Battery Basics

Most large boats and many smaller ones are equipped with more than one battery (or battery banks) so that the operator can control how the electrical power is being used and - more importantly - how it's conserved. Some boats are equipped with a separate battery for each engine system. Others have a "main" battery for starting an engine and a separate "house" battery for onboard lights and appliances with a switch that can be used to connect them together in case of an emergency. Others may use a complex combination of two starting systems and a house battery. Whatever type of system is used, its sole purpose should be to ensure that adequate battery power is always in reserve.

There's no reason for owners of boats to not have this feature. Additional battery capacity and connective wiring can be easily installed on most boats. If your needs require a complex system, or you aren't comfortable with electrical modification, most dealers and boatyards will be happy to do the work for you. But installing a separate reserve or house battery along with a battery switch and wiring is a do-it-yourself project that most boaters can do with normal hand tools and a bit of preparation. In its most basic form, the resulting system can be just two batteries and a switch that allow you to use either or both, or shut everything down completely. 

Plan, Plan, Plan

The minimum material list for installing a reserve system includes a three-way battery switch, a battery, a battery box with cover, and two or more new battery cables. Like any do-it-yourself project, it makes sense to spend as much or more time planning than actually doing the job. Scope out the requirements of the task before you start. Trace the path of existing battery cables to understand how the system is wired. Record the size of the cable - usually marked on the insulating jacket - so that you can match it exactly when ordering the new cable for installation. Look for locations for mounting the extra battery and a three-way selector switch, keeping in mind that battery cables should be kept as short as possible to maintain electrical performance and keep costs down.

It's important for the additional battery to be mounted in an enclosure that secures it firmly to the boat. This provides a cover to prevent metal objects from coming in contact with its terminals and causing a short circuit. Placing it in a convenient location will make installation easier and will be helpful during maintenance time. The new battery must also be made of the same construction as the existing one, whether it's a standard lead-acid, absorbed glass mat (AGM) or a gel type. Different battery types have specific and different charging requirements, and shouldn't be mixed. If you're using the new battery for house purposes only, you should buy a deep-cycle type, but if you'll be using it occasionally for starting, you should consider buying a dual-purpose or starting battery, since high-current draws can do damage to a deep-cycle battery when starting the boat.

An optional component for installation is an automatic charging relay or battery isolator, two names for similar, solid-state devices that allow all batteries to be charged by the alternator, regardless of how the battery switch is set. Without one, only the batteries that are currently selected at the switch will be charged. If the battery selector switch is in the ALL, BOTH or 1+2 position (depending on how the switch is marked), both batteries will be charged, but if only battery ONE is selected, battery TWO won't be charged. An ACR or battery isolator also protects the boat's alternator from possible damage if the battery switch is operated with the engine running. Marine equipment catalogs and manufacturers' websites provide a wealth of in-depth information about the selection of battery switches and isolators to help you choose correctly.

Take The Plunge

Begin by disconnecting the existing battery(ies) to ensure that no inadvertent short circuits happen while you're working on the boat. Wires or cables that you physically disconnect should be marked so they can be reconnected properly.

Mount the new battery box using the strongest hardware possible. It must support a considerable weight and be able to take the strain of bouncing around in rough waters. Make sure you know how deep your mounting holes will go and what's on the other side before you begin to drill. Any holes that you drill in the boat should be filled with marine-grade silicone sealant before the screws are inserted in order to keep moisture out and lock the screws in place. If the battery must be mounted on a sloping surface, cut wedge-shaped shims from pressure-treated wood, and use them to level the box. 

If the boat already has an on/off battery switch, then the new three-way switch will likely fit in the same hole or mounting. Otherwise, the switch should be mounted in a protected spot where you can get to it easily, but not in the bilge or engine compartment where it would be inaccessible in case of a fire. Many boatyards and battery dealers can make up custom cables for your boat, but it's important to make sure that they use only marine-grade wire, which will be clearly marked.

Connect the new battery using the basic diagram shown at top left or a suitable variation from the instructions that come with the new switch. The negative connections of the batteries aren't switched, so they should be directly connected together and grounded. Remember, battery cables aren't protected by fuses or circuit breakers and carry high currents. Their lugs must be positioned correctly and wrapped with insulating tape to avoid contact with adjacent metal or each other, and their connections must be tight to avoid the potential for arcing and fire.

Battery cables should be securely tied to points along their route or bundled with other wiring to ensure that they won't move while the boat is under way. Flopping cables can cause connections to loosen up or danger of insulation becoming chafed or cut. Don't anchor wiring to the shift, steering cables or other mechanisms that may move when used.

Recheck the wiring before turning the system on for the first time. Start by turning on low-wattage devices such as lights to check the power supply from each battery switch position. Follow up with high-current uses by starting the engine. If the lights dim radically or the engine turns over slowly, you may have a loose connection somewhere. Otherwise, you're set to enjoy your boat with one less thing to worry about.

Quick Tips

> Plan tasks and check the requirements for installation.

> Trace the path of existing battery cables to understand system components.

> Don't mix the type of batteries used in the reserve installation.

> Secure cables tightly to non-moveable fixtures.

> Mount additional batteries in a securely enclosed location.

MOUNT IT Place a three-way battery switch in an accessible cubby.

FIND THE PATH Trace the path of the cables in the circuit, and connect your batteries to the switch by using this diagram for a basic two-battery system.

Caption

INSULATE IT Wrap cables correctly with marine-grade markings and lugs.

Caption

ANCHOR DOWN If you need to mount the battery on a sloping surface, cut wedge-shaped shims from pressure-treated wood, and use them to level the box.

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