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A Basic Secondary Electrical System

In most vehicle applications where high DC voltage requirements exist, IF space is available, using a SECONDARY ELECTRICAL SYSTEM to supply ALL non-stock high amp components is, in our opinion, by far, the BEST solution to the problem.

A secondary electrical system is simple to understand.

In addition to the stock alternator, a high-amp alternator (or multiple high amp alternators), along with suitable voltage regulation controls, is installed on the engine with its output connected to a high capacity, deep cycle battery (or batteries). Thus creating an electrical supply circuit which is separate from the primary battery's electrical system. This second electrical system is then used to supply the electrical needs of all high amp nonstandard/special equipment which has been installed (i.e., winches, high power audio systems, radio equipment, DC to AC power inverters, video equipment, etc.).

Since this is also the best way to install welding equipment on a vehicle, our power generating units have been designed to perform perfectly in this type of secondary electrical system. Best of all, we have developed bracketing accessories and installation methods which may help you in implementing a second alternator installation (or more sophisticated multiple alternator secondary charging systems). We have even developed a method for easily adding a single second alternator to a typical vehicle engine. We call this a piggyback installation, and it is the most popular method of installing our alternators when they are to be used as part of a secondary electrical system.
CLICK HERE for a diagram of this type of installation

We also have a number of pictures available which illustrate secondary alternator installations in welding applications. However, the same installation methods are used for secondary alternator installations. CLICK HERE to go to our Web Site Map for descriptions/links to pictures of typical welding system installations.

If you are planning your piggyback installation into a vehicle with sufficient under hood room, and if you have a serpentine belt drive system for under hood accessories (alternator, PS, AC, etc.) consider Building an Add-A-Pulley accessory which provides a means to securely attach a second V-pulley in front of any existing alternator's serpentine pulley - providing an easy mechanism to drive the secondary alternator which is being installed. Call or e-mail for instructions

Our A300A and TBUCK bracketing accessories are also popular installation accessories, for 150A unit installations, and can further simplify the installation process.

NOTE: For 200A units, similar generic bracketing accessories are available.

Finally, for a vehicle installation, you may want to consider adding an automatic speed control to provide a means to increase engine speed when it is necessary to obtain full power from your alternator when the vehicle is stationary.

A number of considerations exist which should be addressed before selecting, and fitting, a high-amp alternator to an existing vehicle.

Mechanical installation is one factor:

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.)

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

A suitable drive pulley will have to be obtained -- and possibly 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.

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 needs to produce.

Min. Alternator Continuous Duty Output Amps
Maximum Vehicle Electrical Load
(Use 80% of stock alternator capacity)
50% of Total Battery Bank Capacity
(in Amp-Hours)
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 if you 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:

264 Amps
64 Amps
(80% of 80A)
100 Amps
(50% of 100AH)
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 264 amps CONTINUOUSLY.

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 by AT LEAST 120% (150% would be better for long term reliability).

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





Substituting the values of our example:

316 Amps
264 Amps





From our final calculation, we see that the proper alternator for this application would need a charging capacity of 316 amps. (A 300 Amp alternator would be OK.)

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

In this situation, assuming adequate space in the engine compartment, installing a second alternator dedicated to supplying 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). CLICK HERE for more on this.

These formulas are offered as a guideline. They are most applicable to vehicle applications where high ambient air temperatures exist (say, 130 to 170 degrees Fahrenheit) and where conventional, high quality, voltage regulation equipment will be employed.

Some minor deviation from these formulas may be possible -- depending on individual conditions. For example, by using a very sophisticated high end voltage regulator combined with an alternator, engine compartment, and battery temperature monitors, it would be possible to lessen the effect of the 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.

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.

Other Serious Considerations:

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.)?

  • Have you selected an appropriate voltage regulator for the type of batteries in use and the high current alternator which is ultimately selected?

  • Will it be necessary to monitor/control battery temperature when charging?

  • Have you replaced stock wiring with large enough cable to insure safe operation.

    Will you void your vehicle's warranty by installing a non standard charging system.

    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 -- when operating at its maximum rated output current.
    Therefore, for a 150 Amp alternator, operating at full output, the expected engine load would be about 5-6 hp.
    If using a gasoline engine instead of a diesel, with a slightly lower 2.5:1 drive ratio, figure on a requirement of about 1 hp per 15 amps of output power -- i.e., 10-12 hp for a 150A alternator.

    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.

    Will a provision to disable or reduce charging current (and, therefore, mechanical load on the driving engine) during times where maximum engine performance is required, or at other times when minimum engine power is available (i.e., starting, idling in traffic)?

    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. (i.e., installing or building a suitable tachometer).
    (CLICK HERE for a link to a nice set of design plans showing a way to do this).

    For Other Ideas/Considerations,
    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, 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 (i.e., a small battery and alternator for vehicle operational requirements, and a second high output alternator and 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. CLICK HERE for more info.


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