Typically, boats have an engine start battery and an auxiliary battery, whilst some have a third battery for a Windlass or Bow Thruster.

 

The question is, how can they be charged from a single alternator?

 

Options are:

 

RELAY
Parallels two batteries when the alternator is charging and seperates them when the engine has stopped.

 

1,2 BOTH OFF SWITCH
A battery is selected for charging, although generally the Both position is opted for and after charging, select the battery to supply your DC Loads.

 

VOLTAGE SENSITIVE RELAY
Essentially the same as a Relay with additional features - Prioritising, Charging, Bi-Directional, Protection.

 

LOW LOSS BATTERY ISOLATORS
Allow for two or three batteries to be charged at the same time from a single alternator.
The voltage drop across the isolator is less than 0.1v.

 

SPLIT CHARGE DIODE
Can charge up to three batteries.
However, the voltage drop across the diode is proportional to the current and can be up to 0.7v.
To compensate for the voltage drop, a battery management system must be used as this will raise the alternator voltage to ensure the optimum charging voltage to the batteries.

 

The most important point to note, whichever option you choose, unless the correct charging voltage is applied to the batteries, generally 14.0v, then the batteries will not adequately charge.