reading /proc/acpi/battery on newer linux versions does not work with current code
so even when unplugged the battery plugin reports it's on AC power.
see this link for the fix in ubuntu, and the actual patch below
http://utnubu.alioth.debian.org/scottish/by_maint/pkg-xfce-devel@lists.alioth.debian.org/large/xfce4-battery-plugin/xfce4-battery-plugin_0.3.0-1ubuntu1_packaging.patch

--
/home/jani/hack/ubuntu/xfce/packages/orig/battery/orig/xfce4-battery-plugin-0.3.0/panel-plugin/libacpi.c
    2003-09-0+++ libacpi.c   2005-10-07 18:05:53.000000000 +0300
@@ -272,12 +272,29 @@
   FILE *acpi;
   char *ptr;
   char stat;
+  DIR *acdir;
+  struct dirent *ac;
+  char *name;
+  char acstatus[128];
+
+  acdir = opendir("/proc/acpi/ac_adapter");
+  if (!acdir)
+    return -1;
+/* Works with ACPI versions newer than feb 2002, but this is for 2.6 kernels
only anyway */
+  while(ac = readdir(acdir)) {
+
+    name = ac->d_name;
+
+    /* skip . and .. */
+    if (name[0] == '.')
+        continue;
+    sprintf (acstatus, "/proc/acpi/ac_adapter/%s/state", name);
+    if (!(acpi = fopen (acstatus, "r")))
+        return -1;
+    break;
+  }

  if (!(acpi = fopen ("/proc/acpi/ac_adapter/0/status", "r")))
    if (!(acpi = fopen ("/proc/acpi/ac_adapter/ACAD/state", "r")))
      if (!(acpi = fopen ("/proc/acpi/ac_adapter/AC/state", "r")))
                                                                               
                       


Reproducible: Always
Steps to Reproduce:
1. 
2. 
3.