Once you've rendered it to your browser, do a view > source and make sure
the ID is still txtBC.  

If you are using master pages, the control is most certainly called ctl00_txtBC
and if you have the control in a user control it will be prefixed with the
name of the user control followed by an underscore.  If you are emitting
the script from your CS or VB code, you can use the ClientId property of
the control to find out what the browser will ultimately see it as.

Dave Bush
http://blog.dmbcllc.com

if (document.getElementById('<%=txtBC.ClientID%>').value.length == 0)
{
   // do something...
}