presumably the 7.2 volt battery pack is recharged when the external supply is in use. This suggests that about 9 volts will be needed as fully charged NiMH cells will peak at about 1.5 to 1.6 volts per cell. Also, in my car and in my aircraft, the 12 volt systems are usually 13.8 to 14.2 volts when the generator/alternator is doing the business so regulating down to 11 volts is not difficult.
Rans6.....