Features April 15, 1999 Issue

When Do You Need A High-Output Alternator?

If you’ve added batteries to power more electrical equipment, your stock engine alternator may be inadequate to charge them quickly and efficiently. Here’s how to size up your electrical system.

In our last report on high-output alternators (HOPA), in the September 1994 issue, five leading brands were tested. The conclusion was that there is little difference in design, construction or performance, and that buyers could base their purchase decisions on mounting options, dealer support and price. Because so many readers ask about replacement alternators, it’s time to revisit the subject. And because previous tests were not particularly revealing, let’s concentrate on how to put together a battery charging system that works on your boat.

Rate of Charge Is Vital
Fifteen or 20 years ago, the typical 35-foot boat had a fairly simple electrical system—house lights, navigation lights, VHF radio, stereo and a bilge pump. Two 80-amp-hour batteries—one for starting the engine and one for everything else—were adequate. And you could charge them with the stock 35-55-amp alternator that came with your engine. Assume the engine start battery is always charged. Say, you discharge the 80-amp house battery by 50% (generally, the maximum recommended). If the 35-amp alternator puts out 12 amps at idle, you’ll replace the 40 lost amp-hours in about 3-1/3 hours, more quickly if the engine is turning faster than at idle.

Today’s typical boat is more likely to be loaded with power-hungry equipment such as 12-volt refrigeration, autopilot, radar, watermaker, maybe even an electric toilet or sanitation device. On such a boat, about 400 amp-hours of battery capacity is required. At 50% discharge, you’ve got 200 amp-hours to replace. With the same 35-amp alternator, you’re looking at 16-2/3 hours at idle. Intolerable! The answer is a bigger alternator and more sophisticated regulator.

Charging Batteries Correctly
Automotive-type marine alternators are primarily designed to charge starting batteries. These batteries supply tremendous amounts of current to run a starter (in the hundreds of amps) but only for brief periods, resulting in a very low net amp-hour use. The starter battery is quickly brought back to full charge by a relatively modest alternator current output, and at a set voltage not great enough to damage the battery with prolonged engine running.

While this is fine for starter batteries, it is inadequate to charge house (or deep-cycle) batteries designed for a deeper discharge over long periods of time. The reason is that they have different charging needs than a starter battery. House batteries have a different internal design that allows for repeated deep discharges.

There are deep-cycle batteries which can also serve starting duties, and a case can be made for buying these because one of the cardinal rules for battery bank systems charged by the same alternator is to use the same types, brands and ages (if possible) for optimum battery system life.

For greatest efficiency, deep-cycle batteries should have high charging current and voltage as well as more complex charging profiles known as multi-stage regulation, normally done in three distinct stages. There are actually different voltage and current profiles for various types of deep-cycle batteries such as common lead acid cells (14.4V during the bulk phase), gel cells, and more exotic AGM (Absorbed Glass Mat) cells (both about 14.1V at bulk). Comparable capacity gel and AGM batteries can accept a far greater charging current than lead acid batteries, and really need a high-output alternator to live up to their quick charge capabilities. The advent of computer chip-controlled, multi-stage, preprogrammed and programmable voltage regulators makes the added complexity very manageable. This can become critical with AGM batteries, which almost look like a dead short when an alternator “sees” one in a discharged state, and to protect the alternator a regulator is needed to prevent overheating and shortened life.

Without a multi-stage charging system, house batteries often end up undercharged, no matter how long they are charged, because the typical voltage regulator is designed to prevent overcharging starting batteries when engines are run for prolonged periods.

When researching alternators, you may be confused by manufacturers’ specifications that list differing sets of standards, particularly output ratings at very different temperatures. Different temperature ratings make valid comparisons impossible because temperature has a dramatic effect on both component life as well as current output. Try as best you can to compare ratings based on the same standards.

System Assessment Overview
The first step is to assess your amp-hour needs. Typically, this is done on a 24-hour use basis, which results in a daily use rate, say 100 amp-hours (multiple the current draw of each device times the hours it runs each day and add the totals for all devices). You then size the house battery capacity using a 50% discharge rule. Next, determine the alternator size required for expected use, again employing a conservative rule of thumb described below. This is followed by choosing a compatible regulator. Lastly, you need to determine whether you can install your proposed battery/alternator/regulator system or need to factor in professional installation costs.

The existing belt size on your engine is a good indicator of how big you can go without making major modifications. For example, a 3/8" belt limits you to about 75 amps alternator capacity, and a 1/2" belt to about 100 amps. Larger alternators may require dual belts, which means new pulleys, too. Check with your engine maker for what size alternator can be safely handled. High-output alternators place significant loads on the engine, which makes belt tension extremely important.

Sizing the Battery Bank
For sizing your house battery bank, we recommend observing the above-mentioned 50% rule, which means not discharging your house bank beyond 50% of capacity. To discharge more than this significantly reduces the life of the batteries.

(Note that different types of batteries have different capacities for abuse. For example, gel cells theoretically have fewer discharge cycles than a typical lead-acid wet cell, but have a greater ability to tolerate abuse, such as being left in a discharged state for prolonged periods, and faster charging capability.)

If you need 100 amp-hours of capacity per day, double the bank size for long battery life by the 50% discharge rule, and then add an 85% quick recharge factor (the last 15% of the battery charge takes the bulk of the recharge time regardless of the alternator). Figure your usable capacity will be reduced 5% to 10% due to aging, and you end up with a conservative recommendation of three to four times the original 100 amp-hours’ daily use.

Sizing The Alternator
Alternators should be sized at 25% to 40% of the amp-hour rating of the house bank (unless you have major DC loads while the engine is running). Going with more alternator capacity than this is not recommended. Batteries can only “drink” up charging current so fast, and it varies with the batteries’ state of charge. This is called the acceptance rate. Unfortunately, many battery makers don’t list acceptance rates, but it’s worth asking.

With a standard alternator, it’s hard just to meet the maximum acceptance rate of an entire system. What happens if you exceed it? Well, providing you have a properly operating regulator, preferably with a battery temperature probe, nothing drastic. A regulator prevents excess voltage and current from cooking the battery. A temperature probe on the battery will provide insurance; if the battery temperature exceeds a predetermined limit (about 125°F), charging may be terminated until the temperature is again in the safe zone.

It is important to distinguish between real world and rated alternator outputs. Many alternators are rated at SAE—77°F, which is a much lower temperature than most engine rooms. The temperature rating to consider is 122°F, or even 200°F if your engine room runs hot or you are in tropical climates. Check it with a thermometer. You may need additional cooling, vents or forced air circulation. Alternators are available that put out their rated current at a constant 200°F, but they are the most expensive. That said, if you use a temperature-underrated alternator in a high temperature environment, it will fail in short order.

It’s best to select an alternator at the high end of the acceptable range so you don’t need to run it at 100% capacity. This will result in a cooler running, longer-lasting unit. For example, if you need 125 amps, get a 150-amp alternator and set up the pulley ratios to develop 125 amps at the desired rpm.

Physical Considerations
High-output alternators come in two basic sizes. Small-frame models are replacements for standard alternators and come in ratings up to 150 amps. As long as you match the single- or dual-foot mounting configuration, it should be a simple job to install, though a longer mounting arm might be necessary.

If more current is required, two small-frame units may be used or a single large-frame model which is available in currents up to 275 amps. Large-frame models require much more complex mounting arrangements and professional installation is recommended.

If installed on a gasoline engine, an approved, ignition-protected alternator must be used to assure that any sparks in the alternator cannot ignite flammable engine room vapors.

An important consideration is the mounting configuration of your engine. Most US and European engines have single-foot alternator mounts, while Japanese engines have dual-foot mounts. Alternators are available with both types, so be sure to specify the correct one.

The pulley used with the alternator must be sized to the specific installation. To do so, first determine the engine rpm at which you desire most charging to take place—say 1,000 rpm at anchor. Set up the pulley ratio to get the required current at this speed. Check the maximum engine rpm generally used for propulsion to be sure the alternator will not overspeed at that rpm. Keep the conservative alternator rating previously recommended in mind so the alternator will not have to run at 100% capacity most of the time. Manufacturers provide graphs to show output vs. rpm. Normally, alternators can run at speeds up to 10,000 rpm.

As an example, suppose you need a maximum of 100 amps to charge a house bank. Using our conservative sizing rule, select a 120-amp alternator and set it up for your normal charging mode—say at anchor and 1,000 rpm. If the alternator puts out 100 amps at 3,000 alternator rpm, you need a 3:1 pulley ratio. If when running under power the maximum engine rpm is no more than 3,000 rpm, the maximum alternator rpm will be no more than 9,000, within the alternator’s limits.

With regard to rpm measurements, remember that if you use anything other than the existing alternator pulley-to-crankshaft ratio or an alternator with a different number of poles, you will have to do some recalibrating to obtain correct engine rpm if the tachometer rpm is derived from the alternator electrically, which is most common. Some alternators have different speed ratio taps, and some tachometers have adjustments. In some cases, if your tach isn’t adjustable, you may have to live with erroneous readings. Or buy one that is compatible with the new alternator.

If you can afford it and want to reduce stray current corrosion, consider an isolated ground alternator. This has a separate connection for the negative terminal. Normally, most alternators are simply grounded through the alternator case and bolted to the engine.

Coatings on alternator cases have been the topic of some controversy. Conventional wisdom says that any coating, such as paint, only hinders heat transfer—the big enemy of an alternator. Because Balmar powder coats many of their alternators, we asked why. They said that they have found the powder coat beneficial on several counts. First, it reduces corrosion and its inherent by-products, which can cause contamination of brushes and slip rings. Second, the smooth powder coat surface reduces oil and dirt buildup on the case, which are big heat retainers.

Powerline (Hehr) plates some of their alternators with a nickel coating to reduce corrosion of the aluminum case. Case coatings (or lack thereof) should not be a show stopper. More importantly, look for cooling fans which are directional, as they are key for cooling and much more effective than bi-directional fans.

Other factors to consider are non-corrosive brush holders, plated brush springs and impregnated windings.

Choosing a Regulator
The last consideration is a multi-step regulator. To skip this step and go with a simple single-stage regulator is constraining the system’s potential. Multi-step regulators have been around for some time, but the latest top-of-the-line models have the reliability and built-in programs for just about any setup as well as programmability to wring out every possible amp from the system. Their two big advantages over automotive-type regulators are higher charging current for longer periods as needed, and bulk-float-acceptance-equalization phases for more complete charging and longer battery life.

In the February 1, 1997 issue was a report on the Balmar ARS II, Cruising Equipment Alpha and Hehr Powerline Aqualine. All three performed similarly. For the money, we felt the Aqualine was the Best Buy. On Viva, our Tartan 44 test boat, we have the Alpha as part of the Link 2000R, which also manages our Heart Freedom inverter. We keep the Aqualine as a backup.

The voltage regulator is a key component of the charging system, particularly where large battery banks, charging at anchor or AGM batteries are factors. They may be single-stage units built into the alternator or stand-alone, multi-stage microprocessor-controlled units. Using Balmar models as examples, though others certainly are available (see the Value Guide on page 6), let’s briefly look at some examples.

The first is the single-stage, built-in regulator. These are generally not adjustable as they are inside the alternator. They are best suited for small battery banks and powerboats which are under way more than at anchor, so the batteries can receive constant charging.

The next step up is the single-stage external regulator like the Balmar BRS 2 ($120 discount). Its voltage can be adjusted according to the type of batteries being charged. Still, they are best suited for small banks.

More versatile is the multi-stage regulator with three charging stages. The Balmar ARS 3 ($160 discount) is a three-stage regulator with adjustable bulk voltage, absorption time and float voltage. The ARS 3 is designed for larger banks of gel or lead-acid batteries being discharged to 50% and recharged to 85% and can handle large DC loads. The regulator has a built-in delay of 1 minute to let the engine start and warm up without alternator drag. Some brands in this class also have a fourth stage called equalization, which is used periodically (and with caution) to supply an extra high voltage to the batteries to bring them back to full capacity and to dissolve crystals of lead sulfate on the plates.

The top of the line Balmar is the MC 512 ($249 discount). It has both multiple pre-programmed settings accessible by dip switches as well as user-selectable parameters for those who want to “tweak” their systems. In addition to multiple equalization programs, there is a built-in 45-second alternator start delay. There is also a 1 minute soft ramp-up which brings on alternator power gradually. It also helps prevent the engine from stalling if you frequently charge at idle. This is particularly important with small engines for which big alternators can represent a significant load. It also has an “Amp Manager” which can be set to automatically cut back on maximum alternator output to compensate for unusual conditions such as operating in extreme heat, or a small engine charging at idle. Other features include battery temperature probes; an alternator temperature probe (about $35 in the marine discount catalogs) to temporarily shut down the alternator in excessive heating conditions; auto mode, which analyzes the batteries being charged and makes automatic charting adjustments; external alarms and a digital output drive.

Installation of a multi-stage regulator is not beyond the ability of the average handyman, but does require patience and good wiring practice.

Installation Considerations
Devices which use, produce or carry high current are extremely sensitive to top quality wiring materials and practices. Appropriately sized, stranded wire and the best quality connectors are mandatory. Buy good quality crimpers and terminals. Use manufacturer fabricated harnesses rather than skimping.

Less obvious is the need for a top-of-the-line drive belt for the alternator. These high-output units put a terrific strain on belts and need to have high tension or they will not only under perform, but also fail early from the heat generated by a slipping belt, which can cook the grease in the front alternator bearing. Manufacturers can detect this type of abuse and may deny warranty coverage.

Be sure alternator voltage sensing is positioned at the battery bank, particularly if you use silicon diode isolators which have a significant voltage drop. Better yet, use an electronic relay such as the Battery Combiner from West Marine or the Batt Maxx from Wells Marine, the latter being available up to 400-amp ratings or more. (See PS, November 1, 1996.)

A dual-output alternator (to charge two separate battery banks) is another option. Be sure to install fuses or circuit breakers to protect from short circuits, which can cause alternator damage or even a fire. Install a “zap” stopper on the alternator to protect it from transient voltage spikes. It’s another form of cheap insurance.

One option is to keep your existing alternator/starting battery setup (providing it’s satisfactory). Then add a high-output alternator dedicated to the house bank. This gives you two functioning alternators, one for each battery bank, perhaps with an emergency parallel switch. The only downside is installation cost and increased maintenance of two alternators.


Contacts- Ample Power, 2442 NW Market St., #43, Seattle, WA 98107; 206/789-0827; www.amplepower.com. Balmar, 27010 12th Ave NW, Stanwood, WA 98292; 360/629-6100; www.balmarvst.com. Hehr Power Systems, Powerline Alternators, 4616 Fairlane Ave., Fort Worth, TX 76119; 817/535-0284; www.hehrpowersystems.com. Prestolite Electric, Alternator Products Division, 400 Main St. Arcade, NY 14009; 716/492-1700; www.prestolite.com. West Marine, PO Box 50050, Watsonville, CA 95077; 800/538-0775. www.westmarine.com.

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