Wiring A Second Battery: A Boat Owner's Guide

Installing a second battery in your boat is a simple process that can give you peace of mind. It is wise to add a second battery to your boat to ensure you have enough battery capacity in case you drain the first battery. This can be done by connecting two batteries in parallel to make one larger battery bank. You can also build in redundancy and failure protection with two battery banks, battery switches, and charge management devices.

Choosing the right battery type

Battery Purpose:

The first consideration is whether you need a starting battery, a deep-cycle battery, or a dual-purpose battery. Starting batteries provide quick bursts of energy to start the engine, while deep-cycle batteries deliver consistent power over an extended period for electronics, trolling motors, and appliances. Dual-purpose batteries combine both functions but are less efficient than specialized batteries.

Boat Size and Power Requirements:

The size and power needs of your boat will influence your battery choice. A small boat with minimal electronics will have different power requirements than a larger vessel with sophisticated systems.

Reserve Capacity:

Look for a battery with sufficient reserve capacity (RC) to power your systems in case of alternator failure. RC indicates how long a battery can provide 25 amps of power before dropping below 10.5 volts.

Cold Cranking Amps (CCA) and Marine Cranking Amps (MCA):

These ratings are crucial if you operate your boat in cold environments. CCA refers to the battery's ability to deliver amps in cold temperatures, while MCA is relevant for warmer marine environments.

Amp-Hours (Ah):

This rating measures the battery's capacity or how long it can deliver power. A higher Ah rating means the battery can provide power for a longer duration.

Battery Chemistry:

Marine batteries typically come in four different chemistries: flooded lead-acid, gel, AGM (Absorbed Glass Mat), and Lithium Iron Phosphate. Flooded lead-acid batteries are commonly used for starting applications and have a no-spill design. Gel batteries are stable and versatile, perfect for boats as they can handle jostling without issues. AGM batteries are sealed and provide excellent deep cycle ability, making them ideal for boaters seeking fast recharging and reliable power. Lithium Iron Phosphate batteries offer superior performance, a longer life cycle, and weigh significantly less than other types.

Battery Size:

The size of the battery will depend on your engine's requirements. As a general rule, factor in one amp for every cubic foot of engine space, and most boat owners will want between 300-500 CCAs for starting their boat.

Weight:

Consider the weight of the battery, as heavier batteries can increase the overall weight of your craft, impacting speed and fuel consumption.

Voltage:

For trolling, you will need a higher voltage battery, such as a 24V lithium trolling motor battery.

Safety and Maintenance:

Look for batteries with safety features such as vented compartments to release hydrogen gas during charging. Additionally, consider the maintenance requirements of different battery types. For example, flooded lead-acid batteries require periodic inspections and topping off with distilled water.

Installing a battery box

A battery box is a specialised type of protective box used for housing a marine battery on a boat. It is important to have a battery box to keep your battery safe and secure. A well-designed box will also help to maintain crucial electrical connections. The constant movement of a boat can be detrimental to an unsecured battery, and an acid spill is the last thing you need, especially in a wooden boat.

Battery boxes are usually made of durable, heavy-duty plastic, but sometimes they are made of aluminium or stainless steel. They are resistant to corrosion, chemical damage, UV radiation, and other types of damage.

Before installing a battery box, you must first choose where to mount it. There is no right or wrong spot for mounting, but you should keep it out of the way while still being accessible. You will also want to mount the box in a convenient location close to the devices your battery will be powering.

Once you have chosen a location, you can decide how to secure the box. Common ways to secure a battery box include tie-down straps, stainless steel screws, and foot clamps. If you use tie-down straps, they will most likely have clamps to tighten the straps against the box. If you use screws, be careful to choose a spot that will support the screws and not cause significant damage to the boat. If you use foot clamps, screw the clamps into the deck and snap the battery box into place.

After the box is secured, place the battery inside and wire it as necessary. Some battery boxes will have holes for the cables, while others will have external terminals and plugs. Connect your devices, chargers, etc., to terminals on the outside of the battery box or thread them through to the battery inside. Once everything is hooked up, test each device to ensure a good connection.

Connecting the positive cables

Firstly, it is important to select the appropriate cables for your setup. The cables should be marine-grade and sized according to your specific application. For most outboards and gasoline inboards, a size 2/0 cable is sufficient. These cables will be used to connect the batteries to the battery switch and the engine.

Before beginning any electrical work, it is crucial to take safety precautions. Disconnect the positive battery cables from the battery terminals to prevent the possibility of a short circuit or electrocution. Wear safety gear, such as safety glasses, and have a fire extinguisher nearby. Be cautious when working with battery acid, and ensure proper ventilation to avoid inhaling fumes.

Now, you can begin connecting the positive cables. Start by connecting the positive terminals of both batteries (Battery 1 and Battery 2) to the corresponding posts on the back of the battery switch. This will allow you to select between the batteries or combine their power output. Ensure that the connections are clean and secure, following the manufacturer's instructions for proper torque specifications.

Next, connect the positive cable from the engine to the output post of the battery switch. This connection will provide power to the engine from the selected battery. Again, ensure a clean and secure connection.

Additionally, connect any positive accessory wires to the output post of the battery switch. This includes wiring for accessories such as navigation lights, spotlights, or other electrical devices. These accessories will now draw power from the selected battery.

After all the positive connections have been made, it is important to properly manage and protect the cables. Use heat-shrink tubing or electrical tape to seal and insulate the cable connections. This will prevent moisture and corrosion, ensuring a reliable and long-lasting connection.

Finally, before completing the installation, verify that all terminal nuts are tight and secure. Consider replacing wing nuts with locking hex nuts for added security. This will prevent accidental loosening and ensure a reliable connection.

By following these steps and taking the necessary precautions, you can safely and effectively connect the positive cables when wiring a second battery into your boat. Remember to consult a professional or a detailed guide if you have any doubts or concerns during the installation process.

Adding a battery switch

Battery switches let you choose one of the two batteries, combine them, or disconnect both with the turn of a dial. When selecting a battery switch, make sure it is designed for two batteries and will handle the amperage when starting the engine. A rating of 250 amps continuous is sufficient for most outboards and gasoline inboards.

• Pick a location for the battery switch that is easy to access and complies with Coast Guard and ABYC standards. The switch should be as close to the batteries as possible and readily accessible.
• Connect the positive cables first. Connect the positive terminals of batteries 1 and 2 to the corresponding posts on the back of the switch, then connect the positive cable from the engine to the output post of the switch. Also, connect the positive accessory wire to the output post.
• Connect the negative cables last. For the dual-battery system to operate properly, you need a crossover cable between the negative terminals of the two batteries. Also, connect the negative cable lug from the engine/ground and the negative accessory wire to either of the negative battery posts, ensuring all terminal nuts are tight.
• Label your batteries clearly as "1" and "2" so you know which is which. This is important information, especially when it comes to diagnosis and maintenance.

Remember, you must never switch the battery switch while the motor is running.

Connecting the negative cables

Firstly, you will need a crossover cable between the negative terminals of the two batteries. This will ensure that the batteries can work in tandem.

Next, connect the negative cable lug from the engine/ground and the negative accessory wire to either of the negative battery posts. It is recommended to replace wing nuts with locking hex nuts to ensure a secure connection.

Make sure that all terminal nuts are tightened to complete the circuit. This will ensure that the second battery is integrated into the boat's electrical system and can be used to start the engine and power other electrical components.

It is important to follow safety guidelines when working with batteries, such as wearing safety glasses and having a water hose nearby to flush away any battery acid. Additionally, always disconnect the positive battery cables before beginning any electrical work to prevent the risk of short circuits and electrocution.

Frequently asked questions