Low Battery Red Screen (Windows)

A tiny, always-on Windows utility that turns your whole screen red when the battery is low and the laptop is not charging - so you notice before the machine dies.

• 🔴 Full-screen red tint across all monitors (translucent, so you can still see your work)
• 🖱️ Zero interference - click-through, never takes focus, no taskbar / Alt-Tab entry
• 🔁 Self-healing - starts at every sign-in and restarts itself if it crashes or freezes
• 🪶 Tiny - ~34 KB exe, no dependencies, no admin rights, no network access

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Install (end users)

1. Open the Releases page and download LowBatteryRed-v*-win64.zip (or just LowBatteryRed.exe).
2. Extract the zip and double-click LowBatteryRed.exe (or Install.bat). That's it - a confirmation box appears and the app is running.
3. If Windows SmartScreen shows "Windows protected your PC": click More info → Run anyway. The exe is not code-signed (that costs money), which is the only reason for the warning. You can read every line of the source here, or build it yourself. Some antivirus tools are wary of unsigned programs that register autostart; if yours flags it, build from source or allow it.
4. Try it: double-click Test.bat → the screen turns red for 6 seconds.

Requirements: Windows 10 / 11, 64-bit (ARM64 PCs run it through Windows' x64 emulation).

Uninstall

Double-click Uninstall.bat (from the zip), or run:

%LOCALAPPDATA%\LowBatteryRed\LowBatteryRed.exe --uninstall

This stops the app and removes everything listed below.

What installation changes on your PC

What	Where
Program + settings	%LOCALAPPDATA%\LowBatteryRed\ (LowBatteryRed.exe, config.ini)
Autostart	HKCU\Software\Microsoft\Windows\CurrentVersion\Run → value LowBatteryRed
Safety-net task	Task Scheduler task LowBatteryRed (current user; at sign-in + every minute; battery restrictions disabled)

Nothing else: no services, no drivers, no admin/UAC prompt, no network connections.

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How it behaves

Situation	Screen
On battery, at or below threshold (default 20 %)	🔴 red
Battery critical (< 5 %) and not charging	🔴 red
Charger connected / battery charging	normal (red disappears immediately)
Just above the threshold	normal (2 % hysteresis prevents flicker at the boundary)
Desktop PC with no battery, or battery status unknown	normal (never shows)

Configuration

Edit %LOCALAPPDATA%\LowBatteryRed\config.ini, save - changes apply within ~2 seconds, no restart needed.

[Settings]
Threshold=20   ; battery % at or below which the screen goes red (1-99)
Opacity=55     ; red tint strength 10-100  (100 = solid red, no see-through)

Invalid or out-of-range values fall back to the defaults / are clamped.

Quick real-world check

Set Threshold=99, unplug the charger → screen goes red. Plug in → it clears. Set it back to 20.

Command line

Command	Effect
LowBatteryRed.exe	install (copy to %LOCALAPPDATA%, enable autostart) and start
LowBatteryRed.exe --test	show the red screen for 6 s, then exit
LowBatteryRed.exe --uninstall	stop everything and remove it
--silent (add to any of the above)	no message boxes
--run / --worker <pid>	internal (supervisor / overlay process)

To check it is running: Task Manager → Details → two LowBatteryRed.exe processes (supervisor + worker).

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How it works

 sign-in ──► Run key / Scheduled Task ──► LowBatteryRed.exe --run   (supervisor, ~no code to crash)
                                              │  spawns / restarts / watches
                                              ▼
                                   LowBatteryRed.exe --worker      (overlay + battery polling)
                                              ▲
                     supervisor dies ─────────┘ worker relaunches it

• Overlay: one borderless window spanning the entire virtual desktop with the styles WS_EX_LAYERED | WS_EX_TRANSPARENT | WS_EX_NOACTIVATE | WS_EX_TOOLWINDOW | WS_EX_TOPMOST, translucent via SetLayeredWindowAttributes. It answers WM_NCHITTEST with HTTRANSPARENT and WM_MOUSEACTIVATE with MA_NOACTIVATE, and is only ever shown with SWP_NOACTIVATE, so it cannot take focus or swallow input. It is re-asserted topmost every 2 s so the taskbar can't cover it.
• Battery: GetSystemPowerStatus polled every 2 s, plus immediate reaction to WM_POWERBROADCAST (plug/unplug, resume from sleep) and WM_DISPLAYCHANGE (monitor changes).
• Crash recovery: the worker installs an unhandled-exception filter that exits instantly (no "stopped working" dialog to block the restart). The supervisor restarts it with back-off, and kills + restarts it if its window stops responding for ~30 s. Single-instance mutexes prevent duplicates.
• Autostart: HKCU Run key (always works, no admin) plus a per-user Scheduled Task as a backstop. The task explicitly turns off "don't start on battery" / "stop on battery", which would otherwise defeat the purpose.
• Per-monitor DPI aware via the embedded manifest, so the overlay is pixel-exact on mixed-DPI multi-monitor setups.

Limitations

• Games/videos in true exclusive fullscreen can draw above any overlay (borderless/windowed fullscreen is fine).
• Windows' lock screen and UAC prompts are separate secure desktops and are not tinted.
• Unsigned binary → SmartScreen warning on first run (see above).

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Build from source

The project is a single C file (src/lowbatteryred.c) built with MinGW-w64.

Linux / macOS / WSL (cross-compile):

sudo apt install gcc-mingw-w64-x86-64 zip     # Debian/Ubuntu
./build.sh                                     # -> build/LowBatteryRed.exe
./package.sh 1.0.0                             # -> build/release/ (exe, zip, SHA256SUMS.txt)

Windows (MSYS2 MinGW64 shell):

pacman -S --needed mingw-w64-x86_64-gcc zip
./build.sh

build.sh honours CC and WINDRES if your toolchain uses different names.

The exe is built with -Wl,--no-insert-timestamp, so the same source built with the same toolchain gives a byte-identical exe (CI checks this on every run; a different MinGW version may produce different bytes).

Tests

tests/run_tests.sh runs the real Windows binary under Wine with a virtual display and drives it from a Windows test program (tests/test_harness.c).

sudo apt install gcc-mingw-w64-x86-64 wine64 xvfb
./tests/run_tests.sh

It uses a test build (-DLBR_TEST) that differs from the release build only in two ways: the battery status is read from a file (LBR_SIM_FILE) so scenarios can be simulated, and timers are faster. It checks:

• overlay covers the full virtual screen exactly; window styles; opacity; click-through hit-testing
• focus is never stolen (foreground/active/focus window unchanged, with a positive control proving the check can detect a steal)
• battery logic: unplugged, charging, threshold edges, hysteresis, critical flag, no battery, unknown status
• crash recovery, killing the supervisor, hang detection, no duplicate processes
• install / upgrade / uninstall, Run-key registration, live config.ini reload, invalid config values
• the generated Task Scheduler XML (captured via a schtasks.exe test double), including the fallback when a logon trigger is rejected

What the Wine tests can't cover: real battery hardware and real Windows DWM compositing (translucency). Use the quick real-world check after installing.

CI / CD (GitHub Actions)

Workflow	Runs on	What it does
ci.yml	every push to main and every pull request	shellcheck → reproducible-build check → full Wine test suite (-Werror) → packaging dry-run. Uploads the test log, the CI-built exe and a run summary.
ci.yml › Native Windows run	after the job above	Experimental, non-blocking. Runs the same harness on a real windows-latest runner and verifies the real Task Scheduler round-trip (task created, battery restrictions off, removed on uninstall).
release.yml	pushing a tag vX.Y.Z	checks the tag is on main, re-runs the full suite as a gate, builds, and publishes the GitHub Release with LowBatteryRed.exe, the zip and SHA256SUMS.txt.

Cutting a release

git checkout main && git pull
git tag v1.0.1
git push origin v1.0.1      # CI tests, packages and publishes the release

Tags containing a - (e.g. v1.1.0-rc1) are published as pre-releases.

Repository layout

src/         lowbatteryred.c, app.manifest, app.rc
tests/       test_harness.c, fake_schtasks.c, run_tests.sh
.github/workflows/   ci.yml, release.yml
dist/        Install.bat, Test.bat, Uninstall.bat, README.txt  (shipped inside the release zip)
build.sh     build the exe            package.sh   create release assets

Sample Outcome

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