nixos-config/home-manager/modules/battery.nix

{ lib, pkgs, ... }:
with lib;
let
  battery-watch = pkgs.writeShellScriptBin "battery-watch" ''
critical_level=20    #percent

while true
do
    if [ "$(${pkgs.acpi}/bin/acpi -a | grep -o off)" == "off" ]; then
        battery_level=`${pkgs.acpi}/bin/acpi -b | sed 's/.*[dg], //g;s/\%,.*//g'`
        if [ $battery_level -le $critical_level ]; then
          ${pkgs.eventd}/bin/eventc critical battery -d "title='Battery level is low!'" -d "message='Only $battery_level% of the charge remains.'"
        else
          ${pkgs.eventd}/bin/eventc notification battery -d "title='Battery is discharging!'" -d "message='Only $battery_level% of the charge remains.'"
          sleep 18m
        fi
    fi
    sleep 2m
done
'';
in {

options.m-0.battery.enable = mkEnableOption "Battery";

config = mkIf config.m-0.battery.enable {
  systemd.user = {
    services.battery = {
      Unit = {
        Description = "Watch battery state and warn user";
      };
      Service = {
        ExecStart="/bin/sh ${battery-watch}/bin/battery-watch";
      };
      Install = {
        WantedBy = [ "graphical-session.target" ];
      };
    };
  };
};

}