In the August 2005 issue Nigel Calder reveals the essentials to a trouble free electrical systems including calculating power demands, battery systems, power saving, must-use calculations and the Four Commandments of on-board power.

Here he adds his advice on sizing a DC system and the calculations he used when looking at the sytsem for his new boat, Nada, a Malo 45.

Rules of Thumb for Sizing a DC System

1. Differentiate cranking and cycling applications and buy the appropriate batteries.

2. Size a dedicated cranking battery according to the CCA requirement of the engine it is to crank.

3. Size a house battery bank such that its capacity is 3 to 4 times the maximum likely amp-hour drain between major recharges. The batteries must be the same age, make and type.

4. Combine all the house batteries into a single bank rather than split them into dual banks.

5. All the batteries in a battery bank must be in the same physical location at the same ambient temperature.

6. In a single-alternator installation, replace the existing alternator with one that has a hot-rated output in amps that at normal charging speeds of rotation is equal to the DC loads running plus 25% to 33% of the rated capacity of all the batteries being charged.

7. In a dual-alternator installation, use the alternator that comes with the engine to charge the cranking battery and add another alternator as in (6) above.

8. Ensure that add-on alternators are externally regulated with an adjustable multistep regulator that has temperature compensation based on temperature sensing at the batteries.

‘Nada’ Calculations

Our goal is to keep the daily load between 100 and 150 amp-hours (at 24 volts). To achieve this and still operate a wide range of electrical appliances (including a DC fridge and freezer, a DC watermaker, and electric winches, bowthruster and windlass) all halogen interior lights have been replaced with fluorescent lights, and I built my own iceboxes from vacuum insulated panels, with additional foam insulation, and double seals on hatches and doors. The galley had to be extensively redesigned to accommodate the added insulation. A wind vane will steer the boat on long passages, removing the autopilot load.

The battery box was expanded to create the space for a 450 amp-hours (at 24 volts) battery bank. The engine compartment was redesigned to accommodate a second, large-frame, high-output alternator (a Balmar 95-24-110), hot-rated at 110 amps (24-volts). It is wound to achieve a high output at relatively slow speeds of rotation.

A 45 watt solar panel is intended to do little more than keep up with the standby drain on the batteries when the boat is not in use, maintaining the batteries in a state of full charge. This standby drain includes the systems monitor and other equipment with a permanently-wired memory function.

Any time the wind speed exceeds 10 knots, an AirX wind generator will make a significant contribution to the energy equation (the AirX is by far the most sophisticated wind generator available for boats).

‘Nada’ has a DC generator with a maximum 200-amp output (at 24 volts). Once the software is perfected (this is still being worked on) it will be wired with an Automatic Generator Start (AGS) such that it kicks on any time the batteries are pulled down to a 50% state of charge, taking much of the user interaction out of the energy management process (and protecting the batteries from damagingly deep discharges).

For a thorough guide to all on board systems you can do no better than buy Nigel’s
Boatowner’s Mechanical and Electrical Manual
published by Adlard Coles; £45.

Nigel Calder’s guide to engines and electrical systems is known around the world as ‘the boatowner’s bible’. The third edition has just been published and is packed with new and updated information. There are plenty of illustrations and diagrams as well as diagnostic trouble shooting charts. Nigel combines expertise gathered as a former diesel mechanic, boatbuilder, and machinist with a clear writing style to make complicated systems easier to understand. The books 820 pages deal with everything from fuses to furlers via props, heads, steering systems and stoves. It’s the next best thing to having a mechanic on board.