float getBatteryChargeCurrent(void);
+    /// @return NaN. Derived from the battery current, which the AXP2101 does not provide.
     float getBatteryPower(void);
+    /// @return NaN. The AXP2101 takes its input from VBUS. ( see getVBUSVoltage )
     float getACINVoltage(void);
+    /// @return NaN. The AXP2101 takes its input from VBUS.
     float getACINCurrent(void);
     float getVBUSVoltage(void);
+    /// @return NaN. This reading is not provided by the AXP2101.
     float getVBUSCurrent(void);
     float getTSVoltage(void);
+    /// The VSYS voltage is returned, which is the equivalent measurement point on the
+    /// AXP2101 of the APS rail of the AXP192.
     float getAPSVoltage(void);
     float getInternalTemperature(void);