# VW Electrics Alternator sizes

Power Requirements formula -> Directions

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:

**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 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:

64 Amps(80% of 80A) + 100 Amps(50% of 100AH) + 100 Amps(Average Stereo Load) = 264 Amps

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 even better for long term reliability).

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

Substituting the values of our example:

264 Amps X 120% = 316 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 -- IF you could find one that would work in your application.)

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 this situation, 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).

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NOTES:
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.

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 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 takes place, the implementation of sophisticated voltage regulation AND protective mechanisms for the alternator and/or batteries become critically important.

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