Big Alternators

Many boat owners consider adding alternator capacity, along with additional battery banks. These often are to power inverters or to just prolong time between charging cycles. When installing larger alternators they’re are many things to consider. Here are some suggestions:

1. What are the specific goals of the improvements to your electrical system?
Many start down the road without a clear idea of where they are going. Determine your loads, load time, and desired charge time and you can figure out just what you need. If you are not familiar with this, email us or give us a call and we can help.

2. How big is your engine?
Smaller engines may have limitations regarding how much power (torque) can be extracted from the forward pulley. Small one and two cylinder engines have limited power available to drive larger alternators. Let’s face it, if your current engine is barely able to move your vessel at the desired speed now, what will it be like when your “big” alternator is working at 100% capacity?

3. How big is the V belt on the engine driving the alternator and how much contact does the pulley have with the belt?
Larger V belts can transmit more power than smaller ones, but it is a little more complicated than that. How many degrees of contact does the belt have with the alternator pulley also affects how much power can be transmitted. Most engines have approximately 90 to 120 degrees of contact, with the belt driving a fresh water pump and the alternator. The V belt on some engines only drives the alternator and that means 180 degrees of contact which is ideal. The less contact, the less power can be transmitted which translates into increasing the number of belts.
The easiest way to increase the amount of power available for an alternator is to double up on the belts. The picture above shows an alternator with a double groove pulley, one for each belt. We recommend that two belts be used on all alternators above 750watts (12volts, 60amps) whenever possible.
Now just because the alternator you installed is working just fine with only one belt, it may not always be that way. Day sailing around the sound, without anchoring out will not discharge the batteries enough to bring the alternator to full output. We have installed large alternators on vessels where there was no additional pulley available and they work fine all along the ICW. When the captain takes the sailboat offshore is when the trouble starts. The second day when the crew goes to charge batteries, the single v belt smokes off the pulleys!!

4. How hot is your engine room?
Another thing to consider is the amount of ventilation that is available to the alternator. Everything on the engine is liquid cooled in one way for another. The alternator is the only thing that is air cooled. The more power being generated by the alternator, the more heat it must get rid of. If the alternator is called upon to produce maximum power at lower rpms, or the ambient air temperature is to high, the unit can overheat causing diode and bearing failures. Make sure that the engine compartment gets enough cool air when charging the batteries. Remember that cooling air is drawn through the alternator from the back of the unit through to the fan in the front. Any ducts that are installed to provide cool air should be directed at the aft end of the alternator

5. Mounting the alternator.
How the alternator is attached to the engine is very important as the higher output units can put considerable loads on the brackets. First, the pulleys must be aligned. You can use a straight edge to place against the front face of the pulleys to check the alignment. You may have to move the alternator forward or backward to bring the pulleys into alignment. Make sure that the pivot foot of the alternator has no lost motion or play in the mounting bolt. If there is any looseness in the foot of the alternator, the vibration will wear the ears of the alternator or the engine housing.

6. Regulators
Regulators control the alternator’s output according to the electrical systems needs. It does this by monitoring the voltage of the DC system. When the voltage drops, it increases the field current, increasing the output of the alternator. If the voltage rises above the set point it reduces the field current, reducing the output. As the speed of the alternator increases, it’s output increases, so to maintain the same output, the field current must be decreased as the rpm increases.

Regulators come in many different kinds. Some are integral, meaning that they are part of the alternator, either bolted in the back or actually inside the alternator. Others are remote, meaning that they are only attached to the alternator by wires and the regulator itself is mounted away from the alternator.

Additionally, regulators do not all function the same. Some are what we call “dumb” regulators, which only control the voltage of the system. Some regulators have small “computers” in them that adjust the charge rate according to time, type of batteries, or battery temperature. They may be called three step or smart regulators. More sophisticated regulators can increase your battery life and decrease the battery charge time. One of the most important aspects of regulators is the type of batteries they are designed to charge. Lead acid batteries, AGM’s, and gelcells all require different charge rates and maximum voltages. Do not mix the type of batteries in your boats system as if they are not uniform, they will not be charged properly.

7. Alternator wiring

Make sure that the wiring for this new alternator is of the proper size. Most engines are equipped with a 55-70amp alternator from the factory. The wiring will be sized for that amount of current. Once you go to 120amp alternator, you should have 2ga. cable connecting it to the boats DC system. If the engine harness utilizes a ammeter at the helm station, it will have to be removed from the system. The distance from the alternator to the batteries should be as short as possible to reduce voltage loss.

Do not forget the ground cable when you are wiring your alternator. Just because the case of the alternator is grounded to the engine, it doesn’t mean that the ground path is able to handle the higher current. Install a 2ga. cable from the case (or ground terminal) of the alternator directly to the ground buss of the boat. This will eliminate any voltage drop through the engine block. Steel isn’t a very good conductor of electricity as compared to copper.