High Output (Dual) Alternator Sizing


Review all of the FAQ links, especially the DC charging link, in order to understand what goes into the requirements stage. When you have a good feel (understanding) of the various factors that are involved, you will probably agree with ZRD's standard solution.

After years of extensive first hand experience, research, and numerous installations, a second alternator is required to ensure that the large house battery bank is kept properly charged. This high output alternator needs to produce in excess of 175 amps of 12 volt DC output. Some of the most significant factors are available charging time, wire sizing, installation, available space, reliability, and cost per amp. With those factors in mind, the ZRD ZCCJE1222 alternator has met and exceeded all of our requirements. There is no compromise in any area with this product. It is as close as perfection gets. It is our most highly recommended and sought after solution.

With the multitude of pulley ratios available between the alternator and power take-off (PTO) pulleys, the following information may vary, but may be used as a guide. The data is from an installation using a 2.7" alternator pulley with a 6" PTO producing a 1.0 to 2.22 engine rpm to alternator rpm. Refer to the output graph with the ratio developed for your specific installation to see how your output scenario may vary from the following.

At normal engine idle, the ZCCJE1222 puts out around 120 amps DC. Before cruising speed is attained for most vessels, 220 amps is possible. With a standard installation, this equates to 120 amps of DC output at 840 rpm and 220 amps at 1500 rpm. As we do not recommend running a power plant engine on the hook (again, review other links), correct normal charging is easily accomplished during location changes or transits while cruising, even at low power settings. This ensures that a strong adequate source of DC power is readily available for all needs. You can personalize this to your situation by looking at the power curve on the Alternators & Regs link and use the standard ratio of 2.6 to 1 to correlate alternator rpm with engine rpm.

A common question we receive from prospective clients is their desire to add a second small alternator (normally found or inherited - the reasons are as varied as the types). Simply, the 99% response is, "do not do it". The normal costs associated with installing a second alternator make the return so little compared to a high output alternator. The only direct savings is the price differences of the two alternators. The difference is normally small. Everything else is the same or even higher for the smaller alternator due to the required specialized manufacturing. This discussion is only dealing with the costs of adding a second small alternator. The previous sections show why the correct solution, regardless of cost, is a high output alternator. Again, our 100% choice is a ZRD ZCCJE1222.

Now one of the most common questions - Why is the First, Start, Engine, Original, Single, Only, or any other name one may call it by, alternator limited to 94 amps? The answer is a combination of simple physics and design. To start with, there is normally only a single sheave V-belt pulley with a 3/8" or 1/2" belt supplied from the OEM engine manufacturer. Regardless of pulley / belt size supplied, any alternator beyond this sizing (94 amps at 12 Vdc) will cause undue stress to the many engine components (bearings, water pump, pulleys, belts, ...). These components were designed by their respective engineers to be subjected to a certain side loading (normally rated in HP) and not more than that. Our research has found that this equates to no more than a 94 amp alternator being installed in this location, regardless if it fits or is made to fit. Some people install oversized alternators in this location and then subject themselves to over wear, belt problems, and premature failures of many parts. If the original manufacturer had designed this area to carry higher loads, they would have supplied a double pulley - not just on the crankshaft, but also on the water pump to ensure equal loading. One can add a double or serpentine pulley to the crankshaft pulley, but not the water pump. This also requires special loading and design. This is in fact what ZRD does when we design a Second High Output Alternator solution for our customers. In summary, simply install no more than a 12 Vdc - 94 amp alternator in the standard OEM engine location. There are even some OEM engines that restrict this even further, but not many. If your needs dictate, adding a Second High Output Alternator is the solution.

Bad Idea

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Now one of the most common prospective customer concepts (thoughts) - Gentlemen: I currently have a sailboat with a xxxx (engine manufacturer) yyyy (model) and a high-output (100 plus amps) alternator. I am interested in mounting a dual V-belt pulley (crankshaft) on the engine which would then allow a second belt to be attached to a dual-V pulley (also replaced) for the alternator. Either ZRD or I can supply the alternator dual-V pulley. the problem is that the current setup only has about a 70-degree "wrap" on the alternator and under load I am shedding significant belt dust. The second V-belt would give me nearly a 180-degree wrap on the alternator. I am not interested in running this second belt around the fresh-water circulation pump on the engine. That works just fine with the existing setup and I don't desire to further side-load that pump/shaft. I am simply trying to give the alternator more "belt" to work with.

Let us slowly analyze this concept.
• First, most did not have the opportunity to discover and read this specific FAQ or they did not pay attention to the previous paragraph.
• Engine manufacturer (other than Westerbeke), model is irrelevant.
• Someone installed an alternator exceeding 94 amps on a single 1/2" v-belt (assuming they are not on a 3/8" - even worse).
• Only a single sheave PTO would be required.
• Adding a second belt does not change the 70 to almost 180 degrees of wrap. It just allows 2 belts at 70 degrees.
• Odds are that this PTO sheave probably has less than a 1% chance of even being close to being in any proper position.
    -To allow a second belt to be attached connecting PTO sheave and alternator second sheave.
    -To allow a second belt to avoid obstacles in the way.
    -Many other problems, no need to list all of them.
• Two belts go around the alternator sheave.
    -Belt one goes around the crankshaft pulley, then the fresh water pump,and then the first sheave on the alternator.
    -Belt two goes around the crankshaft PTO pulley and then the second sheave on the alternator.
    -Obviously (unfortunately, not to all), two belts of different lengths would be required.
    -Tensioning both belts equally is not possible regardless of tensioner used.

Even if we assumed a 1% scenario existed, this last item is a 100% deal breaker.
Having two different size belts would require you to get custom made belts to be within a few hundredths of an inch in length.
Being that the alternator would be the tensioning device common to both belts, even the 1% rule would be slim allowing both to tension equally like a matched set.

The bottom line (as they say), is that this is a reasonable expectation, but unfortunately, it has no realistic possible solution as initially intended. The only choice is to Contact ZRD to discuss your current configuration and needs and allow us to provide you a first rate solution - with or without a second alternator. It comes under the scenario - "Been there Done that".

A recent email describes and highlights much of the above information.
This past summer I had a 105amp alternator installed ... The marina mechanic said the load put on the engine with this size alternator would not be a problem. About 3 months later the fresh water circulation pump on the engine failed. I talked to several people at a boat show and all agreed that the alternator put too much load on the pump and caused the failure. I came across a ZRD advertisement and after reviewing the ZRD web site I felt that your company should be my first stop to help solve my problem. Having spent over $1,000 on the alternator and replacement pump I need to correct the current set up so I do not have problems in the future. Can you provide a feasible solution to resolving the load issue?" Our obvious answer was Yes. Vessel owners and vendors typically overdo the alternator in the first (OEM) position - even if they do not need it. Too many vendors oversell or sell the wrong equipment to their customers - just to make a sale and generate revenue. ZRD provides only proper solutions. Our customers take that to the bank.

Choosing between Extended Runtime and Standard DC Generators
Basically it comes down to the difference between someone that operates their vessel like a distance cruiser and someone that desires to use their vessel more like a floating condo. ZRD is unable to control a user's demand or needs - or should we. Therefore if a user's needs require “excessive use” (greater than 45 minutes to 1.5hrs /day {temperature dependent} of continuous Generator Run/Recharge time without proper complete cool down), ZRD highly suggests the choice to purchase an extended runtime vs. a standard version to prevent equipment damage. One could choose to operate a standard unit version in its reduced output mode to prevent damage. This is the only reason ZRD installs external regulators on our standard High Output Alternators and Generators. Operating a standard unit version in its reduced output mode requires longer run times to accomplish the same task vs. the run time required by an extended runtime version. The downside of running a standard version in reduced output mode is slightly extra fuel consumption and additional greenhouse gas & noise production. The only downside of extended Runtime DC Generators are their initial upfront cost to purchase, but if this is your "normal" need/requirement, in the long run it is the proper choice. It is very similar to someone using an engine OEM alternator vs. upgrading to a ZRD larger OEM replacement alternator or finally installing the ZRD High Output Second/Dual Alternator they should have initially purchased when they purchased their vessel.

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