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ZENAĻ DC Power Generators

Choosing a High Power Alternator

A Few Basic Facts about Conventional Alternators

photo of SR150B power generator

ZENA SR150B 150 modular power generator
150A @ 12-14.5VDC / 80A @ 24-29VDC

photo of SR200 power generator

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. The ZENA DC Power Generator is this type of alternator -- 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.

Except in cases where only a little more charging current is required to handle, for example, a few extra lights for trailer towing, or a bit more output to help get a quicker charge during winter months (when a lot of your driving is done at slow engine speeds in icy/wet 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 a single size 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 alternators, 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, alternators also were 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, in many cases, alternators also became 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 you need to provide power for emergency equipment, or to drive a huge CB radio transmitter, or to power that a 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 an 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 are looking for.

Beware of "rewinds" or rebuilt "stock" alternators purported to 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 alternators larger than, say, 200A to 250A 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 mush better to plan on using multiple alternators when you need to produce 300A, 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 used for welding -- continuously producing an arc powerful enough to melt through 3/8" thick steel plate -- and well capable of vaporizing connected 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 with this equipment. 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 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:

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 marine 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 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 OK -- 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 about 12" in diameter by about 18" in length -- 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 routine 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:

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

Other factors:

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.

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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