The boat owner has two main choices:- one 48 volt battery bank for propulsion and to power the boat's house loads via DC to DC converters or a 48V bank plus a separate lower voltage battery system dedicated to running house loads. Read more about the differences below.
48V DC BATTERY ONLY
48V DC Battery Only: pros and cons
The philosophy of a single 48V battery bank is that all your energy is is one source . This allows the boat owner to choose where it is needed most. The electrical designer can then size the battery bank to allow for projected propulsion loads plus house loads such as refrigeration, lighting, navigation, entertainment appliances etc. This way the boat owner, if they choose, can reduce house consumption in order to have extra range on propulsion or the other way around. If the requirement for motoring is low, power consumption in the house can be more liberal.
This arrangement simplifies the electrical circuit with only one Battery Management System required as all charge inputs are 48VDC.
To power the lower voltage electrics, DC to DC converters are used. Converters of sufficient wattage to power all the projected loads, including peaks, are needed. Alternatively, individual converters onto individual loads ie:- water pumps, fridges, radios and instruments may be used. Multiple units can build in redundancy. The converters can be close to the item they are powering; this shortens the cable run on the low voltage side, reducing the voltage drop. Efficiency loss through the DC-DC converters of about 12% must be factored in when designing the boat's house consumption to charging input requirements.
A combination of a larger unit and individual units can be employed and Victron DC-DC converters can be run in parallel to increase wattage.
The Victron brand converters can be remotely controlled and only turned on when necessary. An example of this is the pressure switch on a water pump activating the converter which in turn starts the pump. This reduces standby power drains.
Short duration high current load motors such as an anchor winch can be controlled via a DC motor controller. This eliminates the need for a separate battery and large cables to handle the higher currents that lower voltages require.
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Having only one battery can provide weight and space savings which are important to some. The cost of DC -DC converters and possible motor controller for an anchor winch may exceed the cost of an extra battery. The owner will have to decide his priorities of cost versus weight and space and advise his electrician accordingly.
The single battery system does require the boat owner to monitor the system and ensure they have enough power in the bank to cover any predicted propulsion needs. To assist them, the BMS can be designed to turn off non essential loads once a predetermined state of charge is reached and alert the crew to their power consumption state.
This system is most often the best choice for yachts with modest electrics and I use this system on my own boat to good advantage.
48V DC BATTERY PLUS HOUSE BATTERY
48V DC battery plus dedicated 12/24V house battery pros and cons
The reasons for having 48V battery bank plus a dedicated lower house voltage battery bank are many.
Some owners prefer to have the 48V battery there only for propulsion and use the house bank only for house loads knowing they can't drain the propulsion bank should there be a mistake managing the house consumption.
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If, in a refit, the 12 or 24v system is already there and it is easier to keep it.
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The 48V propulsion bank is of minimum size and by adding 12/24V house bank it is a way of increasing the boats energy storage.
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Rules for racing or commercial use may require a seperate supply.
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Whichever reason it is, the extra battery has to be charged and maintained and there are several options available.
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The extra battery will need its own power meter for state of charge and If the battery is lithium then another BMS (Battery Management System) will be required. A battery isolator and necessary current protection in the way of fuses or circuit breakers will also be required.
A DC-DC converter can be used as a battery charger from the 48V battery to keep the house battery full. A system can be designed so that it will only charge the house battery until a predetermined state of charge is reached in the 48V battery (say 80%) before turning off. This ensures the propulsion battery never falls below a certain point due to house load drain.
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For example; when the 48V bank is fully charged and the house bank requires power, the DC- DC converter charges the house battery. Once the 48V battery falls below 80%, the control system will turn off the DC-DC charger. When it rises above 80% again, the 48V bank will resume charging the house bank. The house bank will rely on its own storage capacity until the 48V battery has excess power to spare. Assuming the power source, loads and mode of use of the craft (day tripping, prolonged cruising or racing) have been correctly accounted for, this should keep the house battery near full capacity all the time.
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There is an efficiency loss through the DC-DC converters of about 12% and this must be factored in when designing the boats house consumption to charging input requirements.
Solar panel systems can be split with some panels charging the 48V bank via a regulator and the remaining panels charging the house battery via its own regulator. This adds some redundency into the system but means the ability to replenish the 48V battery has been reduced.
This shortcoming can be overcome with more complication. The Victron regulators can sense which voltage they have been connected to. A double pole changeover switch can be installed in the power line of the regulator to each battery bank so you can select which battery bank you wish to charge.
A designated mains charger can be added to charge the house bank and can be used when mains are available. Boats with a 48V generator may chose, while the generator is running, the to run the mains charger off the inverter and take advantage of power while the generator is in operation. This is not the most efficient way, as for the inverter to convert to mains 240V AC there is about a 10% loss. Then the charger to battery may inncur a further 10% loss. Another way may be to add or increase the DC-DC convert wattage taking the 48V DC output directly from the generator or 48V mains charger output to the house battery.
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The 48V DC generator may have a 12 or 24 Volt alternator as part of its system which may be able to be used to charge house bank.
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As you can see, the options are many, and the reasons for using one method over another is for the owner to choose depending on how they wish to operate the boat.
WHICH SYSTEM IS BEST FOR YOU
The boat owner needs to work out the boat's total house loads, their consumption and how they intend to use the boat, then factor this into the system.
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If you have already decided which system is preferred, single or dual batteries, then it is a matter of adding up the house loads plus propulsion load and ensuring the charging inputs will replenish the power used in the most practical and cost effective way.
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If you wish to make a decision based on technical information, again you need to know the total house loads and consumption, propulsion power prediction and intended use. Then you, or your electrician, can put these parameters into both systems and see which one comes out best for you in terms of cost, weight, practicality of installation and ease of management.
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Just as no two boats are the same nor will be the electrical systems. Similar boats with different owners, uses and expectations can require very different systems. Even the choice of primary power source will vary depending on where you are in the world. Solar, wind, mains power and generators all have their place in the varied world of sailing. All of these differences is what makes boating interesting.
Betts Boat Electrics can be contracted for boats in Australia to do a technical assessment of the pros and cons of each method for your boat. There is no right or wrong way. The best way is the way with which way the owner feels most comfortable.
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Written by Mal Betts
Betts Boat Electrics
21/10/2018
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