posted 06-01-2015 08:40 PM ET (US)
It was common for decades in small boats to have two batteries as sources of power in a single circuit primary power distribution system. The configuration of the batteries was controlled by a simple switch with positions OFF-1-BOTH-2. Either battery 1, battery 2, or both were selected as a source to power all loads.Today, with boats having many more accessory loads such as electronics, pumps, and other electrical accessories, this legacy arrangement is not used as much as it once was.
Today on small boats a more typical configuration for two batteries is to maintain two completely isolated electrical power distributions, one for each battery. One battery, called the main battery or battery one or the engine cranking battery is connected only to the outboard engine. This battery supplies all the electrical power to the outboard engine for starting the engine and for operating any engine loads that require more current than can be supplied by the charging system of the engine.
The second battery, called the house battery or battery two or the auxiliary battery, is connected only to non-engine loads. This battery supplies all the electrical power to loads like engine instruments, chart plotters, depth sounders, radios, GNSS receivers, other boat electronics, house loads such as house lighting, navigation lighting, pumps, anchor windlass, motorized gear like down riggers, and so on.
By separating the loads into two groups, battery one (or the main battery or the cranking battery) will always be available for engine starting without being run down in charge by other loads. Battery two (or the house battery or the auxiliary battery) stays isolated from the engine loads, and supplies power to other devices without being affected by engine starting sags or spikes.
The problem with isolating loads and batteries comes in distribution of the engine charging current. Modern outboard engines, such as the Evinrude E-TEC (and others), provide for auxiliary charging outputs from the outboard engine's alternator. Having two charging outputs permits the two batteries to be maintained at charge without having to combine the batteries in parallel.
On modern engines that lack the desirable feature of dual charing outputs, the two batteries can be temporarily and automatically placed in parallel for charging by an automatic combiner relay (ACR) which is operated by a voltage sensitive relay (VSR) circuit. This is not quite a nice as having two alternator outputs, but can work well in many cases with outboard engines lacking the dual alternator output feature.
So the "best" configuration is to abandon the old-fashioned one-source switch, and re-wire the boat to use a more modern approach. If your outboard engine does not have two charging outputs, add an ACR/VSR device.
If you don't follow that approach, then manually operate the OFF-1-BOTH-2 switch as desired to distribute both the load current and any charging current as appropriate.