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		The 1-2-Both battery selector switch Most boats come out of the factory with a battery selector switch and two equal battery banks. This is indeed the cheapest and the simplest way to set up your batteries.  All that is needed is a switch, some batteries, wires and voila! What could be simpler. The fact that it is cheap and simple accounts for the fact that it is used on the majority of production boats, however that doesn't necessarily mean that it is the best solution. Advantages of the 1-2-both battery selector switch layout Its simple Its cheap Boatowners are familiar with it Disadvantages of the 1-2-Both battery selector switch layout It requires frequent user attention If not operated correctly it can cause all the batteries to run down If accidentally switched to OFF with the engine running it can fry the alternator Its not always obvious what is going on. When used with equal battery banks it can be wasteful

How is the 1-2-Both battery selector switch operated?

Normally one battery will be designated the start battery.  When starting the engine the switch is turned to this battery.  Once the engine is running the switch is turned to the Both position so that the engine can charge both battery banks. The operator needs to remember not to turn the switch through the OFF position or that would cause the alternator to burn out. (A switch with an alternator field disconnect is designed to stop this)  If the operator forgets to turn the switch at all then only the start battery gets charged.  When the engine is stopped and the boat is sailing or at anchor the switch is turned to the house battery position.  That way only the house battery can run the loads and the start battery is kept fully charged for starting.  If the operator forgets to do this and leaves the switch at the Both position then both batteries can get discharged and then it may not be possible to start the engine.
 

Other ways of using this switch arrangement.

You may hear other ways of operating battery banks with a 1-2-both switch.  One frequently heard method is to run one bank as a house bank on odd days and the other bank as the house bank on even days.  This method presumes that both banks are the same size.  The problem with this method (apart from the problems outlined above) is that at any one time a large chunk of battery bank is doing nothing but "resting".  While resting a battery bank sounds an interesting concept I dont think it is anything like the same as its owner resting.  There isn't any real evidence that resting batteries does anything whatsoever for their performance and having two equal sized battery banks is in most cases wasteful.
 
Whichever way the switch is used the Both position is intended to be used to provide some extra power if the battery being used for starting is unable to do the job.
 
Is there a better way?

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There is a better way

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