4G: Electrical Systems, DC
4G1: DC Concepts
Each year it seems more folks are confused by the operation of electrical systems on
yachts than by any other subject! Don’t feel discouraged if something isn’t clear: you’ve got
company in your confusion. So let’s try to cover some theory here first.
Most of the equipment on any boat is run by 12 volt DC electricity from the boat’s
batteries. This is true because DC should always be available: we have batteries aboard even
when there is no shore power! If the batteries aren’t run down, everything should work, just
like in the family car.
Since the batteries are used so much, we have to replenish, or charge them. The most
important way we do this is by alternators on the ship’s engines. In most cases the engines
will provide enough electricity to run everything, and still have some energy left over to add
back to the batteries, that is, to charge them.
Ah, but what if the engines aren’t running? Then, the batteries are slowly depleted until
they have “run down” and there is no more electricity stored in them . . . a big problem,
because then we not only can’t run all the neat stuff on the boat, we can’t start an engine to get
more electricity.
So a good skipper and crew has “electrical power management” in mind whenever they
turn an electrical gadget on or off!
It is with this concern that we can cite a reality: If we need more electricity than the
batteries alone must provide, and if the propulsion engines aren’t running, we will need to get
our electrical power from an alternative source! That’s the most important reason why we plug
the boat in to shore power or use the generator: To keep from running down the batteries. For
by using battery chargers getting their power from shore power or the generator, we can keep
the batteries charged, or, at least, from getting too low.
In modern, luxury cruising boats, however, there is another important factor: Some of
the “goodies” we like to have on board such as hair dryers and microwave ovens require
ordinary household electricity. This is 120 volts AC. It is different from DC. So if we want to
use these things when we’re not at a dock, we must have another way to get 110 volts AC,
and for this we use the generator, or an “inverter”, an amazing high tech gadget that takes 12
volts DC from the ship’s batteries and makes it into 110 volts AC!
So here’s what we’ve got:
• A lot of stuff running on 12 volts DC with that electricity from the batteries;
• To keep the batteries from running down, we have alternators run by the engine, and
battery chargers that get their power from shore power or the generator;
• For the stuff that runs on 120 volts AC, we have shore power, the generator, or, for
making AC out of the batteries’ DC, the inverter.
Section 4G: Electrical Systems, DC 4.16
