Thoughts on Electricity on Board

By Bob Humphreys
Westerly Fulmar, Cajun Gold

An American philosopher once said: ‘A lot of things have happened in the 20th century, and a great many of them plug into three pin sockets on the wall’.  A yacht has access to many of these things – or does it?  To benefit from the likes of plotters, fridges, heating and lighting, and cold beer, you need quite a lot of power.  That may not be a problem in the marina where you have abundant 240v from shore power.  But what about when you are on the move?

Most rely on 12V from the reliable supply from lead-acid batteries. A normal 12V yacht battery can provide you with around 100 amps for an hour or so. That is around 1.2kWh. Put another way, this is one bar electric heater for an hour or a 100W yacht fridge for a day or so without charging.

The alternator on the engine is there to charge your batteries when the engine is running. But it will probably be the kind that you find on a car engine and will not be designed to fully charge your batteries. An ‘Adverc’ controller connected to its output controls the alternator which can then fully charge both your engine and domestic batteries.

Care of your batteries

The 12V batteries will run all the usual D.C. equipment; instruments, lights and so on, but care needs to be taken not to over discharge them. There is a limit to how many times you can do this; every time this happens their capacity is lessened. Never let their voltage fall to 11.2V if you want to avoid this happening. Batteries should never be left in a discharged state for long periods.  This causes hardening of the soft lead-peroxide, which is converted into lead-sulphate, causing a reduction in plate area and loss of power capacity.  This will also happen if the acid level falls and the top of the plates are exposed to air.

Shore power

Figure 1 – Incoming shore power

Shore power can be used to run both run a battery charger and to feed the 240v sockets. I used to fit Victron chargers. Their charging output is controlled by the state of charge of the batteries, and it is safe to leave everything connected and working, you won’t overcharge them. There are separate charging circuits for the domestic batteries and the engine start battery on ours, (which is the blue unit) and is rated at 20A. As a guide, a battery should be charged at a maximum rate of 10% of its ampere hour capacity. Some sources say it can be at  up to 20% but, in my view, a 100AH battery should not be charged at more than 10 amps.  Gel batteries can be charged at a much higher rate, but they need a specific design of charger; if over-charged their capacity is immediately reduced.

The 240v supply needs the same safety measures on a yacht as in a house. The incoming shore power lead needs to first feed an RCD (residual contact device). As long as the current flowing in the live and neutral wires is the same it won’t trip. But if a fault causes an imbalance, then it will break the supply.

 After this you need fused outputs. These take the form of MCBs, miniature contact breakers, i.e. re-settable overload fuses.

Figure 2 Installation in the boat

The incoming mains lead is terminated in the white round junction box (see figure 2). From here it is fed to the RCD (the white rectangular box on the left) and then to a dual MCB unit – the box next to it on the right.  The left hand MCB feeds the square black junction box on the far right. One output from here feeds the dual 13A socket and another feeds the battery charger via a single socket. 

Galvanic Isolator

Any shore power arrangement should include either an isolation transformer (heavy and expensive) or a galvanic isolator, to stop stray earth currents corroding exposed metal on the hull. With an isolation transformer there is no ‘copper’ connection to the shore power supply; it is made through the magnetic field of the transformer. Our galvanic isolator is the unit on the far left on the main diagram. The incoming earth wire goes straight to it, and its other terminal has a wire connected to the main earth point on the yacht.

Inverter power

The inverter converts low voltage 12V into 240v. Power into it is roughly the same as the power out, they are very efficient. This means that the input wires to the inverter need to be sized for the maximum current of our unit, in this case 125A, so they are very thick and incorporate a fuse in the positive one. These are the red and black wires that you can see here. The output of the inverter is used to run the Sony chart plotter, and if needed provide power to re-charge mobile phones, run a TV and so on. Although it can supply a considerable amount of a.c. power, this would quickly flatten the batteries and therefore needs to be used carefully. Never-the-less it is a very useful bit of kit to have on board.

Figure 3 Power sockets

The lefthand dual socket (see Figure 3) is the one which is live when shore power is available, but if we need 240V when underway – the right hand one is supplied from the inverter. The inverter on/ off switch, seen on the far right of the diagram, is situated beside the navigator’s seat. The two black switches are the main battery isolation switches.

Figure 4 Second battery installation

The second domestic battery is installed under the navigator’s seat. Note that it is in a separate plywood box and is strapped down (See Figure 4). The positive lead has a cover but the negative one is also the main ship’s earth and doesn’t really need one.  The two domestic batteries give us a total capacity of around 200AH.

And finally, noting that 240v can be fatal through shock whilst with 12V, heavy load currents can cause fire:  if you are at all unsure of what you are doing with the electrics on your boat – don’t do it; get a professional in!