Estimation and Installation of the required Battery Capacity
The electrical power supply on board of small sailing vessels should be planned carefully in advance of it's installation. Later changes especially capacity enhancements will often result in serious installation and reliability problems.
Estimating the Energy Requirements
To plan the battery capacity to be installed, the supply requirements have to be estimated for different situations. On a 3-week transatlantic journey, the electrical supply requirements will be different from the requirements during a 3-week anchoring period.
During anchoring the auto-pilot or GPS receiver will not be used, whereas under sails, the required energy for the cabin lights may be lower, depending on how the crew is spending their "off-guard" and "stand-by" periods.
The following scheme should be elaborated for each different situation (journey under sails, anchoring periods, ...) to determine the worst case daily energy requirements:
Determining the Battery Capacity
For longer journeys without the possibility of recharging the battery from an external (mainland) power supply, the generated energy should be higher than the consumed energy. This requires on-board energy sources such as the alternator (driven by the in-board engine), solar panels or wind generators!
To provide for some days of battery backup (days with no energy input) multiply the daily required energy with the number of backup days:
Capacityeff [Ah] = backup-days [days] x daily-required-energy [Ah/day]
This is the required energy that must be made available for on-board usage. The installed battery capacity however, must be chosen about 50% higher:
Capacitybat [Ah] = Capacityeff [Ah] x 2.0The factor 2.0 accounts for two effects:
The energy density of lead-acid batteries is about 35Wh/kg. For a 12V battery, each Ah of energy results in a battery weight of 0.3kg. An installation with daily energy requirement of 180Ah and 2 days of backup will result in 160kg of lead-acid batteries with 540Ah installed capacity supplying 360Ah effective battery capacity.
Typical cost for a wet-cell lead-acid battery is 1.5 Euro/Ah. So the total cost for such a battery system will be about 800 Euro. An equivalent AGM-system will cost about 50% more (2.5 Euro/Ah).
Assembling the required Battery Capacity
The required battery capacity usually can only be obtained by assembling commercially available batteries in a parallel operation. The capacity of the parallel battery pack is the sum of the capacities of the individual batteries.
When connecting batteries in parallel, old and new batteries should never be mixed and only identical types of battery should be used (same voltage, same capacity, same type and manufacturer). Cable lengths should be kept as short possible and the cable size should be large enough to avoid a measurable voltage drop (less than 0.2V) between the batteries. As a simple rule a cable size of 0.05mm2 per effective installed Ah capacity should be used (e.g. 25mm2 cable size for 540-Ah installed capacity).
Large battery capacities are not very practicable due to the weight and volume of the required lead-acid batteries. Therefore another approach would be to rely on the alternator of the in-board engine to recharge the batteries when no line power from the mainland is available. However the loading of lead-acid batteries takes time. Fast charging with excessive bulk charge currents, will damage lead-acid batteries.
So if the in-board engine should not run for hours a day just to recharge the batteries, the only alternative is to reduce the energy requirements for the autarchic periods on sea. For these periods, the electrical energy supply must be managed in the same way as water and food supplies are managed. Obviously, solar panels or wind generators can substantially improve the energy situation and thus the quality of life on board.
The in-board boat engine for sailing yachts, normally comes with a starting battery and corresponding alternator as a complete unit. The ship builder has - or should have - taken care of special installation aspects concerning safety, maintenance and the special maritime environment in which this unit is operating.
The situation for the on-board power supply batteries is usually different. Traditionally leisure sailing yachts are equipped with a rather low battery capacitance for the on-board power supply. In many cases the battery capacitance will have to be increased substantially to obtain a "blue-water"-suited sailing vessel.
Since the required battery capacity may finally result in a large weight of batteries, special attention on the installation of those batteries should be paid.
Considering the total weight of the required batteries, they should be installed as low as possible in the vessel and they should be tightly fixed! The best position would be in the bilge, just above the keel provided this is a dry place!
An important aspect is what happens to the batteries if the vessel is flooding. Safety equipment such as the radio system should remain operable to broadcast emergency calls. Also the electric bilge pump should remain under power to work under such situations. So the installation of the batteries should be such that the operation of the on-board power supply system remains fully intact even in emergency situations. This can be obtained e.g. by mounting them in an extra compartment, which should be watertight protecting the system from bilge water but which must also provide the required amount of ventilation.
The following gives a rough estimation of the expected maximum (average) current in the power circuit supposing a full charge or discharge in not less than 10 hours:
I10h = effective-installed-battery-capacity [Ah] / 10h
The cable size for the connection between the individual batteries, the alternator and the battery charger should be dimensioned as follows (provided these connections are not longer than 2m!):
Acable = 0.50 [mm2/A] * I10h [A]
Copper cables have a specific resistance of ρ=0.018 Ohm/m/mm2. The the resistance of a 10m cable connection with a size of 18.0mm2 is 0.01 Ohm. Making a closed circuit including the battery charger over this distance results in a total cable resistance of 0.02 Ohm, yielding a voltage drop of 0.72V (6%) at 36 Ampere charging current.
This is intolerable, and illustrates the importance of short and oversized cables for connecting the batteries with the alternator and the battery charger.
Advanced Battery System for YachtsThe battery system on a typical 40-ft blue-water yacht could consist of the following components:
The heavy-duty service battery will have the largest capacity and will typically consist of two identical batteries connected in parallel. A battery charger with 3 independent outputs will be used for recharging this system. The negative poles of the batteries are connected to the ground rail through a shunt (one for each battery system) Positive poles are NOT connected together. Each battery system operates as an independent system.
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