Nominal 7.4
Operating 6-14 negative gnd (battery connect)
11-16 negative gnd (external dc jack, charging)
So as I interpret that:
Nominal voltage of the internal battery is 7.4V
You can apply 6 to 14V to the input terminals from the battery,
and the rig will still operate correctly.
You can plug a source of 11 - 16V to the DC input jack to charge
the battery and operate the radio on external DC. (Likely with a
built-in regulator to charge the battery.)
So you could make an adaptor and run the radio straight off the
Bioenno battery rather than the internal one (assuming that the
maximum voltage of the Bioenno was less than 14V).
Applying 12V to the external DC connector shouldn't be a problem
for running the radio, charging the battery, or both.