Splitting laptop power supply to charge 3 NiMH battery packs, how to protect from overcharge?
NiMh batteries requires 1.4V per cell to charge full, the data provided indicates it's a 6 cell pack. I assume you have limited component on hand, I give you the below circuit.

It will charge the battery to about 90% full and you can leave it to charge overnight; without damaging anything. Make sure the resistor is 2W and the diode is rated 2A for safety. Do make sure the BAT1 terminal is at 8.3V max after long period of charge.

Technical:
Reduce R1 to 1 ohm will increase the charging current, allows faster charging time. D1 is normal diode, not schottky diode; some may spec 0.6V forward voltage. You may want to start the V1 (dc-dc) voltage at 8.7V to have a feel first.







simulate this circuit &ndash; Schematic created using CircuitLab