Joe Coons
Boat electrical systems, like boats, are a compromise. They also are perhaps the least-understood of a boat's components. I'm not talking about the 110-volt systems that larger boats have, for except for a shore-power switch and/or generator, these are pretty much like those at home (unless you have an inverter). Instead, I mean the DC systems, usually 12 volts, that start our engine and run most our lighting, accessories, navigation, and communications facilities.
The compromise has to do with (A) how many batteries the boat has; and (B) how the batteries are controlled and charged.
In the earliest years, boats had but one battery for everything, but as we all know, the risk then was that the battery would get low during anchor-time activities, then not be able to start the boat's engine, an outcome that is very undesirable! So boats began to be equipped with a second battery, and a switch to select between them. The switch could be set to either battery or both batteries, allowing the user to prevent running down both batteries when anchored always leaving one for starting, but charge both when underway.
The trouble with this "switch" scheme is that the operator can forget to change the switch, and so a battery may not get charged by the engine, or, conversely, both may be run down.
So the next improvement was the "isolator", which allowed both batteries to always be charged by the engine, whether they were switched on or not, and then one battery was always for the "ship", and the other for starting. But there was a problem here, too: when the engine is running, its alternator or generator which charges the batteries is told what to do by a "regulator", which senses the voltage, but can only do it one battery at a time! For safety, the battery sensed was the one most important --- used for starting --- so it was not uncommon for a boat's starting battery to be fully charged yet the ship's battery used for everything else is less than full.
What I have done on my own boat came from a suggestion by Don Rasmussen of Rasmussen's Marine Electric: I put an oil pressure switch on the engine. When the engine is running, its switch closes, and in turn closes a 200-amp, continuous-duty solenoid so that my ship's and starting batteries are paralleled. Now both batteries are sensed by the regulator, and my alternator keeps charging until both batteries are "topped". This solved all my "ship's battery-not-fully-charged" problems.
In the October issue, I'll tell you more about DC considerations for twin-engine boats.
Happy charging --- and cruising!
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Last updated 8/27/97 by SCR