There are, of course, chips just for this purpose.  I think they
generally watch for either a zero or negative dV/dt, depending on battery
chemistry.  They will also often watch battery temperature, since there
is a rapid rise in cell temperature on continued charging after the
battery reaches full charge.  Each of these techniques assume a constant
current charge.
        Comapnies that come to mind are Maxim and maybe Linear
Technology.  You could, in the interest of reducing chip count, do
soemething similar with a PIC.


Harold