This is a bug in the composite battery / icon mismatch. I think we should just remove the composite battery and have the icon represent the primary battery.

(In reply to comment #0)
> My laptop is plugged in, at full power, but my wireless mouse is about to die
> (well, actually it's not, but upower thinks it is).

That's fixed in upower git IIRC.

>The shell status area shows
> an "about to die" battery icon, but then if I open the menu, the Battery
> section shows the same icon, but labeled "Fully Charged", and there's no
> indication (even in the submenu) that it's talking about two different
> batteries.
> 
> (I'm not sure it should show the "about to die" icon anyway... 99% of the time,
> the status area battery icon refers to my laptop battery, so it's confusing to
> have it suddenly switch to being about my mouse battery. Maybe it should show
> two battery icons in this case. Or not indicate the mouse battery state in the
> status area at all, and only do the notifications.)

What's the output of:
gdbus introspect --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power
?

(In reply to comment #2)
> That's fixed in upower git IIRC.

yup, found that bug already

> What's the output of:
> gdbus introspect --session --dest org.gnome.SettingsDaemon.Power --object-path
> /org/gnome/SettingsDaemon/Power
> ?

trimming the dull introspection bits:

node /org/gnome/SettingsDaemon/Power {
  interface org.gnome.SettingsDaemon.Power {
    methods:
      GetPrimaryDevice(out (susdut) device);
      GetDevices(out a(susdut) devices);
    signals:
    properties:
      readonly s Icon = '. GThemedIcon battery-caution-symbolic gpm-mouse-000 battery-caution ';
      readonly s Tooltip = 'Mouse (1%)\nLaptop battery is charged';
      readonly d Percentage = 99.616858237547888;
  };
  interface org.gnome.SettingsDaemon.Power.Screen {
    methods:
      StepUp(out i new_percentage);
      StepDown(out i new_percentage);
    signals:
    properties:
      readwrite i Brightness = 100;
  };
  interface org.gnome.SettingsDaemon.Power.Keyboard {
    methods:
      StepUp(out i new_percentage);
      StepDown(out i new_percentage);
      Toggle(out i new_percentage);
    signals:
    properties:
      readwrite i Brightness = -1;
  };
};

Ended up fixing it in bug 709572. Thanks for the debug output.

*** This bug has been marked as a duplicate of bug 709572 ***