They are generally packs of (probably C type) 1.5V Li-MH cells soldered/welded together to give a 18V battery. They are usually shrink wrapped to keep them together.
If you can open up the battery case and find a specialist industrial battery supplier you can probably replace the whole battery set. There is one in central Bristol, look for a Panasonic, Chloride or Yuasa agent/distributor. Have a look at Maplins or RadioSpares web sites to find an image of what you are after.
WinUser, November 2008
