Melting plastic, wires warm to the touch, uncontrolled zooming electrons--all signs your boat’s electrical system is not to be taken for granted.

SAILJazz member and electrical engineer Norm Bernstein offers some insight on electrical systems based on thirty years of boating and professional repair. Check out Part One of the installation here: http://www.sailjazz.com/editorial/read/58

Running a battery bank in parallel is possibly the most controversial topic on sailboat e-mail lists! I think it's a bad idea, both from a technical, as well as a strategic point of view. Certainly, two brand new batteries of identical type and capacity can be operated in parallel, and the nominal capacity of the 'bank' will indeed be twice the capacity of each battery. For some period of time, they will continue to operate in this fashion; during recharge, each battery will draw the same current, and their electrochemical conditions will be identical. However, as the batteries age due to manufacturing tolerances and so forth, their electrochemical condition may differ.

At some point, the weaker battery will begin to draw measurable current from the stronger one, and the total capacity will diminish. There is a possibility that one of the batteries will develop a shorted cell, and the result will be that the other battery will be dumping a huge amount of current into the damaged one; a possibly consequence of this happening can be boil-over of electrolyte, extreme heating and the release of gas. The released gasses can ruin the interior of your boat, and there’s a slim but palpable danger of explosion. Granted, it rarely happens and probably won’t in the first few years of battery life, but why take a chance? Is there any strategic advantage of running your batteries like that? I think not!

There's another problem with running batteries in parallel. Let's assume you've got a pair of 4D batteries, which you are using sequentially. When one of them becomes depleted, you start your engine, and the alternator attempts to recharge the battery. Chances are good that the alternator will be working fairly hard, in the first hour or two, to supply the amount of current that the battery will accept. Eventually, the terminal voltage rises, the current reduces, and you reach a fully charged condition.

Now let's assume that you've got two 4D batteries, in parallel and you deplete them to the same level as in the previous example. The problem now is that you need twice the current to recharge the 'bank' in the same time period, but alternators are not linear devices, and you won't get twice the current, because heating will lower the maximum output, and you're asking the alternator to do twice the work as in the previous example.

OK, I can already expect that someone will be saying 'yes, but if I use the batteries sequentially, and recharge them sequentially, it will take twice the running time on my engine!' Actually, that's not true, because the alternator output is nonlinear and it will be more efficient at lower operating temperatures and power outputs. You'll actually get a faster cumulative recharge, when measured in amp-hours replenished. Do the math, and you'll see what I mean.

The tricky thing with electrical systems is that it’s not a one-size fits all topic. A heavily used Moorings boat in the Caribbean will likely have different demands and maintenance than weekend cruiser.

The self-perpetuating cycle This occurs when the owner decides he needs more capacity and installs a huge set of additional batteries. But now he needs the ability to recharge the batteries, so he installs an expensive high output alternator, and now he needs an even larger charger at the dock to keep things fully charged.. and it goes on and on and on. I realize that there are those who have different energy needs, but I’ve spent 20 years on a succession of sailboats using the stock electrical systems (I did add a starting battery to one of those boats), and had no problems supplying all the power I needed for refrigeration and other house loads, while cruising. The strategy with which you use your energy systems matters a lot more than a lot of expensive and, in many cases unnecessary electrical 'toys'.

A better strategy If you agree with what I've said so far (and some may still not be convinced, but bear with me), let me suggest a far better strategy. Use two battery banks, each of which consists of a single battery with sufficient amp-hour capacity for your nominal loads, large enough to last long enough for the worst case interval between recharge. For my typical cruising pattern, a 4D battery is adequate. Each day when you shut down the engine in the evening, switch to the opposite battery. This means that the battery you were using during the day--when you probably were using your engine intermittently--is fully charged, and held in reserve, and you're using the other battery for house loads. The next day, you'll replenish the battery you were using the previous evening. By following this pattern, your batteries will wear evenly, you'll never deeply deplete either one, and there will be no need to operate them in parallel and risk destruction.

"But wait a minute," someone is bound to say that, when they chartered sailboats in the Caribbean, the charter boats always had both batteries being operated in parallel. Yes, this is true, but you have to realize that a charter company can't expect a customer to observe sensible battery switching recommendations and furthermore, batteries are comparatively cheap and easily replaceable, on a schedule. For charter use, it makes sense for them to just tell the customer not to touch the battery switches, and replace the batteries every year or two. We're not likely to do this on our personal boats, because we're not generating charter revenue!

How do I get my third (starting) battery recharged? Sailors seem to have this dilemma, because the stock boat wiring doesn't clearly provide a way for getting a battery used exclusively for starting recharged. The trick here is to use a 'diode' from the main alternator output. I use the quote marks here because a true 'diode', while workable, isn't ideal; the 0.7 volt drop through the diode means that the starting battery won't get a full recharge. The alternative is a 'smart diode', sometimes called a 'battery isolator', 'battery combiner', or even 'Echo charger'. These devices use either voltage-sensing relays or large power FET transistors to create a one-way path for current to flow, without suffering the loss of voltage that a true diode provides.

Starting the engine is battery intensive, but only for a short amount of time.

In actual practice, however, an ordinary diode would probably be fine. Engine starting is a current-limited phenomenon, more dependent on the ability to produce an enormous current than a high voltage. I know this seems counter-intuitive (power = volts times amps, after all) but a starter motor is yet another nonlinear device, which is why a somewhat depleted battery can still start an engine--you don't need a full charge to do it.

Inverters Inverters are used to generate a 120VAC output from a 12VDC source, in order to operate some appliance. I wish someone would tell me just what 120VAC appliance is essential or even desirable on a sailboat that would justify the high expense and poor efficiencies of an inverter. I know one boater who spent $1000 to put in a large inverter on his boat, in order to operate his microwave. Isn't Jiffy Pop on the stove adequate? Admittedly, there are those who enjoy the convenience of high power appliances like microwave ovens. However, the impact on the boat’s house battery is severe, and the expense for the large batteries, larger alternator, band a big inverter makes the cost of supporting such a convenience ridiculously high.