24 Volt DC Electrical System

Iron Lady (being a US boat) has 4 different (but integrated) electrical systems.

First, a 240 Volt AC system for high amp devices (air conditioning, watermaker, speed oven, induction cooktop, electric hot water heating elements, washer and dryer). Despite the fact that we are a US boat, all of these systems will run on either 50 or 60 cycle so we can plug directly in to shore power just about anywhere in the world.

Second, a 110 Volt 60 cycle AC system for powering US small appliances and our computers.

Third, a 12 volt DC system for engine and genset starting and powering some of our electronics.

More about all of these systems in future posts.

The backbone of Iron Lady, however, is her 24 volt DC electrical system.

The 24 volt DC system powers most of our everyday systems directly. 24 volt service is controlled from two panels – one near the helm station for things that are frequently turned on and off. This includes basement lighting, stabilizers 24 to 12 volt converters, deck lighting, PC inverter (forward), nav and anchor lights, forward toilet and holding tank, and various electronics (Furuno gear, Comnav pilot, AIS, etc), heater fans and windlass controls.

Also on this panel we have meters for viewing alternator output (amps and volts) as well as a battery monitor which gives us state of charge (like a gas gauge if you will), current amp usage, total consumption and time remaining. These are located about midway down the panel in a row starting at the left with the round battery monitor.

The second is located in the office area in the aft companionway and is for devices that are normally left on. This includes all the lighting thruout the boat (halogen), refrigeration and freezers, autopilot pumps, 24 V alternator regulator and alternator field switches, fire control, engine alarms, Kabola heater master switch, fuel control system and pumps, engine room lights, potable water pumps, bilge pumps, aft toilet and holding tank pump, shower and sink drain pumps, oil change pump, and deck wash pumps, and PC inverter (aft).

Additionally, other 24 volt devices include the anchor windlass, the aft deck winch and the bow thruster.

The primary supply for this system is a very robust traction battery system consisting of 12 cells each rated at 2 volts, which in series provides us with 1200 amp hours of 24 volt DC power. 6 cells are located on the port and starboard side of the boat in the forward part of the basement.

Thus we can really run most of the boat directly from the battery bank and leave the 100 V and 240 V systems off unless they are needed. This saves a significant amount of power since these systems all have significant overhead even when power is not being used directly. Under most circumstances, we run the boat for 24 to 48 hours without recharging the batteries.

Well obviously there comes a time when the batteries need to be recharged. This is accomplished in a number of different ways.

The main engine has two very powerful alternators mounted to it (the two big red units). In combination, these two alternators pump out over 300 amps and are rated for continuous duty at this output. This translates to roughly 7.2 KW of power – more then enough to recharge the batteries in a timely manner.

Part of the secret here is that the diodes which rectify the output are remotely mounted on heat sinks and are fan cooled – most alternators have the diodes mounted directly on the alternator and the heat generated by the alternators slowly kills them.

The regulator that governs the output of the alternators is a smart regulator (located between the two diode blocks with the fans on them) and has 3 charging stages which are tailored to the traction batteries (bulk to bring the batteries up quickly, absorption to fill them to capacity, and float to maintain them). This design maximizes the life of the batteries.

The second means to charge the batteries is thru 3 2.5 KW Victron inverter/chargers. As the name implies, the Victron units are both a battery charger and inverter which converts 24 volts DC into 240 volts AC (50 or 60 cycle). Thus the inverters can take power from the batteries and alternators and convert it into 240 volt power to run our high amp systems. More on that in another post.

For our purposes here, the inverter/chargers (which are smart like the regulator) provide up to 210 amps of 24 volt DC charging to the battery bank. There are two sources where the inverters get their power to do this – shore power when the boat is connected at the dock or the 11.6 KW diesel genset when we are away from the dock.

Our typical cycle when at anchor is to run the genset every day or other day for 2 to 4 hours. During this time, the batteries are charged, we run the air conditioning if it is hot, do laundry and,if needed, make water with our reverse osmosis desalinization plant. If we are moving about, the output from the engine alternators is more then sufficient to do the same thing so we do not need the genset. As an example, in the last 3 months of cruising, we have only put 70 hours on the genset as we are in the temperate clime of New Zealand and have been underway every few days. Last year in the tropics our usage was higher as you would anticipate.

A few more details about the system. All of the above (the alternators, the batteries, the electrical panels, the inverters and other high amp devices (bowthruster, windlass, and winch) are all connected together thru a primary 24 volt DC buss. For protection, each of the circuits are fused with fuses ranging from 300 to 400 amps to where they connect to the buss. There are also main breakers/disconnects on each of the breaker panels. While many manufacturers and designers don’t go to such lengths, each branch circuit is protected by a 2 pole breaker – both the positive and negative side of the loads are disconnected when the breaker is tripped.

Further, there are heavy duty Blue Sea disconnects mounted on the basement bulkhead so each of the major circuits can be switched off for maintenance or in an emergency. While the disconnects can be manually switched, there are also remote switches to control them mounted under one of the cushions in the salon.

That about covers the 24 volt DC system. I will try to cover the other electrical systems in future posts.

Best