Try using an Op-amp in a comparator configureation.
fix one input to a zener diode with the cut off voltage of the charger.
this should give you a constant charge cct. that charges the batt whenever its voltage drops below the voltage fixed by the zener. However if you want to build a cct for spike/rapid charging and then tricke charging, its going to be another project on its own.
Take care as jrz126 said what's the type of batt you wish to charge, cos if you try to charge the wrong type of batt with the wrong charger you might end up in hospital.
So take care.