Just connect your 12Vdc power supply in series with a resistor (say 1 ohm 50W) to the 7.2V pack for 30 seconds and then use the voltmeter to check the pack voltage. Repeat this process until the pack voltage reachs around 7.2V.
If you don't have a resistor, just directly connect the 12Vdc power supply to the 7.2V pack for 10 seconds.