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Cross Charge Diode
In general it is a good idea to minimize the number of battery banks that are used. Typically, there should be a house bank, and a dedicated starter battery. Think carefully before adding more banks.
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 . . . this time to eliminate voltage transients that occur when high current house loads turn on and off, i.e. trim tab motors.
We have a small Group 24 gel battery wired directly to our diesel furnace . . . it's no good to turn power off on a running furnace!
Keeping such batteries charged is simple. Use an Ample Power Cross Charge diode. It's a large Schottky diode mounted on a heatsink. The diode is rated for 200 Amps, so it can handle the current from most alternators. Do not substitute a regular silicon diode as their voltage drop will degade battery life.
As usual, make sure the wire size is appropriate for the maximum Amps that the alternator could deliver to the battery if it were empty. Fuse as appropriate.
The Table below shows typical voltage drop at different currents. A normal silicon diode used in isolators will exhibit twice the voltage drop, or more.
Presented below is a schematic diagram. The Ample Power Cross Charge Diode is simple to use and very effective.