Some Marine Electrical System Basics
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Introduction

Perhaps Malthus was right (you remember Malthus: he put the dismal in the dismal science that we know as Economics). He claimed that demand eventually outstrips supply. This certainly seems to be true onboard boats, where your original electrical system inevitably proves inadequate to run all of the electrical loads that you lust for. Inverters, electric windlasses, galley appliances, entertainment systems? they may make boating more fun, but they extract a toll.

We don't want to paint an excessively bleak picture, since there are a variety of new technologies and products which make meeting the challenges of these electrical loads practically child's play. We think that four developments merit special attention:

 - Sealed gel and AGM batteries
 - Inverter/chargers which produce silent, quality power
 - Smart chargers, whether regulators on alternators, battery chargers, etc.
 - Amp-hour meters which also show current flow and
   voltage in one package

Where do I start?
How do you figure out which components you need to operate your boat effectively? There are six categories of electrical components:

 1 - DC Loads use the stored electricity
 2 - Batteries store electricity
 3 - Wire and Distribution Panels connect the system and protect
      it from damage
 4 - Chargers and alternators charge the batteries
 5 - Monitors display the status of the system
 6 - Inverters, generators, solar panels and wind generators produce
      AC charging current.

Your "ideal" system may or may not require all of these components. What's important is that the total system meets your daily electrical needs with sufficient battery capacity and charging ability.

What's really important?
OK, here it is: the Holy Grail of Boat Electrical Systems. Pay attention!

Boats should have two battery banks: one dedicated to starting the engine; the other reserved for house loads. Use the house bank for all DC loads, period. Don't add separate battery banks for specific loads, other than to start the genset.

House loads and engine loads should be run through separate battery switches, not a single 1-2-BOTH switch. Allow the banks to be combined in an emergency with an Emergency Parallel Switch which has a removable key.

Charge batteries in parallel using the Battery Combiner or switches; discharge in isolation.

Your house battery bank should be 3/4 times your periodic consumption.

Create electrical paths with as few serial conductors as possible.

Ground your alternator using a stout wire to the common ground point in addition to the engine block, or use an isolated ground alternator.

Avoid battery isolators, analog volt meters, and ferroresonant chargers.

Use marine-grade wire in ABYC-recommended wire sizes. Err on the large size.

Sailors should select an alternator that produces substantial power at low RPM. Power boaters should select an alternator designed to run at their cruising engine RPM.

Make sure your engine has enough ?belt? to drive the alternator you select.

Use smart multi-step chargers and regulators to protect your battery investment.

Do not leave batteries discharged, or charge them at a high voltage, for extended periods.

Identical batteries connected in parallel which do not self-discharge are probably fine. Doesn't hurt to have a fuse between them, though.

Use flooded batteries, gel batteries, or AGM batteries, but limit yourself to one type on the same boat.

Provide a means to cross-connect battery banks in an emergency.

A fresh set of large gel or AGM batteries can solve a variety of problems.

Equalize flooded batteries every 30 cycles or a few times a season. Disconnect any voltage sensitive loads before you do, however.

Replace batteries when their actual capacity drops to 2/3 of their of their peak capacity.

Protect circuits with fuses or circuit breakers, including the alternator. Fuse batteries near the positive terminal.

Voltage drop is the enemy. Learn how to find offending connections and fix them.

Use a combination amp-hour/voltmeter/ammeter on the house bank.

Alternators should have an output 30% as large as flooded batteries, and 50% as large as gel or AGM batteries.

Battery chargers should have an output of 6-10% of your battery capacity, plus the average DC loads when at dockside.

Inverters need to be run from a house bank that has a least 20% as many amphours as the inverter has watts, e.g. a 1,000W inverter should be run from at least a 200Ah battery bank.




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