The best method I know of for detecting a fully charged NiMH cell is Delta V. The pack needs to be discharged bellow 80% or so or this method won't work. Basically you apply a constant charge current and simply monitor the voltage until the voltage goes down. As a NiMH charge the cell voltage rises steadily to it's peak charge point and then drops just slightly, detecting this drop in voltage can be a little tricky, because it's not much of a drop. The higher the charge current the more pronounced this dip is. Typically after this chargers go into a trickle charge of 1/10th to 1/20th C