Trying to detect if a battery is charged
You could use a circuit similar to the following (It was not optimized though).
In principle The output capacitor simulates the battery and its voltage is constantly compared to the input voltage through a voltage divider.
If the capacitors's voltage + some tolerance exceeds the reference one, the transistor M1 is turned off and the transistor M2 is turned on.
The op-amp counts also with some positive feedback(\$R_8\$) in order to add some hysterisis to the input.





If you want to simply diconnect the supply voltage from the battery once it is charged (therefore no need for a dummy load), you can get rid of the components \$R_2\$, \$Q_1\$, \$M_2\$ and \$R_3\$.