The problem with preparing a boat for cruising is that you can only imagine what you will need. Not until you are well away from the dock will you really know. A case in point is the need for a high-output alternator and three- or four-step regulator.
Let’s deal with the alternator first. Next month we will take a critical look at multi-step regulators.
For the sailor, daily consumption is not what directly determines the size alternator you should install. Rather, the determining factor is the size of your house battery bank. The rule of thumb is that conventional batteries will not accept a charge at greater than around 25 percent of battery capacity. So if your boat has a 100-amp-hour house battery, there is no benefit to having an alternator larger than 25 or 30 amps. For a house bank of 440 amp-hours—about what we get from four six-volt golf-cart batteries, four deep-cycle Group 31 batteries or two 8Ds—an alternator with a hot output greater than 110 amps is going to be a waste.
In truth, the relative benefit of a 110-amp alternator over, say, a standard 65 or 80 amp alternator is probably less than you imagine. Let’s stick with our 440-amp-hour house bank as an example. A second battery bank has no effect on the numbers unless you discharge both banks between recharges, not the normal (or wisest) charging regimen. Otherwise, the second bank is normally fully charged or only nominally discharged and does not materially increase the charge acceptance level the alternator experiences.
So, if we discharge the house bank 50 percent between recharges, what decrease in total charging time does a 110-amp alternator provide over a 65-amp alternator? The precise charging profile will vary with the type and condition of the batteries, but we will not be far off if we anticipate that the 110-amp alternator will put out maximum amperage for not more than 10 or 15 minutes. Because the higher-output alternator drives up battery voltage more quickly, the regulator begins sooner to cut back alternator output to a level compatible with declining battery acceptance. At the end of the first hour, expect the large alternator to have pumped around 80 amp-hours into the battery bank.
The 65-amp alternator will likely maintain full output for longer than the big alternator, but during the first hour it will always be charging at a lower rate. As a consequence, it takes the small alternator an additional 20 minutes to achieve 80 amp-hours of charge. That 20 minutes is the difference. For the remainder of the charge, the outputs from both alternators will be essentially the same.
Due to battery inefficiencies, we need to put back about 20 percent more energy than we take out, so we need 264 Ah of charge to fully charge our half-discharged 440-Ah bank. However, by the time the charge level has reached 90 percent, the battery bank is accepting so little current that replacing that last 10 percent takes hours. So house batteries, unless they are charged by some alternative source, tend to get 100 percent charged only when the boat is motoring. When charging at
anchor, a 90 percent charge level is the maximum practical target. Taking a 440-Ah bank from 50 percent discharged to 90 percent discharged necessitates around 210 Ah of charge, allowing for inefficiencies. Even the big alternator takes more than three hours to achieve this charge level; the smaller alternator gets there just 20 minutes later. Saying the same thing another way, however long it takes the 65-amp alternator to reach the 90 percent charge level, you will shorten that time only around 20 minutes by installing a 110-amp (or larger) alternator.
Gel and AGM batteries have a different acceptance profile, so if you have either of these types aboard, the benefits of a big alternator can be somewhat better. However, rapid charging comes at relatively high cost, both in the original price of closer-to-the-edge batteries, and in the shorter battery life that inevitably results from high-current charging. For recharging conventional wet deep-cycle batteries, we need to temper our expectations from a high-capacity alternator.
Adding insult to injury, the potential benefit is even less when the batteries are not deeply discharged. This is likely to be the situation if your boat has mechanical refrigeration that requires running the engine daily. With a discharge level of around 30 percent, expect the big alternator to cut back almost immediately. Within five minutes the output level will be below the peak level of the smaller alternator. The charging profile for the remainder of the charge from both alternators will be nearly identical. Net improvement in total charging time will be just five minutes.
One potential benefit of a larger alternator is that because it will normally be operating at a lower percentage of capacity, it should enjoy a longer life. This can be a decisive factor if you are headed for far-flung destinations. However, if most of your charging will be done at anchor, low-RPM performance may be an equally important consideration. In general, lower-output alternators have more windings because the wire can be thinner, so they give better low-RPM performance.
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When you are sitting at anchor listening to your engine run day after day, reducing engine time by even a few minutes can be a significant benefit. And other than the additional cost, there is generally no significant disadvantage to having excess alternator capacity. In fact, presumably adding a high-capacity alternator also gives you an on-board back-up in the form of the original alternator. However, if you expect that doubling alternator capacity will halve engine time, think again. In most circumstances you should expect no more than a 15 or 20 percent improvement and that only if you discharge your battery bank 50 percent.
Come back here next month when we will take a companion look at multi-stage regulators—how they function and if they can significantly reduce recharge times.
