you could probably hook in a 7.2v external source by using a power supply with a connector or plug interface . or stuff it inside space permitting
respect polarity , current needs and voltage rating.
if volts are a bit high you can wire in a couple of diodes to drop it (check diode specs for voltage drops& amps rating x diodes needed.)
For silicon diodes, the typical drop is 0.7 volts
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