Direct current electricity (DC) is the lifeblood of almost all recreational powerboats. Without it, our engines won’t start, our radios won’t talk, our navigation units won’t navigate, our lights won’t illuminate and, perhaps worst of all, our toilets won’t flush. Direct current electrical systems are relatively easy to understand and far less lethal than alternating-current (AC) systems to work on, so many boaters can do their own DC repairs and modifications. But this comes with the caution that you’ve got to understand DC basics, read the manufacturers’ instructions regarding connections, wire sizes, circuit breakers and loads. From time to time, Boat Digest will explore DC basics by focusing on DC components. This season we have batteries from Optima and solid-state bilge pump switches from Water Witch in three of our product test boats on Long Island Sound. We’ll be reporting on them soon. This month, we take a look at battery switches, battery isolators, and battery cables—where the whole DC process begins. Battery Cables and Switches Over time, the terminals at the battery and its switches get cruddy—especially in saltwater environments—and one sure way to mess up a good day on the water is to ignore the build-up of that crud until excessive (heat-generating) electrical resistance builds up at the connections. So, if you do nothing else at the beginning and midpoint of your boating season, check the electrolyte level in the batteries, clean the battery terminals, check the condition of cable lugs at the battery end and at the switch (or isolator) end, and be sure they well secured. If the heavy-gauge battery cables are frayed at the lugs, strip back the insulation until you get to bright metal and crimp on new lugs. (You may ask the yard to do this, because the job requires a special tool.) Never cheat on the size (gauge) of the battery cables. The fatter the cables, the less resistance they offer and the better they’ll do cranking your motor or supplying your electronics. The yard or ships’ store can help you with your selection, using tables that look at the current load vs. length of cable run. Whether you have one battery or several banks of batteries, you need the capability to switch them in and out of your boat’s DC circuits. That’s the function of the marine selector switch and its associated cables. Switches are rated for voltage and the amount of current they can carry continuously and momentarily. Typical voltage ratings for marine battery selector switches are 6, 12, 24 or 32 volts. The continuous current rating for a selector switch is always less than the momentary load rating. For example, one commonly used marine selector switch is rated at 230 amps for continuous operation and 345 amps for momentary operation. Your switch should be able to handle your starter’s cold cranking draw. Ask customer service at the engine manufacturer for that number or their recommendation for the switch rating. Never use a switch with a lower rating than the one installed at the factory. Over the seasons, the internal works of your switch can be gummed up by the same corrosive elements that get to the exposed battery lugs. Replacing a battery master or selector switch is not difficult, but you want to tag all the wires and cables as you remove the old switch so you get things back together properly. In a single-motor, single-battery installation, the switch is usually called the master or disconnect switch. When more than one motor or more than one battery is installed, the switch is called a selector switch because it enables you to select which battery serves the load or to link up both batteries (or banks) in parallel to handle balky engine cranking. In any case, a modern replacement switch should include an alternator field disconnect switch. This is important. You should never disconnect (turn OFF) a battery switch while the associated alternator is running. To do so can fry the alternator’s diodes—an expensive inconvenience. Old switches carried an embossed warning against moving the switch to the OFF position while its charging engine was operating. Newer models of switches have terminals for alternator field disconnect leads. As you turn the knob from ON toward OFF, the device first opens the alternator field circuit and then opens the main circuit. That way the alternator stops charging before the battery goes off line. Good disconnect and selector switches are fabricated with heavy-duty sliding contacts. Read the product sheets. Heavier sliding contacts mean longer life. Also, with selector switches, you’ll usually see the term “make before break” listed as a feature. That means that the switch works in such a way as to avoid arcing and momentary current interruptions as you move from one battery to the other or as you hook up both. Battery Isolators Many folks with a single-motor opt for two batteries—one for cranking and one for running electronics, lights and even trolling motors. A slightly bigger boat could have two motors and three batteries—one each per engine for cranking and a third house battery for lights, navigation, water pumps and the like. Selector switches can be set up in many ways to accommodate these installations and in such cases you’ll also want a battery isolator in the circuit between the alternators and the batteries. Think of a battery isolator, installed between the alternator and the battery, as an automatic traffic-control station for direct current. An isolator is installed between the alternator(s) and the battery(s). Consider, for the moment, an outdrive boat with a single engine and two batteries. The battery selector switch will have four positions—OFF, 1, BOTH, 2. Typically, operators use one battery to start (crank) the engine and the other as an accessory battery to handle the electronics, entertainment systems and lights. The idea is to have the cranking battery available to get back home even if the accessory battery runs down while you’re on the hook listening to music and feeding your expensive bait to the local fish. Without a battery isolator in the circuit between the batteries, the discharged accessory battery could draw down the charged cranking battery leaving you with two discharged batteries. The isolator, actually a network of diodes, keeps the batteries from discharging each other while allowing them to take a simultaneous charge or to work in parallel for a tough start. Some boat manufacturers install a selector switch and two batteries at the factory but leave out the isolator. Installing one is simple, but read the literature and ask for help at the marine parts counter. Battery isolators are rated by the numbers of alternators and batteries they serve, as well as the current they handle. Do your homework when selecting an isolator. They differ for alternator and generator setups, so get technical help. Once you’ve made the selection, the hook-up is relatively simple. So, as you begin the season include the front end of your DC electrical system—batteries, cables, isolator and switch—in your spring checkup.