Volts, Watts, Amps, and Ohms: 12-Volt Terms for New Boaters

If you’re new to boats, or even if you’ve been around them for a while but have been avoiding the electrical bits, sooner or later you’re going to need to become more familiar with DC electricity than you are if all you know how to do is jump-start your car. To help get you going, here are some very basic definitions you’ll need to understand.

DC means Direct Current. It’s the kind of electrical current produced by batteries. Batteries found in cars, trucks, RVs, and boats are almost always 12-volt DC.  Flashlight batteries are DC, too. In fact you can create 12-volt potential by linking together just eight 1.5-volt flashlight batteries – AA, C, or D (1.5 x 8 = 12).

Fuses are rated for a certain maximum amperage flow, above which they will 'blow,' interrupting the circuit. Amp ratings are shown on the fuses above. Circuit breakers work the same way but 'trip' when they're overloaded.
Fuses are rated for a certain maximum amperage flow, above which they will ‘blow,’ interrupting the circuit. Amp ratings are shown on the fuses above. Circuit breakers work the same way but ‘trip’ when they’re overloaded.

Basic 12-Volt Definitions

  • Voltage/Volts = the amount of potential energy available to push electrical current. Since electricity is invisible, it’s convenient to picture voltage as the potential pressure in a water system. For a battery-powered system, think of a water tower with a big tank on top. It’s drained by gravity, and the way water flows out of it depends partly on the volume and weight of the water in the tank at any given time (the “voltage” equivalent), and partly on the characteristics of the drain pipe. In an electrical system, wires and other conductors are the equivalent of water pipes.
  • Amperage/Amperes/Amps = the flow of electrical current through conductors like wires. Think of it as the amount of water flowing past a single point in a pipe at a given time.
  • Wattage/Watts = the amount of energy expended, or used. Think of it as the water needed to fill a glass (a few watts) or a swimming pool (lots of watts).
  • That’s as far as the plumbing analogy goes, though, because unlike water systems that run from source to drain, electrical systems run in “circuits,” in other words in a circle, from power source to usage (“load”) and back to the power source, with switches and fuses or circuit breakers in between to interrupt the flow of electricity as necessary.
  • Ampere-hours (Ah) = the current in amperes multiplied by the amount of time it flows. Batteries have ampere-hour capacity ratings that give a general idea of how many amps can be drawn from the battery for how long. In a perfect world, a battery rated for 90 amp-hours would be able to give you 90 amps for one hour, 45 amps for two hours, one amp for 90 hours, and so on. In reality, you can and would use only a portion of those amp-hours before the battery should be charged again.
  • Ohm = a measure of resistance in a wire or other conductor. Resistance is determined both by the wire’s length and its thickness, or gauge. The thicker the wire, the more easily current will flow through it. Resistance always creates heat, and the greater the resistance, the more heat. Try to put too much current through too small a wire, and you can create enough resistance to start melting things and causing fires. This can happen even in a simple 12-volt system, so always use common sense and generous wire gauges.

In most cases it’s unrealistic for a boatowner to measure for ohms, simply because boat gear manufacturers don’t usually offer a baseline of resistance to measure against.  Instead we measure for an abnormal drop in voltage in a circuit, which would indicate corrosion, too small a wire size, or a poor connection.

Now, here are some of the easiest equations you’ll ever have to use:

Volts x Amps = Watts (example: 12 volts x 5 amps = 60 watts)

Watts / Volts = Amps (example: 60 watts / 12 volts = 5 amps)

Amps x Time = Ah (example: 3 amps x 5 hours = 15 Ah)

If you read the owner’s manual for any piece of electrical gear, or the stamped information on the gear itself, you can usually discover what it needs for energy input and how much energy it uses. Most marine gear, whether a chartplotter, a bilge pump, a windlass, or an electric windshield wiper, will tell you its current draw in amperes. Then it’s a matter of arithmetic to find out if your 12-volt system can handle the task, and for how long.  (Note: there’s also marine gear made for 24-volt systems, but those are usually on larger or specialized boats; the great majority of small pleasure-boats use 12-volt systems, and in any case the DC principles are the same.)

12-Volt Safety Issues

Working with 12-volt systems is easy, and relatively safe, compared to 120-volt AC. But 12-volt systems are far from benign, for several reasons.

First, a 12-volt battery can deliver a whopping big discharge of current all at once. That’s how a 12-volt battery can run a starter motor to get a car or a boat running. That discharge of current can be violent, as you’ve noticed if you’ve managed to bridge the the terminals with a wrench or a wire. If you have a metal watch strap or a ring on, and that metal becomes part of the short circuit, you can get hurt. If you’ve used jumper cables, you’ve probably seen a good-sized spark as a clamp comes in contact with a terminal. So be alert to the danger of short circuits and sparking.

Working with marine 12-volt batteries is relatively safe, but there are precautions and habits to pay attention to.
Working with marine 12-volt batteries is relatively safe, but there are precautions and habits to pay attention to.

Second, when you produce a spark in the wrong environment, you can cause an explosion. Batteries produce hydrogen and oxygen when they’re charging  — no problem if the battery is well-ventilated, but a potentially explosive mixture if the battery is in an enclosed, poorly ventilated space or container.

Third, inadequate wiring, unfused wires, poor connections, and corrosion can cause electrical fires, which in turn can catch other flammable material like cloth or paper on fire. If you get involved in a 12-volt installation on your boat, follow the instructions, use big enough wire with short enough wiring runs; use fuses where necessary, and never run wiring under or through any flammable material.

Fourth, the liquid electrolyte in flooded-cell batteries is mostly sulfuric acid, which will eat your clothes and burn your skin, and can blind you if you get it in your eyes. When inspecting and testing the flooded cells in your battery it’s a good idea to wear glasses or other eye protection, and rubber gloves. When you pry off the cell covers with a flat-bladed screwdriver, do it gently and carefully, and make sure the battery is on a stable surface. If you get electrolyte on you, rinse thoroughly with fresh water.

These warnings aren’t meant to scare you off — far from it. Working with 12-volt systems is relatively safe, but you must read the directions, use common sense, and be alert. That’s a pretty good combination of habits for any project.

Good References

Electrical Resistance on Boats: Keep That Voltage Drop in Check

The 12 Volt Bible for Boats by Miner Brotherton and Ed Sherman

Powerboater’s Guide to Electrical Systems, Second Edition, by Ed Sherman

Boatowner’s Illustrated Electrical Handbook by Charlie Wing


This article originally appeared on Boat Trader in January 2016.

 

Boat Wiring: Use Good Terminals and Tools

Maybe you’ve seen this at your marina or boatyard before: an able-bodied weekend warrior hanging upside down in a bilge or wet locker mumbling obscenities as an electrical project gets underway. It’s a frequent sight, because basic electrical system upgrades and repairs are one of the most common do-it-yourself projects we undertake as boat owners.

This ratcheting crimper places a precise amount of pressure on the crimp fitting while also making two crimps—one on the wire and one on the wire jacket—at the same time. Photo by Gary Reich.
This ratcheting crimper places a precise amount of pressure on the fitting while making two crimps—one on the wire and one on the wire jacket—at the same time. Photo by Gary Reich.

And while most folks start these projects with the best intentions, many of the materials they use are often ill-suited for life in the marine environment. With that in mind, let’s take a look at one area where using the wrong parts is a recipe for disaster: electrical terminal fittings.

Electrical crimp terminals do exactly what the name implies—they provide an attachment point for a wire where it terminates. Good examples include a ring terminal where a wire is screwed to a fuse block, a spade terminal connecting lead wires to a removable depthsounder or fishfinder, or a butt connector joining two pieces of wire. These terminals are crimped down on the wire using a tool engineered for that purpose, and provide a good connection between the wire and its termination point. So why not just use the cheap terminals found at your local auto parts store or Radio Shack? Three words: corrosion, vibration, and moisture.

Let’s start with corrosion. The crimp terminals you generally find at your local auto parts shop are what you might call “mystery meat,” meaning it’s difficult to know what type of metal they’re made of. I’ve seen them made of anything from mild steel to zinc, and neither of those materials is good at much of anything electrical, save for sacrificial zinc anodes. Tin-plated copper terminals are what you’ll want to look for at your marine store. Tin is highly resistant to corrosion, and copper is an excellent conductor of electricity. In tandem, they make for a great marine-grade electrical terminal. Plain copper terminals with no tin plating can be found on many boats, but they’re all susceptible to that funky green corrosion that can cause connectivity issues in the future.

A double-crimp, ring-style marine terminal installed on a section of 10 AWG wire. Note the double crimp points as illustrated. Photo by Gary Reich.
A double-crimp, ring-style marine terminal installed on a section of 10 AWG wire. Note the double crimp points as illustrated. Photo by Gary Reich.

Moisture and corrosion-resistance go hand-in-hand, especially if your boat operates in a saltwater environment. Tin plating will definitely help suppress this corrosion, but if you really want to nip it in the bud, use not only tin-plated marine terminals, but tin-plated marine terminals with integral heat-shrink tubing. This integral tubing often is lined with sticky adhesive that melts and seals out the end of the termination when heat is applied. Alternatively, you can apply your own heat-shrink tubing, cut it to size, and shrink it on the connection yourself. If the electrical work you’re undertaking is in a bilge, underneath a center console, or in any location where there’s a remote possibility of water contact, it’s absolutely essential to use these high-quality marine-grade terminals and heat-shrink protection.

An assortment of ring-style marine crimp terminals. Photo courtesy of Ancor.
An assortment of ring-style marine crimp terminals. Photo courtesy of Ancor.

Vibration is an electrical system nemesis we don’t generally think about, but a nemesis it is. Engine and drivetrain vibrations, shock waves from wind and waves, encounters with docks and pilings, bumps and bangs caused by hatches and locker doors — all of these  bad vibrations (not like the good ones the Beach Boys were singing about) can make a terminal fail at its connection with the wire, causing circuit failure or a fire. To battle these connection failures, a good marine terminal uses not one crimp, but two, and is aptly called a “double crimp” terminal. Look for terminals labeled as such. A double-crimp terminal makes one connection onto the bare wire, and another on the wire jacket, which provides excellent strain relief and resistance to the effects of vibration. Last detail: Make sure you use the proper tool when installing double-crimp terminals. There are many different types of tools out there, but I like ratcheting double-crimp tools. Not only do they release when the proper amount of pressure is applied to the fitting, but they also crimp the terminal fitting to both the wire and the wire jacket in one motion. You also can use a standard strip-and-crimp tool, but make sure you crimp at both the wire and the wire jacket.

You’ll find these double-crimp, marine-grade electrical terminals at any good marine supply shop. To find out what size terminal you need for your job, look for the American Wire Gauge (AWG) specification on the jacket of the electrical wire you’re crimping to. For example, “16 AWG” designates a 16-gauge wire, which will use a 16-14 AWG terminal. A “22 AWG” wire needs a 22-18 AWG terminal, and so on. Make sure you use the right size terminal for each job you’re doing, and crimp using the right slot on your tool. And never use a pair of pliers or Vise-Grips.

So there you have it. Next time you’re rewiring a VHF radio or installing a new navigation light, spend the time to find the right terminals for the job. In the long run, you won’t be sorry you did.

Here’s a video that walks you through the steps of how to install a heat-shrink terminal:

Boat Buyers: Beware of Auto Parts

As I mentioned a while ago in this blog, I used to be an auto parts parts guy. I worked in a parts store and a GM dealership, and I can’t even remember how many boat owners came in looking for parts for their boats. Silly boys. Car parts are for cars. Boat parts are for boats.

It always began the same way: The customers would complain about how much the MerCruiser dealer wanted for whatever it was they were looking for. They never knew what year the engine was or any other relevant information it might take to get them the right parts, but they always insisted that because it was a small-block Chevy, they were all the same.

Any original or replacement alternator for a marine engine is required to have ignition protection.
Any original or replacement alternator for a marine engine is required to have ignition protection.

Well, that was fine for points or spark plugs or a water pump, but for everything else, all bets were off.

The lesson last time was that boat wiring in not car wiring. I’m bringing it up again because I’ve seen and heard of more cases recently where non-marine starters and alternators have been installed in boats. This is flat-out dangerous. Why? Because if you buy an automotive alternator or any electrical appliance that’s not ignition-protected, you run the risk of blowing up your boat. Without ignition protection, if for some reason you have an explosive air/fuel-vapor mixture in your engine compartment, and there’s a spark when you crank over that engine, it’s going to ignite the surrounding atmosphere.

Marine starters and alternators are ignition-protected, which means they don’t allow a spark to be emitted from the device. And that will keep you and the people around you from having a very bad day.

So, as you click through the pages of BoatTrader.com, and more importantly, as you go out to inspect prospective used boats, be aware that there are owners out there who have been either too cheap or too ignorant to install safe equipment — and you really don’t want to buy boats they have owned.


 

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