There is a 1800 mAh that is stock size and a 3600 mAh that requires a custom cover, but I haven't heard of anyone using either yet on a 6800. However, this thread
http://pdaphonehome.com/forums/ppc-6...irmed-fit.html over at pdaphonehome has a user who put a battery made for a HTC 3600 in his Mogul, so the batteries below might work.
XDA Forum Discussion of 1800 and 3600 mAh batteries with links
http://forum.xda-developers.com/showthread.php?t=298273