ELECTRICAL
BASICS
The electric system was designed at the outset to ensure AC and DC separation. AC circuits are routed along the port side and DC circuits along the starboard side. The main exceptions are in the engine room where AC cables cross over the the DC side to supply the inverters and charger etc and where DC control circuits are required to activate AC devices. Cables are fed through 3 inch diameter pipes under the side decks. Outside the cable pipes, they are sleeved where required.
DC SYSTEM
The DC system has both 24v and 12v elements. The domestic battery is 24v 920Ah C10 and consists of 12 large 2v cells. These cells are heavy and when I weighed them I discovered that they are 48kg each when empty - and I put 12 in the back of my Golf hatcback!! They were originally designed for use in de-gaussing barges and the flash deperming of warships. They are capable of repeatedly discharging at 5000A, and should have a long life as a domestic battery - at least 15 years. A drawback of the battery is that the cells are lead acid and at high charging voltages will gas off hydrogen and oxygen. The battery is therefore in a large sealed box with forced ventilation during charging to remove the gases and vent them out of the engine room. The provision of an auto watering system and battery cell caps that reduce water loss are also being investigated. The engine starter battery is also 24v, whilst the generator battery is 12v. I also bought 4 12v 200Ah batteries in Holland as temporay house battery - they will be used to power the LEM 200 bow thruster motor. This motor will run on either 24 or 48 volts depending on the revs required and draw up to 600 amps if required. To charge these batteries, Ebay provided another immaculate bargain - a Xantrex Heart Freedom 25 24 volt Combi Inverter/Charger (2500W and 65A) for only £127, considerably cheaper than just a 24v charger and added redundancy with the inverter..
All wiring is generously sized to minimize voltage drop and I have aimed at 3% drop maximum throughout. Whilst I was researching voltage drop, I found that much advice only considered the voltage drop from an item's switch to the item, and did not include the drop from the battery posts - it can and does make quite a difference. I made up a spreadsheet to assist me calculate the culmulative voltage drop and the cable lengths required. It also provided me with a cable numbering system. As a result some cable are really quite large - for example the main feeds to lighting junction boxes are 6mm2 and even the cables to lighting units are 4mm2. Feeds to heavy DC equipment - an electric toilet drawing 20A - are 35mm2 - I wanted 25mm2 but the supplier did not have it and substituted 35mm2.
I also discovered a complete lack of suitable distribution boards/panels to provide large current supplies. These include up to 150A to supply the inverter and 50A to the Xact, and also accept large current input from the battery charger. Plus I needed and the facilites for an "always on" section - for bilge pumps and alarms and a lighting section powered by the XAct. I therefore made up my own panel, as seen in the internal layout pictures. The copper bus bars are 150mm2, and will carry currents of up to 400A - which is more than I intend using. There is a short busbar to feed midi and mega fuses and then other bus bars for fuses below 30A and to provide negative paths. The main battery feed is a parallel pair of 70mm2 cables to a 275A domestic battery switch and it is fused next to the battery to meet the ISO/ABYC rules. There is also an emergency parallel switch to supply the engine battery. Another length of bus bar provides the common ground for both the DC and AC systems next to the engine battery box. The same copper bus bar will be used to make the links for the battery cells - Chloride Industrial Battery Ltd having advised that the cell to cell links should be at least 100mm2.
After much research I decided to use a "Smartbank" to solve the dilemma of how to charge 2 batteries. Using this device, the engine alternator is connected directly to the domestic battery and I ran 35mm2 cables direct from the alternator posts to the batteries - positive to the domestic battery and negative to the engine battery. I may increase them to 70mm2. The Smartbank is connected to both batteries to monitor voltage and operates a heavy duty relay when the engine battery needs charging or is experiencing a load. In many ways it is similar to a battery to battery charger but with one very important advantage - it allows the full alternator output to charge the engine battery if needed, rather than 4 or 10 Amps as with battery to battery chargers. I also learned much about battery charging voltages - above 28.6 volts the amount of additional charge going in with higher voltages tails off dramatically, so whilst an alternator controller will attempt to charge at 29 or more volts, it does not translate into quicker charging. It is also a very cost effective solution - Smartbank and relay is around £100.
The engine battery has a separate master switch and the cable is also fused to meet ISO/ABYC. I decided to mirror auto practice and all systems that are used when the engine is running are fed from the engine battery. These are switched from the wheelhouse panel. Again, I was not particularly enamoured with the commercial offerings of panels and made up my own panel to also match the shape of the engine panel.
Lighting is a mix of 24v DC Halogen and 240v CFL. Downlighters from Eurobatteries are liberally sprinkled across the deckheads and fitted with 24v 20w and 10w lamps and have been wired up in groups. These circuits are fed by the XAct Minor. This device auto switches between the 240v AC supply and the domestic battery supply - which ever is available - and supplies exactly 24v for halogen lighting. Max supply is 50A or 1200W - more than enough for all the lights switched on at the same time. It can also be used as a 24v DC power supply.
AC SYSTEM
Again the main AC distribution boards are a custom designed and built affair. KEI has 4 AC sources - Shore, Engine AC Alternator, Generator and Inverter. The engine alt immediately brings its own problems along to the party - is requires a 12v feed, it is Quasi Sine Wave and centre tapped 120-0-120, but it was a bit of a bargain so I wanted to use it - if the engine is turning I might as well have some 240v AC. To switch all these sources in a logical sequence, I designed a system using industrial contactor pairs with mechanical interlocks and control relays. This system is simple enough to allow automatic changeover between power supplies with complete electrical and mechanical safety features so that supplies cannot get mixed. However, it will be complicated enough to enable me to feed the engine alternator centre tapped 120-0-120 output into the isolation transformer and convert it into a 0-240 v source and then feed the QSW supply to a split load board, with the FSW from the inverter still feeding the other side of the board to provide power to those items that like FSW supplies. When the engine alt is not supplying power, then the inverter can feed both sides of the board, and I will add a device to prevent the charger running off the inverter - the on-off switch!! There are also 2 small QSW inverters to run the fridge and CH system when the barge is left unoccupied and everything is left on 24v DC - and possibly relying on solar panels to maintain the battery state. Diagram of AC supplies and switching is here.
Another advantage of having an isolation transformer was illustrated well during the initial AC circuits installation. Using a DVM, I discovered that the boat yard negative line was 12v positive with respect to earth at the shore supply breaker onboard. On the boat side of the IT, quite correctly, the negative was at zero volts with respect to earth. If I had only fitted a galvanic isolator, the negative, and consequently the entire hull, would have remained at 12v positive with respect to earth.