Charging A Boat With Dual Batteries: The Complete Guide

Charging a boat with two batteries can be done in several ways, depending on the boat's configuration and your personal preferences. The traditional method involves using a battery switch to select one or both batteries for charging, but this requires manual intervention and can be prone to user error. More modern approaches include using split-charge diodes, twin alternators, or voltage-sensitive relays (VSRs) to manage the charging process. Additionally, the type of charger you use, such as onboard or portable, and the way the batteries are connected (in series or parallel) will also impact the charging process.

Charging two batteries in parallel

• Prepare the Batteries: Ensure both batteries have similar capacities and are at a similar state of charge to prevent charging imbalances. Use batteries of the same type (e.g., lead-acid) to ensure compatibility.
• Connecting the Batteries: Use high-quality cables to connect the positive terminal of the first battery to the positive terminal of the second battery. Repeat this step for the negative terminals. Ensure all connections are tight and secure to prevent sparks or poor charging efficiency.
• Attach the Charger: Connect the positive lead of the charger to the positive terminal of one battery, and the negative lead to the negative terminal of the other battery. Ensure the charger's capacity matches or exceeds the combined capacity of the batteries.
• Begin the Charging Process: Double-check all connections, then start the charging process. Monitor the charging closely, especially during the first charge.
• Monitoring and Maintenance: Regularly check the voltage of each battery with a multimeter to ensure they are charging evenly and within safe limits. Inspect terminals and cables periodically for signs of corrosion or damage. Always wear protective gear, such as gloves and safety glasses, and ensure the area is well-ventilated, especially when charging lead-acid batteries.

Note: When charging two batteries in parallel, it is important to use batteries of the same type, capacity, and age to prevent imbalances that could lead to overcharging or undercharging.

Onboard vs. portable marine chargers

Onboard and portable marine chargers are the two types of boat chargers. The choice between the two depends on factors such as boating habits, frequency of use, and budget. Here is a detailed comparison between the two:

Onboard Marine Chargers

Onboard marine chargers are permanently installed on the boat and connected directly to its electrical system. They offer automatic charging by continuously monitoring the battery's condition and adjusting the charging rate. This eliminates the need to carry and connect a portable charger each time. Onboard chargers are recommended for boats primarily used near shore power sources or marinas.

Onboard charging methods include:

• Shore Power Charging: This is the most common method when docked at a marina. It involves using a battery charger designed for marine use to charge batteries from an external power source.
• Offshore Charging: This method uses onboard generators or alternative power generation methods, such as solar power systems, to charge the boat's batteries away from shore power.
• Alternator Charging: The boat's engine alternator can charge the batteries while on the water. However, it primarily maintains the starting battery's charge and may not provide a complete deep cycle battery recharge.

Portable Marine Chargers

Portable marine chargers are compact and versatile. They can be carried on board or used on land to charge batteries when docked. These chargers come in various models, including manual, automatic, and smart chargers, making them suitable for different battery types.

Portable chargers are preferred for boating scenarios that require flexibility and mobility. They are crucial when frequently far from shore power or marinas, enabling recharging wherever you anchor. They are also cost-effective compared to installing onboard systems.

Factors to Consider

When choosing between onboard and portable chargers, consider the following:

• Boating Habits: If you boat near shore power sources or marinas, an onboard charger is more convenient. If you boat in remote locations, a portable charger provides the flexibility to recharge wherever you are.
• Frequency of Use: If you boat occasionally, a portable charger may be more cost-effective than installing an onboard system. If you boat frequently, the convenience of an onboard charger may be preferable.
• Budget: Onboard chargers typically require a higher initial investment, while portable chargers can be more budget-friendly.
• Battery Type: Consider the type of marine batteries you have, as different batteries have specific charging requirements. Ensure that the charger is compatible with your battery type to avoid damage.

Battery switches

There are two main varieties of battery switches: single and dual. Single battery switches have two positions: OFF and ON. Dual battery switches have four positions: OFF, 1, BOTH, and 2. This allows you to select which battery you want to use, or to use both batteries together. Most boats have a dual battery setup, with two identical general-purpose batteries, so a dual battery switch is the most common type.

When choosing a battery switch, it's important to consider the amperage ratings, which indicate how much current can safely flow through the switch. The ratings are given for both continuous and intermittent use. For example, a switch with a 100A rating can handle a load of 100 amps for an extended period of time, while a switch with a 450A rating can handle higher current flows for shorter periods.

There are several options for dual battery setups:

• Battery switches: This traditional method is simple and inexpensive but relies on the user remembering to switch between batteries.
• Split-charge diodes: This method isolates and charges two battery banks independently but may not fully charge the batteries due to voltage drop.
• Twin alternators: This provides a totally independent means of charging each battery and built-in redundancy but requires sufficient engine power.
• Voltage-sensitive relays (VSRs): These devices keep battery banks isolated when not charging but combine them when a charging voltage is detected. This method is simple and reduces the chance of human error.

Some popular battery switches include:

• Blue Sea Systems e-Series Battery Switch: A 4-position selector switch.
• Seachoice Battery Select Switch: A dual 12-50V switch with a disconnect isolator cutoff.
• Blue Sea Systems m-Series Battery Switch: An ON/OFF switch with a knob, available in red.
• Perko 8501DP Medium Duty Battery Selector Switch: A small, red switch.
• Attwood 14230-3 Single Pole 4-Way Selector Marine Battery Switch.
• Blue Sea Systems HD-Series Heavy Duty On-Off Battery Switch: Available in grey.
• BlueStars 350A e-Series Battery Switch: A 4-position selector switch for boats, ships, and yachts.

Remember to always follow safety precautions when working with batteries and consult experts if you're unsure about your boat's electrical system.

Split-charge diodes

A diode allows current to flow in only one direction. Most split-charge diodes contain two separate diodes that, when connected to an alternator, split the charge in two and deliver it to two separate batteries. This means the batteries are totally isolated from each other.

However, diodes create a volt drop of up to 1V as the current passes through, which means the batteries won't receive the alternator's full charge. This can be overcome by using a battery-sensed alternator, which will enable the batteries to charge fully.

It's also a good idea to fit a switch so you can combine the batteries in an emergency. Without this, they will be completely separated and you won't be able to start the engine using the house battery.

Voltage-sensitive relays

When a charging voltage is detected on the start battery, some VSRs will immediately combine the house and start batteries to charge both, while others will wait until a preset voltage is reached at the start battery before combining them. This preset voltage is usually around 13V. Once combined, if one battery is less charged than the other, it will take a greater share of the charge until they are equal.

When the engine is switched off and the voltage falls, the VSR will isolate the batteries once again. Some VSRs also include a ‘low-voltage cut-out’ feature which will prevent the VSR from combining the batteries if either of them is at an unacceptably low charge level.

VSRs are particularly useful when a small inboard or outboard engine is used, which has combined starter motor and alternator wires to the battery. In this case, a diode system will only let the current flow one way, whereas a VSR will allow both charging and starting with only one set of wiring.

VSRs are a simple solution that takes human error out of the equation. However, if one battery is past its best or has a faulty cell, the other may not charge fully.

Frequently asked questions