Current sensing across a resistor leading to the battery or from the battery determining how much charging current is being drawn by the battery coupled to an op amp You use one of the pins to provide a reference voltage the other to detect when battery is fully charged the output of which you couple to a transistor operating a relay which pulls in or out disconnecting the charger circuit
Look up charger circuit on the net lots of examples of how to
OR
If you wanted to go down the micro processor road then you could use a 12F675 and use the ADC module to determine when you battery is fully charged operating some type of electro mechanic switch or other power switching device
Or you could use a MAX 712
LINK BELOW
http://www.rason.org/Projects/gelcell/gelcell.htm
