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). 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.

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.

Bonding

Second only to discussions about one house bank or two, are discussions about bonding or not bonding the electrical circuits. The simple fact is that all electrical circuits have to have a common ‘ground’. On shore, this is often the earth. Bonding means connecting all the ground points together with an extra run of wires.

In all cases in a steel hull, the DC system must be a "floating ground" (DC negative bus) type of system, with an insulated return, fully isolated from the hull and all the hull fittings. This means that no electrical items (including common appliances) have a local ground to the hull and lamps should be double pin. Instead, all ground returns are tied to a Common Grounding Point (CCGCommon grounding point).

For example, all engine fittings are double insulated. The engine is electrically isolated from the hull via flexible mounts and flexible coupling. A grounding wire runs from the alternators to the DC negative bus. This might seem confusing, because the CCG itself is grounded to the hull. However, a CCG avoids stray electrical currents running through the hull and causing electrolysis. It also provides a grounding point for the lightening-protection system.

Having selected the voltage, the next phase in the design of the electrical system is to determine the requirements for the DC battery primary system – the house bank.

Charging Systems

Alternator

Each engine (if there is more than one) will have a high-capacity dual-output alternator and multistage regulator, with separate charging circuits for the starter and house batteries. A backup manual switch and regulator are provided. The regulator must be suited to the type of battery: Flooded cells require an equalization charge after the main charge; whereas gel and AGMAbsorption glass mat cells usually do not. Typical vendors are: Ample Power, Balmar, Hehr Power Systems, JackRabbit Marine and Sea Air Land Technologies, Inc.

If the boat will be unattended for periods at least one engine must autostart on a schedule to keep the batteries charged.

Trickle Charge System

In case the main charging system fails while the boat is unattended, a DC trickle-charge system can be provided. Trickle charging is also a good idea because there are usually parasitic loads on a battery system that will slowly discharge it. Deep discharge batteries do not want to be trickle charged at a high rate: 3%percent is recommended. Thus a boat with a house bank of 1000 AHAmpere-hours (A*H) requires a trickle charge of 30 AH.

Wind turbines and solar panels are ideal for a trickle-charge system; although they are not suited as a main power source. Unfortunately, as a main power source, each of them has a significant performance drawback in the context of a small- to medium-size boat. They simply need too much real estate.

Shore Power Charger

See ACSecondarySystems#Inverter/Charger.

Distribution

Controller

The controller for engine charging and inverter/charger is a single point of failure, so a back-up controller or a bypass system should be provided.

Distribution Panel

For easy access the AC/DC distribution panel is located in the pilothouse. Remote latching relays disconnect the batteries in the event of an electrical fire.