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Installing a New Battery Bank

The silence of this anchorage is only disturbed by the sounds of the captain's angst when he discovers the engine won't turn over due to a dead battery.
It’s a windless Sunday morning and you awake to the sounds of nature coming to life all around. The whole family has really enjoyed this remote cove where you’ve been snugly anchored for the weekend. Ah, life on the boat! You eat breakfast, savor your coffee in the cockpit, and then start readying the crew for the trip home. As the kids strike the sail cover and stow their gear, you turn the key to start the auxiliary engine.

Click. Click. And then you hear nothing.

‘Oh no!’ you wince. There’s only one thing happening here. The battery is dead!

This is a pretty common scenario as we sailors often misjudge just how much power gets used on our boats when we’re at anchor and unplugged from the dock. Reading lights, anchor lights, marathon margarita sessions with the 12-volt blender, all of these things take their toll. But by adding a second, isolated battery or bank of batteries you can avoid this scene. It’s an easy, smart move, and in almost all cases you can do it yourself. Here are the steps you need to follow to make this electrical upgrade.

Choosing a Battery    Batteries come in many different sizes and types, and are rated according to their capacity. When selecting a battery for your second battery bank, your first consideration is how you plan to use that battery. If your new battery bank will be used for cranking the engine, then you’ll want to choose a battery designed for that purpose. Cranking batteries supply a high amount of power for a short period of time and are not intended to be deeply discharged. For powering house loads, such as lights, electronics or refrigeration, a deep cycle battery is your best choice. This type of battery is designed to last when put through the rigors of being deeply discharged and then recharged over and over again. Deep-cycle batteries are manufactured using several different technologies, all of which are acceptable for marine use.

The battery box aboard the authors' boat doubles as the navigator's seat. More importantly, it contains the batteries securely, and provides the necessary ventilation through a stainless-steel plate.
You can choose between a traditional Wet Cell battery that will require periodic topping off with distilled water, or opt for a maintenance free Gel or Absorbed Glass Mat (AGM) battery. The Gel and AGM batteries provide the benefit of a longer life when compared to traditional Wet Cell batteries. As they require no maintenance, they’re also a good choice if your mounting location doesn’t provide easy access. They are, however, much more expensive than traditional Wet Cell batteries and these benefits need to be weighed against their higher initial cost.

When choosing a battery size and capacity for your own boat, your anticipated power consumption along with the amount of space available for mounting the battery itself are important factors to consider. If the purpose of your upgrade is to allow you to overnight more comfortably on a small boat, then the addition of a single Deep Cycle battery like a group 27 or group 31 should suffice. If you’re anticipating extended stays away from the dock, or have dreams of serious cruising, you’re going to want to choose a series of heavier, larger, and more highly rated batteries for your new bank. On Serengeti, our 46-foot cruising boat, our house bank consists of six wet cell six-volt golf cart batteries. Our second battery bank, which is reserved solely for emergency cranking, is comprised of a single group 31 battery. For more information about how to estimate your own daily power needs, you may want to check out the article, Creating a 12-Volt Spreadsheet, by Tom Woods, here at SailNet.

Mounting the Battery    In general, batteries are best mounted in a level position when your boat’s not heeled over. With Wet Cell batteries this is imperative, and with the others it just makes sense to keep them level if you can. All batteries should be tightly strapped down to eliminate the chance of movement or shifting while underway. One of the reasons for this is that batteries are heavy, but the other equally important reason is that the terminals on top of the battery must be protected from incidental contact with anything metal. You can accomplish this most easily by installing rubber boots over the lugs and terminals, or by placing the whole unit in a covered and ventilated battery box. A battery box offers the additional benefit of containing any acid should it accidentally spill out of a Wet Cell battery. Special plastic battery boxes are readily available in various sizes to meet this requirement.

"If allowed to accumulate in a non-vented area, hydrogen gas can become the fuel for an explosion should a spark somehow enter the equation."
As Wet Cell batteries charge, hydrogen gas is given off in the process. In low concentrations, this does not present a problem. However, if allowed to accumulate in a non-vented area, this hydrogen gas can become the fuel for an explosion should a spark somehow enter the equation. For this reason, it’s imperative to choose a mounting location that will allow any gas to dissipate. If you locate your battery down low in a locker or under a seat, it’s a good idea cut a hole and add a locker vent if one is not already present.

A battery is best located away from any bilge water, and in a spot where it will not adversely affect how the boat sits in the water. Also, a shorter cable run from the battery to the switch will be more efficient and less expensive. Lastly, keep in mind that you’ll need access to the batteries from time to time to check water levels and confirm that all connections remain tight.

Making Battery Cables    Making your own battery cables is very simple providing you have a few basic tools. You’ll need a hacksaw, cable lugs, heat shrink tubing, a hair dryer, lithium grease, a lug crimper or swage-it tool, and a razor knife, along with your new battery cable. The first step in making your own battery cables is to spec out the wire diameter needed for your application. Wire diameter is a function of the amount of power the cable will carry and the round-trip length from the battery to the load (battery switch in this case) and back. Once you have this information, you can use a simple chart like the one below to determine your own battery cable size.

Minimum Wire Size (AWG) Selector Table




Round-Trip Length of Conductor (Feet)

If you want your new battery cables to last the life of your boat, be sure to use only tinned wire. Tinned wire is a lot more expensive but it does not corrode and lose its conductivity over time like regular copper. Because of this relatively high cost per foot, it’s important to measure carefully so that you don’t buy too much cable, or even worse find out that you’re just inches short. To help us get our lengths exactly right, we like to use a section of plastic hose to simulate each cable run. We’ve found that hose like this bends and mimics the exact path a cable will take better than any tape measure or piece of string. After we mark each end of the hose, we know that we’ve precisely determined the length we’ll need for the finished cable.

The positive cable that will run to your new battery switch will need to be red in color.  Your negative battery cable should be black in color. This black cable does not run to your new battery switch, but should terminate at a common ground point along with your existing negative battery cable.

Installing a new battery is more simple than you might think, and it requires a minimum of toolsjust those shown here.
To make sure your connections remain tight, both ends of the battery cables are fitted with lugs. Lugs are specially designed pieces of metal that crimp onto the ends of the cable to provide a flat circle that slides over a threaded stud and is held in place with a nut. When purchasing lugs, make sure you get the right diameter hole so that the lug will fit correctly onto the threaded terminal. The terminals on batteries are sometimes different sizes so that you don’t inadvertently switch the positive and negative cables by mistake. Check your individual battery and battery switch carefully to determine the lug sizes you will need.

Fitting a lug to a cable end is pretty easy. Simply strip back about one and a quarter inches of the insulation with a razor knife, then slide a three-inch long piece of heat shrink tubing onto your cable. The lug is placed over the exposed wire and snugly crimped in place using a lug crimper or swage-it tool. Apply a dab of lithium grease around the connection, and then slide the heat shrink tubing over the greased connection that you’ve just made. Using a heat gun or hair dryer, apply heat to shrink the tube down tight. The lithium grease helps to repel moisture from the connection and will further prolong its integrity.

"For most sailors, the lower capacity switches can handle far more current than we’d ever use on board."
Battery Switches    Battery switches that are designed to control two battery banks normally have four positions: OFF, 1, BOTH, or 2. This type of switch allows positions for drawing power from or supplying charging current into each battery bank individually, or both at the same time. Some switches are rated for higher amps than others, but for most of us sailors the lower capacity switches can handle far more current than we’d ever use on board.

One feature that you may want to look for when shopping for a battery switch is a model that offers an alternator field disconnect. A field disconnect turns your alternator off when the battery switch is in the OFF position. When using a standard battery switch without this feature you risk frying the alternator diode should a crewmember inadvertently turn the battery switch to OFF while the engine is running.

For cost and efficiency, battery switches are normally mounted somewhere near your batteries. For convenience, try to locate your switch in an area that’s both easy to see and use. Many switches are designed to be surface mounted so that the incoming and exiting cables can be visible or hidden from view behind a bulkhead depending upon your preferences. 

The battery switch above, which the authors installed on their own boat, does not have an alternator field disconnect, so they're always careful to shut the engine down before they switch the battery to "Off." 
When it comes time to wire the battery switch, remember that we’re only going to be dealing with the positive (red) cables. Detailed wiring instructions are almost always included with the switch, so it’s just a matter of following those directions. Looking at the backside of a standard battery switch capable of switching two battery banks, you’ll find three connections that need to be made.  The lug on the positive (red) cable from battery No. 1 attaches to the terminal marked 1. The positive (red) cable from battery No. 2 likewise attaches to terminal 2. The third lug on the back of the switch is reserved for the power out cable. Depending upon how your boat is set up, this power out lead could run to a positive bus bar, your electrical panel, and possibly to your engine’s starter motor as well.

So, with your new battery securely in place and a handy battery switch installed that allows you to control the power in and out of each battery, you’re all set. It will only take a weekend to free you forever from that dread of turning that engine key only to hear a disappointing click. We promise this addition will help you get your boat away from the dock more often and you’ll quickly begin to discover many more of the hundreds of wonderful places that only your sailboat can take you.

Suggested Reading:

Battery Bank Design by Kevin Jeffrey

Trouble Shooting Your Electrical System by Tom Wood

Standard On Board Charging Systems by Tom Wood

SailNet Store Section: Battery System Components

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