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Choosing a High Power Alternator

A Few Basic Facts about Conventional Alternators

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ZENA SR150B 150 modular power generator150A @ 12-14.5VDC / 80A @ 24-29VDC
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ZENA 200 amp. modular power generator 200A @ 12-14.5VDC / 125A @ 24-29VDC

A "standard" automotive or marine alternator is designed to produce a certain amount of power when the engine of the vehicle or vessel for which it is designed is idling.

This is the amount of power that the vehicle needs for its basic onboard equipment -- engine ignition/controls, fuel pumps, instrumentation, lights, climate control systems, etc.

This type of alternator is usually identified by the maximum amount of power that it can produce when the engine is operating at high speed.

However, it cannot produce this amount of power for more than a few minutes without overheating and potential damage.

The most power that such an alternator can be expected to provide continuously is about 1/2 the maximum rated output power -- possibly as much as 2/3 for a conventional "heavy duty" design.

Special design alternators which can produce their full rated output continuously are available. They are used in eergency veshicles, special utility vehicles, first responder vehicles, etc.

The ZENA® DC Power Generator is this type of alternator. It is built to a standard of construction that far exceeds that of the standard alternator.

Alternators made by companies like Leece-Neville and Niehoff are also designed to produce fully output power continuously -- under normal loading conditions.

Installing a high output alternator may be quite a bit more complicated that it was in the 1960's, 1970's, or 1980's when most American cars were equipped with an alternator with a case that was large enough so that it could be easily rebuilt, or replaced, with an alternator that appeared identical, but produced as much as double the current provided by the vehicle's original alternator.

Modern alternators are a much different prospect.

Typically, for a given application (vehicle/engine), the alternator output is designed to exactly meet the vehicle's electrical needs when operating at low engine speeds -- and the alternator is built to be as physically small and light as possible (often making cooling under high load conditions very difficult) to help the manufacturer achieve better vehicle fuel economy.

Often, with these "stock" alternators, charging power produced at low engine speeds is kept relatively low (or even turned off) to reduce the load on the engine, again to achieve better fuel economy.

With the addition of computer controls in vehicles, and on vehicle engines, srock alternators were also changed to act as protective devices for the installed computer systems -- further limiting their maximum charging capacity -- and making it even more difficult to rebuild or replace them with non-stock higher amp units.

Also, stock alternators have also become active participants in the engine's computerized electrical controls systems.

In fact, their interaction with the vehicle electrical system is so critical that, in most cases, replacement of a stock alternator with one that is not supplied by the vehicle/engine manufacturer will void the vehicle warranty and may also make the vehicle impossible to service with factory diagnostic equipment!

Making things even more complicated are newer sealed batteries such as gel cells and AGM (absorbed glass mat) -- batteries (typically, after market components, that require different charging voltages and techniques than those used for the common lead acid battery).

These batteries are different enough so that simply replacing an existing lead acid battery with a new sealed type battery in a vehicle equipped with a stock alternator with a built in, nonadjustable voltage regulator may require an alternator change just to protect the new battery from too high of a charging voltage!

But you STILL need a high amp charging system to power that super stereo system you installed, or to provide power for emergency equipment, or to drive a huge CB radio transmitter, or to power that huge winch that you installed, or to supply current to an inverter producing on board AC power for tools and equipment, to drive an air conditioner on your RV or trailer, or even power your RV (or trailer's) complete, electrical system.

Don't give up hope. It's certainly possible. BUT, to do it right may be a bit more difficult and/or expensive, than you thought. (Possibly a LOT more expensive and difficult if you have a very small vehicle.)

Don't expect that replacing a modern existing/stock alternator with a high current alternator is a simple bolt-on retrofit.

Almost certainly a true continuous duty high amp replacement alternator will be significantly larger than the alternator that it is replacing.

If it's not larger, it's unlikely that it will be able to provide you, continuously, with the additional current that you're looking for.

Beware of "rewinds" or rebuilt "stock" alternators which purported can provide significantly more current than they did originally.

It's possible to build such an alternator, and to see some additional current at low speeds -- but it's highly unlikely that it will survive in a high demand environment where operation at full power for more than a few minutes is required.

Want an easy way to check on this? Just ask the rebuilder how long of a warranty that they provide on their modified stock alternator.

Don't expect to find an air cooled 300A (or larger) alternator that is also small enough to fit into your engine compartment.

Because waste heat produced by an alternator when supplying severe duty loads increases with the square of the current generated, air cooled 12 volt alternators larger than, say, 200 Amps to 250 Amps become very large in order to cool properly. For example, about 12" in diameter by about 18" in length would be typical measurements for a 300A air cooled unit.

Reasonably sized, liquid cooled, 300A and 350A units are available (most are used in bus service applications) -- but you have the extra complexity of providing the cooling system required to use this type of alternator.

It's much better to plan on using multiple alternators when you need to produce 300 amps, or more.

Don't expect that any alternator capable of producing more than 80A, including so called marine units, will not also be capable of being a source of ignition in case of malfunction, OR wiring failure, OR a failure to take protective measures during installation.

When you consider replacing a low amp alternator with a high amp device that can produce more than, say, 80A in output current -- particularly when contemplating use in vessels that use flammable (gas) rather than combustible (diesel) fuels -- you must take care with your installation so as to protect your self from fire hazards inherent in the design of such devices.

For example, our 150A alternators are capaile of use for welding -- continuously producing an arc powerful enough to melt through 3/8" thick steel plate -- and well capable of vaporizing wiring which is too small, too loose, or improperly terminated. Not at all comparable to a low amp stock alternator designed for simply recharging a small starting battery and supplying power for a few low current engine controls, engine systems, running lights, and basic engine instrumentation.

In short , don't expect a high amp alternator (ours or any other alternator with similar specifications) to be inherently safe to use as a simple "bolt in" replacement for a low amp stock alternator in an automotive, or marine, environment without taking extra care, during the installation process, to insure that you have fully protected yourself and your vehicle, or vessel, from the ability of such a device (even when functioning normally) to produce a powerful arc and/or to melt a piece of thick cable -- when/if conditions are right.

Proper fusing, positive ventilation fans (properly operated and installed), "cold" alternator start and/or delayed/reduced charging current output, temperature sensing voltage regulators/controllers, etc. should not be overlooked.

If your vehicle (or boat) is still under warranty, replacing your stock automotive (or marine) alternator with anything other than another alternator made by the vehicle's, or vessel's, manufacture will likely void your engine and electrical systems warranty.

Also, if your vehicle or vessel was built after 1990, and if the vehicle or vessel relies on computer circuits for engine or other systems control, and if proper maintenance requires the use of automatic diagnostic equipment supplied by the vehicle's or vessel's manufacturer, replacing your stock alternator may make it impossible for you to obtain service.

Often, any change to the electrical system can affect the ability of the diagnostic equipment to function. And/or the chance of damage to the diagnostic equipment caused by changes in the electrical system may cause the service provider to refuse to work on the vehicle or vessel.

A number of other considerations exist which should also be addressed before selecting, and fitting, a high-amp alternator:

Mechanical installation is one factor:

As stated previously, the existing alternator is likely to be considerably smaller than a high output replacement. (Be wary of alternators "rebuilt" to produce higher outputs. Sometimes this can provide excellent results. But, often, results are less than satisfactory. If possible, speak to others who have used these units in identical applications.)

A true high amp unit may mount differently -- requiring a specially fabricated conversion mounting bracket.

A suitable drive pulley, for your application, will have to be obtained -- and/or may need to be modified slightly to fit properly.

The ratio between the alternator drive pulley and the primary engine drive pulley (usually the crank pulley) must be checked to insure that maximum alternator output will be available at your desired engine rpm AND that alternator over-speeding does not occur at higher engine speeds (a VERY IMPORTANT consideration).

Don't expect to get high output current at low engine rpm. If you need high current at low engine speeds you may need to install a generating system (possibly multiple alternators) with a capacity large enough to provide the power that you need at the engine speed that you desire. For example, you may need to have a 600A charging system to provide 200A when operating at low speeds -- even though you may never use the extra amperage produced at higher speeds.

A high amp alternator will take more engine power to drive that the stock alternator that it may be replacing. Sometime A LOT more power. In most cases this means that a heavier V belt, multiple heavy duty V Belts, or (best choice in most cases) serpentine belt drive may be required to drive the new alternator.

With some engines, installing a pair or alternators on the engine on opposite sides of the engine may be necessary if the engine is one with main bearings that is sensitive to side loading. For example, by placing one at the 2-4 o'clock position and one at 10-8 o'clock position load on the engine bearings can be virtually eliminated,

Do you know how large an alternator that you need?

Is your chosen high-amp alternator replacement big enough for the job?

To determine the minimum size of alternator that you need for a typical automotive or marine application, use the two simple formulas/calculations shown below.

The first formula calculates the minimum amount of CONTINUOUS DUTY output current that your alternator (or your charging system) needs to produce, at engine idle, for vehicles -- OR at the desired auxiliary engine operating speed for a marine or of grid battery charging applications:

Min. Alternator Continuous Duty Output Amps
(At your planned operating speed)
Maximum Vehicle Electrical Load
(for vehicles, typically, 40% of max stock alternator output amps.)
50% of Total Battery Bank Capacity
(in Amp-Hours -- NOT cold cranking amps)
Average Current Required to Supply Non-Stock Loads

For example, assume that you have a vehicle with a 80 amp alternator, and you are planning to charge a 200 amp-hour battery bank, AND that you also have an exotic stereo system in your vehicle which requires an average of 100A to operate.

We can enter this data into our formula, and solve it:

232 Amps
32 Amps
(40% of 80A)
100 Amps
(50% of 200AH)
100 Amps
(Average Stereo Load)

From the formula, we find that an alternator properly sized to handle this load would have to be able to produce at least 232 amps CONTINUOUSLY -- when operating at engine idle for vehicles -- OR at the desired auxiliary engine operating speed for marine battery charging applications.

To determine the actual size of the alternator that should be fitted to handle the load that we have defined, multiply the minimum alternator continuous duty output (at the required engine operating speed) by AT LEAST 120% (150% would be even better for long term reliability).

Required High-Amp Alternator Capacity
Min. Alternator Continuous Duty Output Amps
120 - 150%

Substituting the values of our example:

278 Amps
232 Amps

From our final calculation, we see that the proper alternator for this application would need a charging capacity of 278 amps. (A 300 Amp alternator would be even better -- IF you could find one that would work in your application.)

The formulas above are offered as a guideline. They are most applicable to vehicle applications where high ambient air temperatures exist (say, 120 to 160 degrees Fahrenheit) and where conventional, high quality, voltage regulation equipment will be employed. They also assume that you will be using a high quality external voltage regulation system that has the capability to monitor alternator temperature and moderate charge current should unexpected alternator overheating occur.

However, as stated previously, a single 300 Amp alternator will probably be much too large to fit into the engine compartment as a replacement for the stock alternator.

A typical air cooled, continuous duty/heavy duty 300A alternator is VERY LARGE and VERY difficult to fit in a typical automotive engine compartment. For this reason, companies like General Motors who need 300 amp alternators in vehicles such as busses have to use liquid cooling to keep the package small enough to fit to their engines.

In cases where even larger battery banks are used (say, 400 AH and larger), or where large AGM batteries are employed, or where the battery bank is made up of more than two units wired in parallel, or where deep discharge (say 50%) of the battery bank is routine, the implementation of sophisticated voltage regulation AND protective mechanisms for the alternator and/or batteries become critically important.

Don't Despair -- Viable Options for Getting the Amps That You Need Exist:

Good Electronic Controls:

For example, by using a very sophisticated, high end, voltage regulator combined with an alternator, engine compartment, and battery temperature monitors, it is often possible to lessen the negative effects of large battery bank capacity on the defined charging system -- possibly to the extent that a smaller 200A alternator might be appropriate for the example used in the formula.

Installing a Secondary Alternator:

For the example above, assuming adequate space in the engine compartment, installing a second alternator dedicated to supplying JUST the non-stock loads (with an additional battery or batteries installed to support the non-stock load) would provide the best solution (considering cost, operating efficiency, and protection for critical engine control systems that can be seriously damaged by low system voltages).

We can help here!

The same features that make our modular alternators so excellent for welding make them ideal for just such use! We even have an extensive catalog of bracketing accessories that can often make the job even easier. (CLICK HERE to view some bracketing accessories.)

CLICK HERE to take a look at some typical ZENA welder installations. A second alternator installation is basically the same thing -- just without the welding cables and controls.

Other factors:

What modifications will have to be made to your stock electrical system (charging indicator, means of starting/exciting your new high current replacement alternator, fusing, oversize wiring, type of voltage regulator, etc.)?

    1. Have you selected an appropriate voltage regulator for the type of batteries in use and the high current alternator which is ultimately selected?
    2. Will it be necessary to monitor and control battery temperature when charging?
    3. Will it be necessary to monitor and control alternator temperature when charging?
    4. Have you replaced stock wiring with large enough cable to insure safe operation.
    5. Will you void your vehicle's warranty by installing a non standard charging system.
    6. Is your engine large enough?
      For example, if you are driving your alternator with a diesel engine, and assuming adequate flywheel mass and a 3:1 engine drive pulley to alternator pulley ratio, a high efficiency, high current, 12V alternator will require about 1 hp per 30 amps of output power + about 1-2 hp for engine operating/muffling loads -- when operating at its maximum rated output current.
      Therefore, for a 150 Amp alternator, operating at full output, the expected diesel engine load would be about 7-8 hp.
      If using a gasoline engine instead of a diesel, with a slightly lower 2 to 2.5:1 drive ratio, figure on a requirement of about 1 hp per 15 amps of output power + about 1-3 hp for engine operating/muffling loads -- i.e., 11-14 hp for a 150A alternator.
    7. Is your charging system large enough to provide a proper load for your engine when being used only for charging at an appropriate engine operating speed?
      For example, if you are driving your charging system with a diesel engine you want the system to provide enough load so that the engine will be operating within its normal operating parameters (average alternator load when charging between 20-80% of engine's rated output power at the speed chosen for charging).
      Operating with too small of a load, for extended periods, can contribute to greatly accelerated engine wear. ALSO, operating at too low of an engine speed (regardless of load effects) can be bad for your engine -- carbon deposit buildup, poor cooling, lack of proper lubrication.
      We recommend that the minimum engine speed used for high amp charging be kept well above the point at which full engine oil pressure is produced and/or above the point at which full cooling system function is realized.
    8. If your vehicle has voltage sensitive computer engine system controls, your should determine if your high output replacement alternator must be equipped with special protective devices (such as avalanche diodes) to insure that over voltage conditions can not exist -- even if a defective voltage regulator or battery is in the circuit.
      NOTE: Avalanche diodes are used/required on most vehicles with computer controlled engine systems.
    9. Will a provision to disable or reduce charging current (and, therefore, mechanical load on the driving engine) during times where maximum engine performance be required, or at other times when minimum engine power is available (i.e., starting, idling in traffic)?
    10. Are there any other signal outputs from your stock alternator that may not available from a replacement high current alternator?
      For example, some vehicles rely on the stock alternator to produce an output for a tachometer.
      If this is the case, you will have to make provision for this in your alternator installation plan.
  • Other considerations for a high amp charging system

  • Take a look at electrical systems in commercial vehicles which have been specifically designed and/or modified to support high electrical loads, such as ambulances.

    • For example, will your application, like that of an ambulance, or other emergency vehicle, require a method of maintaining a high engine idle when the vehicle is stopped (either parked, or in heavy traffic)?

    • If you need this, we have low cost engine speed control accessories, such as our ASC1 which can help.

    • Should you have a dual charging system with multiple batteries? For example, a small battery and alternator for vehicle operational requirements, and a second (third or fourth) high output alternator supplying a large capacity battery array for high current accessory loads?

    If you need this, we can help here as well. In addition to alternators designed for this sort of use, we can help with system design information, and with bracketing accessories which can help you simplify system fabrication and installation. Call us for more info.


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