Thank you, it's a viable option.
There is a third way to control the screen brightness, though. You see, I use redshift, and with redshift you can automatically control the screen brightness (and do this separately for day and night). But I don't use it because I don't need. Well, I didn't need. Nevertheless, the power manager does in general a bit more than mere brightness control.
It seems that kernel initializes the SMBus correctly:
$ sudo dmesg | grep 00:1f.3
[ 0.269155] pci 0000:00:1f.3: [8086:9ca2] type 00 class 0x0c0500
[ 0.269171] pci 0000:00:1f.3: reg 0x10: [mem 0xf7318000-0xf73180ff 64bit]
[ 0.269190] pci 0000:00:1f.3: reg 0x20: [io 0xf040-0xf05f]
[ 8.755485] i801_smbus 0000:00:1f.3: SPD Write Disable is set
[ 8.755527] i801_smbus 0000:00:1f.3: SMBus using PCI interrupt
And also it correctly identifies that the battery is off:
$ sudo dmesg | grep -i battery
[ 0.576492] battery: ACPI: Battery Slot [BAT0] (battery absent)
So, xfce4-PM should display a picture in the tray for an empty battery, but it doesn't. When I start it via the terminal, it writes a lot of nonsense:
** (xfce4-power-manager:2582): WARNING **: 16:08:34.279: Failed to get name owner: GDBus.Error:org.freedesktop.DBus.Error.NameHasNoOwner: Could not get owner of name 'org.freedesktop.PowerManagement': no such name
** (xfce4-power-manager:2582): WARNING **: 16:08:34.280: Failed to get name owner: GDBus.Error:org.freedesktop.DBus.Error.NameHasNoOwner: Could not get owner of name 'org.xfce.PowerManager': no such name
(xfce4-power-manager:2582): xfce4-power-manager-WARNING **: 16:08:34.283: Unable to connect to session manager : Verbindung zur Sitzungsverwaltung fehlgeschlagen: SESSION_MANAGER environment variable not defined
(xfce4-power-manager:2582): GLib-GObject-WARNING **: 16:08:34.505: ../glib/gobject/gsignal.c:2614: signal 'Changed' is invalid for instance '0x5566a23628e0' of type 'GDBusProxy'
(xfce4-power-manager:2582): xfce4-power-manager-WARNING **: 16:08:34.544: could not map keysym 1008ffa8 to keycode
(xfce4-power-manager:2582): xfce4-power-manager-WARNING **: 16:08:34.556: It seems the kernel brightness switch handling value was not restored properly on exit last time, xfce4-power-manager will try to restore it this time.
(xfce4-power-manager:2582): xfce4-power-manager-WARNING **: 16:08:34.557: Unable to set the kernel brightness switch parameter to 0.
(xfce4-power-manager:2582): xfce4-power-manager-WARNING **: 16:08:34.724: ../panel-plugins/power-manager-plugin/power-manager-button.c:858: invalid property id 2 for "show-panel-label" of type 'GParamInt' in 'PowerManagerButton'
(xfce4-power-manager:2582): xfce4-power-manager-WARNING **: 16:08:34.724: ../panel-plugins/power-manager-plugin/power-manager-button.c:858: invalid property id 3 for "presentation-mode" of type 'GParamBoolean' in 'PowerManagerButton'
(xfce4-power-manager:2582): xfce4-power-manager-WARNING **: 16:08:34.724: ../panel-plugins/power-manager-plugin/power-manager-button.c:858: invalid property id 4 for "show-presentation-indicator" of type 'GParamBoolean' in 'PowerManagerButton'
(xfce4-power-manager:2582): Gtk-CRITICAL **: 16:08:34.785: gtk_icon_theme_has_icon: assertion 'icon_name != NULL' failed
(xfce4-power-manager:2582): Gtk-CRITICAL **: 16:08:34.797: gtk_status_icon_set_from_icon_name: assertion 'icon_name != NULL' failed
That's its usual whining, it writes the same in Debian (where it works correctly) except for the last two strings related to icons. So, for some reason, it has icon_name == NULL somewhere in its entrails. Thus, the question is, why this happens and how to make the manager happy.