However, small buffer batteries can be used to power electronics, particularly the radio, where both the radio and the buffer battery are mounted high in the vessel. Some people insist on using a separate battery for the anchor windlass. If used, this battery should be isolated so that it doesn't discharge in parallel with the house battery. A windlass only draws high current when the anchor is stuck to the bottom, so the need of a separate battery for it is questionable. On the other hand is a bow thruster that can draw high currents for extended periods.
Peak load demands probably mandate a battery local to the thruster motor.
A battery for engine controls is another case where a small battery can be used . . . We have a small Group 24 gel battery wired directly to our diesel furnace . . . Keeping such batteries charged is simple. Use an Ample Power Cross Charge diode. It's a large Schottky diode mounted on a heatsink.