diff --git a/monitor/batteryIcon.py b/monitor/batteryIcon.py
index 089892b..63d7f91 100755
--- a/monitor/batteryIcon.py
+++ b/monitor/batteryIcon.py
@@ -7,10 +7,14 @@ management IC over I2C (bus 1, address 0x64) -- the same registers the
 oneUpPower kernel module uses. This means the icon works for an ordinary
 user (member of the ``i2c`` group) without sudo and without the kernel
 module being loaded. Updates every 5 seconds.
+
+When running on battery it also warns on low charge and powers the system
+off when critically low, mirroring the oneUpPower kernel module.
 """
 
 import sys
 import time
+import subprocess
 
 import smbus2
 from PyQt5.QtWidgets import QApplication, QSystemTrayIcon, QMenu, QAction
@@ -24,6 +28,10 @@ SOC_REG = 0x04          # state of charge, percent
 CURRENT_HIGH_REG = 0x0E  # bit 7 set => running on battery (not plugged in)
 UPDATE_INTERVAL_MS = 5000
 
+# Auto-shutdown while running on battery (mirrors the oneUpPower module).
+SHUTDOWN_SOC = 5   # power off at or below this percentage
+WARN_SOC = 10      # warn the user at or below this percentage
+
 
 def read_battery(bus):
     """Return (percent, plugged_in) read over I2C, or (None, None) on error.
@@ -115,6 +123,8 @@ class BatteryTray:
             time.sleep(2)
 
         self.bus = smbus2.SMBus(I2C_BUS)
+        self.shutdown_triggered = False
+        self.warned = False
         self.tray = QSystemTrayIcon()
 
         # Context menu
@@ -155,6 +165,8 @@ class BatteryTray:
             tooltip = f"Battery: {percent}% ({state})"
             self.tray.setToolTip(tooltip)
             self.status_action.setText(tooltip)
+
+            self.check_power(percent, plugged_in)
         else:
             # I2C read failed - IC not reachable
             icon = make_battery_icon(0, False)
@@ -162,6 +174,35 @@ class BatteryTray:
             self.tray.setToolTip("Battery: N/A (I2C read failed)")
             self.status_action.setText("Battery: N/A (I2C read failed)")
 
+    def check_power(self, percent, plugged_in):
+        """Warn on low battery and power off when critically low.
+
+        Only acts while running on battery. `systemctl poweroff` works for
+        the active desktop session without sudo.
+        """
+        if plugged_in:
+            self.warned = False
+            return
+
+        if percent <= SHUTDOWN_SOC and not self.shutdown_triggered:
+            self.shutdown_triggered = True
+            self.tray.showMessage(
+                "Battery critical",
+                f"Battery at {percent}% - shutting down now.",
+                QSystemTrayIcon.Critical, 10000)
+            try:
+                subprocess.run(["systemctl", "poweroff"],
+                               check=True, timeout=10)
+            except (subprocess.SubprocessError, OSError) as e:
+                print(f"poweroff failed: {e}", file=sys.stderr)
+                self.shutdown_triggered = False  # allow retry next cycle
+        elif percent <= WARN_SOC and not self.warned:
+            self.warned = True
+            self.tray.showMessage(
+                "Battery low",
+                f"Battery at {percent}% - save your work.",
+                QSystemTrayIcon.Warning, 10000)
+
     def run(self):
         return self.app.exec()