Charging Two Boat Batteries: Stance And Setup For Success

Charging two boat batteries simultaneously is a complex process that requires careful consideration to avoid costly mistakes. The traditional method involves a battery switch that allows users to select one or both batteries. However, this approach relies on the user's memory and can lead to both batteries being drained if forgotten. More advanced methods, such as twin alternators, split-charge diodes, and voltage-sensitive relays (VSRs), offer improved convenience and safety but may be technically challenging to install. Proper crimping, tinned marine-spec wiring, and correct gauge wires are essential for a safe and effective setup.

Battery switches: A cheap and simple solution, but relies on the user to switch between batteries

Battery switches are a cheap and simple solution for charging two boat batteries. They are a traditional way to switch between batteries, allowing the user to select one or both batteries. This method has been used for decades and is a straightforward process.

Battery switches are connected to large cables that carry the high electrical current produced by boat batteries. They are an essential component for every boat as they disconnect the engine and standard loads from the boat's batteries, preventing battery drain while in storage.

The two main types of battery switches are single and dual. Single battery switches have two positions: OFF and ON. Dual battery switches, the more popular option, have four positions: OFF, 1, BOTH, and 2. This allows the user to select any combination, including having both batteries connected simultaneously.

When charging two batteries with a dual battery switch, it is important to remember to switch it to the correct battery. This means switching to one battery to start the engine and then back to the house battery when the engine is turned off. While the 'both' option is useful in combining the batteries, it carries the risk of forgetting to switch back to the house battery, resulting in two flat batteries.

Battery switches are a straightforward and affordable option for charging two boat batteries. However, they rely on the user to remember to switch between batteries correctly, which can be a drawback if not done properly.

Split-charge diodes: A 'fit and forget' way to isolate and charge two battery banks, but may result in batteries not being fully charged

Split-charge diodes have been used to charge two battery banks since the 1960s. They are a 'fit and forget' solution, allowing you to charge two battery banks independently and in isolation from each other.

A diode is a device that 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 and deliver it to two separate batteries.

However, this method is not without its drawbacks. 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 and may never become fully charged. This problem can be mitigated by using a battery-sensed alternator, which will enable the batteries to charge more efficiently and reach a full charge.

Additionally, it is advisable to install a switch that combines the batteries in an emergency. Without this switch, the batteries will be completely separated, and you won't be able to start the engine using the house battery.

Twin alternators: Provide a totally independent means of charging each battery, but require sufficient engine power

Charging two batteries on a boat is a common practice, as it allows you to run various electrical equipment without worrying about not being able to start the engine the next morning. One way to do this is by using twin alternators, which provide a totally independent means of charging each battery and offer built-in redundancy in case of a battery or alternator failure.

When considering twin alternators, it's important to ensure your engine has sufficient power to accommodate the additional load. The engine should be able to handle another alternator, and be powerful enough to manage the extra load. This setup can be advantageous as it provides totally isolated battery banks, meaning you won't accidentally discharge the start battery.

For example, PBO's editor, David Pugh, installed an extra 10A alternator on his 13hp Lombardini engine. This setup completely separates the start and house batteries and their circuits, but they can be combined in case of failure using a switch. This also ensures that each battery receives the right amount of charge.

In another instance, a boat owner with twin diesel engines, a starting battery for each engine, and a house battery bank, encountered issues with their battery isolator. They had to seek advice on whether to use two isolators or a single one that could handle two alternators. This highlights the importance of proper setup and compatibility when dealing with twin alternators.

When using twin alternators, it's essential to consult experts or professionals to ensure the correct configuration for your specific boat and engine setup.

VSRs/ACRs: Voltage-sensitive relays keep battery banks isolated when not charging, but connect them when charge is available

VSRs, or Voltage-Sensitive Relays, are a type of charge management device used on boats to control the charging of two battery banks. They are also known as ACRs (Automatic Charging Relays) and work by using a relay combined with a circuit that senses when a charging source is being applied to either battery. When a charge is detected on the start battery, the VSR combines the house and start batteries to charge both. 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 isolates the batteries once again.

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.

When choosing a VSR, it is important to consider current management, remote control, and manual override options. Some VSRs can also be configured for start isolation, which provides temporary isolation of house loads from the engine circuit during engine cranking, protecting sensitive electronics from voltage spikes.

It is also important to note that VSRs should only be used if the batteries have similar chemistries. Mixing battery types, such as AGM and calcium batteries, can shorten their lives and lead to poor performance.

Battery isolators: An older method that uses diodes to allow charge to flow to batteries, but not between them

Battery isolators are an older method for charging two boat batteries. They are simple devices that allow one charging source to send a charge to multiple batteries while preventing current from flowing back. They are "one-way check valves" that allow current to flow to the batteries but not between them.

Battery isolators use diodes or relays to cut the batteries off from each other and prevent them from discharging at different rates. When a battery isolator is used, each battery can be charged and used separately from the others in the system.

However, one disadvantage of battery isolators is that diodes cause a voltage drop, which wastes charging energy, creates heat, and causes batteries to be undercharged. This problem can be corrected by using alternators with external voltage sensing. Nevertheless, the voltage drop and heat generation remain issues.

Battery isolators are also less effective than battery separators, which are more sophisticated gadgets with embedded microprocessors that monitor the batteries and distribute the charging current as required. Battery separators can determine which battery needs the most charge and only charge that one, making them more efficient and precise. They are also better suited for modern battery technologies like AGM and lithium-ion batteries due to their exact charging requirements.

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