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Need Help Choosing A High Output Alternator?


Do you need help choosing an alternator for your boat? Are you considering a high output alternator made especially for the marine industry? If so read on.

Help choosing a marine alternatorA high powered marine alternator is designed and created to produce only a certain amount of power. This is the power that you need especially when the engine of the boat is idling. This power is what is needed to use the basic on-board equipment, such as;

  • fuel pumps,
  • engine ignition
  • controls
  • lights
  • instrumentation
  • AV equipment
  • and a few other necessary systems

The standard alternator is defined by the maximum degree of power that it can produce, that is, when the engine is operating at maximum speed but this standard cannot maintain this for more than a couple of minutes. After that it will overheat and cause potential damage. Believe it or not, the most this type of alternator can accomplish is only about half the maximum rated output power.

Getting a new and sealed battery

Another factor that complicates things, and is a reason to consider an alternator change, is when you get a new sealed battery, which comes with components such as absorbed glass mat (AGM) and gel cells. These battery components need different charging voltages and require different techniques than the ones used with common lead acid batteries. They are different enough that merely replacing with a new sealed battery in a boat that has a stock alternator with a non-adjustable voltage regulator can require a new alternator just to protect this new battery from getting too high of a charging voltage.

But the real reason that you are likely replacing your alternator with a high output model is because of all the other things. You need to power the awesome stereo system you just installed, or you have both your tablet, smart phone, and computer that all need to keep charged, or you have emergency equipment that takes a bit too much power than normal, or any number of things that just add up to needing more power that the current alternator can handle. That is really why an alternator upgrade just makes sense. You need an alternator with higher amperage.

So how do you choose the right one?

At the core of this question is figuring out how many amps you expect to be needed for the job. A very basic equation that can be used is formulating: the maximum vessel electrical load plus 50% of the total battery bank capacity, plus average current that is required to supply all of the non-stock loads. This should roughly help you determine minimum continuous duty output amps needed for this new alternator.

Let’s look at an example. Let us assume you have an 80 amp alternator, an easy assumption since it is a common “standard” amperage. To calculate the maximum vessel electrical load, a common tactic is to assume that it is 40% of this. So this comes to 32 amps.

Let us say that the total battery bank capacity is 180 amps. So 50% of this is 90 amps. And to keep it simple, let us say that the non-stock load all comes down to one electrical device amounting to 100 amps of extra current.

So 32 amps plus 90 amps plus 100 amps equals 222 amps. Keep in mind that this is the amps that would need to be continuously produced by this new alternator. This is just the power that is needed for when you are idling or when you need to run just the auxiliary engine operating speed for your basic marine charging applications.

To determine the size of the alternator you need in the real world, the one you actually need fitted that will handle the load, common practice is to multiply this minimum continuous duty output of the alternator by at least 120% to 150%.

So multiplying the minimum continuous duty output by 120% puts us at 266.4 amps. That is our final estimate. So what is likely to happen is that you would want 300 amps of capacity to make sure you cover your bases.

What is important to note is that you most likely can’t fit a 200 amp alternator into the engine compartment. It will be too large to straight out replace the previous stock alternator. Most likely a continuous duty, heavy duty 300 amp alternator is going to be 12 inches by 18 inches, which is very difficult to fit in your standard engine compartment.

It varies. If this is the case though, assuming you have the space or the space issue is solved to do so, installing two alternators that cover the roughly 300 amps you want, will suffice. This could mean just adding an alternator to cover specifically the non-stock load.

With all of these considerations in place, you are well on your way to determining and selecting the high output alternator you need. Before you know it, you will be well on your way to worry-free power usage on your boat, coasting off into the sea with that stereo system loud and clear.