Boats with more than one battery – Various systems exist for ensuring that an engine start battery is never flattened, but most follow one of two arrangements. Either the engine battery is a dedicated unit that is never used for anything other than starting the engine, with domestic (‘house’) batteries fulfilling all other functions, or the boat has two battery sources which can be interchanged at will. • Dedicated engine batteries are often smaller than the ship’s domestic supplies. They are typically switched on and off independently. If they’re off, the engine won’t start. With most dedicated systems, the charging relays or diodes are arranged so that as soon as the engine fires up, the alternator charges the start battery first. This ensures that it is always ready for next time. As it approaches peak capacity, charge is progressively spilled over into the domestic batteries. With such a system, you don’t have any thinking to do. Just make sure that all batteries are ‘on’ and fire up the engine. So long as the charging system is working, you’re in business. Dedicated batteries should be wired up so that in the unlikely event of the engine unit going flat, the domestic banks can be switched in to start her up. Such switches are rarely tested and have been known to fail. Hence the jump leads…
• Interchangeable batteries are often of equal capacities and controlled by a 4-way switch offering ‘Battery 1’, ‘Battery 2’, ‘Both’ or ‘Off’. What you elect to do with these options is a matter of personal choice, but you must decide whether to use one battery as ‘engine start’ and stick to it, or whether to change them over from time to time to share the hammering dished out by domestic demands. In either case, make sure that ‘Both’ is selected while the engine is actually running, so as to charge everything. If one battery is too flat to start the engine, it’s generally better to use the other on its own rather than opt for ‘Both’, because the dead one may compromise its opposite number.
Power consumption
The basic principles of electrical power storage and consumption are an integral part of managing a yacht or motor cruiser. The simplest way to think of this is to imagine a battery as a storage dump containing units called ‘amp hours’. An electrical consumer such as a light bulb, a fridge or a GPS receiver uses flowing current, which is measured in amps. If it pulls five amps, then in one hour it will use five amp hours. Its demand, or capacity, is expressed in ‘watts’, and the power to shove the amps down the wires to feed it is given in volts. The essence of power management lies in understanding the relationship between these three elements. Fortunately, the arithmetic is as simple as it gets. Here it is: Watts = Volts x Amps
It follows that Amps = Watts ÷ Volts. Combined isolator and selector switch
This means that a 25-watt bulb such as is used for a masthead tricolour lamp in a 12-volt system draws 25÷12 Amps — more or less 2 amps. In one hour, it consumes 2 amp hours.
No battery can really deliver as much as imanufacturers’ claim, and a 110 amp-hour battery’s capacity may be as limited as 65 amps in typical use on a sailing craft. This means that the tricolour will flatten it in just over 30 hours. Now add up everything else that is drawing power in the same way, and you can work out what’s required.
Key type isolator switch
38 | MANUAL OF SEAMANSHIP
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