DCPrimarySystem
The DCDirect current Primary System
12 or 24 Volts
AAmpere (amp), SI unit of electrical current boat'sSecond primary electrical system is DC. In a small or medium size of boat, 48 VDCVolts direct current is uncommon, so the choice lies between 12 or 24 VDC. Generally, 24 VDC is preferable but some equipment may be available only in 12 VDC.
24-VDC is better because for a given amount of energy consumption in Watts, it requires fewer amps running through the wires than a 12-VDC system. The lower the voltage the higher the current (Amps = Watts/Volts).[1] For example, a 100-WWatt device draws approximately 8.3 A at 12 VVolt and 4.2 A at 24 V.
Higher current has disadvantages. The higher the current draw, the thicker the wiring required (and the bigger the spark when you accidentally ground it). Thick wiring is more expensive and hard to install and maintain – think jumper cables for boosting your car.
Higher voltages are also feasible. Large ships have long used a higher voltage DC bus. Some smaller yachts have used 150 VDC. Some of the advantages this yields are smaller wiring, more efficient thrusters and windlasses, and compatibility with most shore power around the world and existing resistive devices like stoves.
Wires also have resistance and, when a current passes through them, this causes a voltage drop along the length of the wire. The higher the current, the higher the drop in voltage. This voltage drop limits the practical length of a wire. The maximum run for 12-VDC wiring is around 30-35 ftFoot, which translates into a boat length of around 50 ft, given a midships battery, and the need to run wires around corners.[2][3]
Also, for various reasons the efficiency of DC-ACAlternating current inverter circuits is better the higher the DC input voltage. Therefore, a 24- or even 48-VDC system is better than a 12-V one. As mentioned, the downside to 24 V is the wider range of equipment available for 12 V.
To accommodate both 12- and 24-VDC equipment, it is possible to design a battery system for 12/24, in somewhat the same way that North American houses have 110/220 VACVolt alternating current. This system would use high-amperage blocking diodes to split the two voltage circuits.
A better approach, if you select a 24-VDC system and some equipment is available only in 12 VDC, is to use individual and inexpensive 24-12 VDC, solid-state, low-noise controllers with voltage regulation. Obviously you should keep some spares on board. Using the individual controllers eliminates the need for an extra wiring system, complexity in the house bank, and dependence on a single set of high-performance diodes.
As you iterate the design to assess total system performance you may hit a wall with lower voltages. For example, above a certain capacity, watermakers are 24 VDC. The key differentiating factor here is the number of people on board. More people need more water; hence a larger capacity watermaker.
The trend in the market is to 24 VDC, so that should be your first choice, anyway. However, if your boat is less than 50 ft, 12 VDC is probably still your best price-performance option.- ↑ https://en.wikipedia.org/wiki/Ohm's_law
- ↑ www.computrols.com/file_download/.../Calculating-Wire-Resistance.pdf
- ↑ http://www.advanced-energy.com/upload/File/White_Papers/ENG-24VDCInstallation-260-01.pdf
