The output of the generator needs to have a POWER DIODE to allow flow of current just one way -- from the generator to the battery.
If you have an alternator -- by definition the energy produced is AC, therefore you need diode rectifiers to convert the AC into DC for the battery to "take" the energy as a charge.
If you have the diodes in the alternator that accomplishes what you want - Alternator - battery isolation when the alternator is "dormant"
The diode has to have the capability of carrying the current that the alternator produces and I suggest 3 times the current being generated to reduce the diode power dissipation ( implies that the diode may need a Heat Sink to keep it cool and alive).
Nando