Hey Gringo, I am not a battery expert or even a psuedo one, just rather safe than sorry.
Your contention that the charge voltage exceeds the potential of the other pack is of course
correct but what happens if the failure of one pack is not simply a voltage imbalance
but a shorted cell, then what will the good pack "see", my contention is that it will use the
positive rail in the receiver to dump its charge through the short as well.
That's why I suggested the blocking diode. Not just because of a pack imbalance.
If the guy asking the question knew all that stuff he wouldn't have asked in the first place.