Online Book: Electrical Systems For Cruising Boats

Most production cruising boats are fitted with inadequate DC electrical systems. Let’s fix that.

Electricity, batteries and how to charge them are the source of more confusion in the cruising world than just about anything I can think of. But suppose you could really understand electricity? Now you can, and it’s not hard. Read on.

The details of how batteries charge and how voltage regulators work together…or not. Practical information that will help make sure you have electricity when you need it.

Just because the battery-supplied systems on cruising boats are low voltage, that does not make them safe. Here are some tips to reduce the chances of a nasty injury.

It’s a common misconception that battery-supplied electrical systems are safe because they are low voltage. That’s wrong. In fact, there are high fire risks. Here’s why, and what to do about it.

Two core decisions we must make when designing a cruising boat electrical system for living aboard full time and making offshore voyages, are the size of the battery bank and which charging sources we will need: generator, solar, main engine, wind, etc.

But the first thing we need to do, before getting into all that fun stuff, is think about electrical consumption and how to keep it reasonable.

In the last chapter we looked at some big loads that take electricity from our battery. In this chapter John covers watermakers and diesel furnaces and shows how system thinking can save us from having to install a silly-sized battery bank.

In the last two chapters we took a deep and considered dive into analyzing the electrical loads on our boats and thinking about ways to reduce said loads through smart systems thinking. Now we get to the payoff: How to calculate optimal battery bank size or, alternatively, how to live with the battery bank size we already have. I have built a spreadsheet to make the whole process easier. And I have updated my thinking on lithium batteries too.

These days we are seeing more and more gear added to boats, much of it AC supplied through inverters from the battery, that demands current (amperage) way higher than even dreamed of a decade ago. But will our electrical system buckle under the load? Here’s how to figure that out ahead of time.

So which is better, 12 or 24-volt DC systems for live-aboard cruising? Like most things, it depends. Here’s a definitive way to determine which is best for your boat and usage.

In Part 1 we learned that it was inefficient, and often impossible, as well as potentially dangerous, to supply the high-load equipment, that so many cruisers seem to want, with a 12-volt system. And, further, that the solution to this problem is either to forgo all very high-current (amperage) gear, or select a boat with a 24-volt system. So let’s look at that.

We all know that any cruising boat should have the engine and house banks separated, but that’s just the start of what we need to know.

Selecting battery master switches and deciding how to wire them is a fundamental part of a good cruising live-aboard electrical system.

A well set up cruising boat electrical system separates the house and engine start batteries. Here’s how to make sure both are properly charged.

Cross-bank battery charging with DC/DC chargers or alternator-to-battery chargers.