Hmmm, down to ships battery power, not good.
It will be remembered by some that one airline, which pioneered many (but certainly not all) long overwater routes (Atlantic mostly)...TWA, had extra ships batteries fitted to cater for this very same potential problem.
PanAmerican (most aircraft)...likewise.
Never ever ever depart with insufficient ships battery backup power.
Note: Even TWA Constellations had extra ships batteries fitted....ditto their B707's, L1011's.