The battery plugin should indicate the current battery state (percent full and charging / discharging) by changing the icon shown, like it is done by the gnome power manager and some other similar tools. One would get a better indication of the current battery state at the first glance, without reading any text or looking close at the rather tiny bar. A fixed icon just wastes space and does not carry any information. In the icon theme I use there are battery-charged.svg for full battery (not chargin / discharging, AC on). battery-missing.svg for battery not present. battery-low.svg battery-caution.svg battery-040.svg battery-060.svg battery-080.svg battery-100.svg for 0%, 20%, 40%, ... 80%, 100% discharging battery. battery-charging-low.svg battery-charging-caution.svg battery-charging-040.svg battery-charging-060.svg battery-charging-080.svg battery-charging.svg for 0%, 20%, 40%, ... 80%, 100% charging battery (AC on). The gnome power manager has similar naming: gpm-battery-100-charged.svg = gpm-battery-charged.svg gpm-battery-missing.svg gpm-battery-000-charging.svg gpm-battery-000.svg gpm-battery-020-charging.svg gpm-battery-020.svg gpm-battery-040-charging.svg gpm-battery-040.svg gpm-battery-060-charging.svg gpm-battery-060.svg gpm-battery-080-charging.svg gpm-battery-080.svg gpm-battery-100-charging.svg gpm-battery-100.svg The plain gnome icon set contains battery-missing.png battery-full-charged.png battery-full-charging.png battery-full.png battery-good-charging.png battery-good.png battery-low-charging.png battery-low.png battery-caution-charging.png battery-caution.png battery-empty.png
Problem so far is that the icon-naming-spec only requires battery, battery-low and battery-caution icons, those icons are noy present in all themes : tango only provides battery-caution, afaict, gnome and adwaita provide battery-$(state){,-charging}.. no real consistencym, so it's not easy to use icons from a userthene, Another option would be to copy/import the icons provided by xfpm.
*** Bug 8734 has been marked as a duplicate of this bug. ***
Fixed/implemented in http://git.xfce.org/panel-plugins/xfce4-battery-plugin/commit/?id=3373e1e0b5369a85d4da1a3db92f55f6ad54ad0d / http://git.xfce.org/panel-plugins/xfce4-battery-plugin/commit/?id=f4d3815267137f9c093ff650943c5d070e31fbf6.