Created attachment 2292 diff between version 0.5.1 and the modified version Can't compil xfce4-battery-plugin on XFCE 4.4.3 on Slackware 12.2, there were some duplicate declarations in file battery.c, just selecting and ordering include diretcives. I've test it on i686 only.
To me this looks like a duplicate of bug # 3938. Can you try the patch from that bug please? It's much more trivial.
*** This bug has been marked as a duplicate of bug 3938 ***