Why invest in a high-output alternator?

Most standard alternators are designed to meet the needs of the engine and associated electrical demands. Yet, in many marine applications, vessel wide electrical loads greatly exceed what that standard alternator is capable of providing.  There are many opinions regarding the optimal battery and alternator sizing but the following information should assist in creating a balanced electrical system.

How many batteries will be required?

For most marine applications, battery capacity should range from two to three times daily energy requirements. This is based on the expectation that your batteries will be recharged on a daily basis. If you plan to go for longer periods without charging, battery capacity will need to increase proportionally.  In other words, if your daily usage is 100 amps, and you anticipate charging each day, your battery capacity should be between 200 and 300 amp hours. If you plan on going for three days between charges, a battery bank providing 600 to 900 amp hours will be required.

Which type of battery?

Just how quickly a battery can be recharged can be determined by the battery's ability to accept charging amperage provided by the alternator or other charge source. This ability to take a charge is called the battery's "acceptance rate", and will vary based on the type of battery you are using. In the marine industry, there are three primary battery technologies you'll find: flooded lead acid, gel and AGM.

Flooded batteries -- which use a liquid medium to carry electrolyte between the battery's plates – can accept roughly 25% of the battery's available capacity at any given point of the charging process.

Gel batteries -- rely on a gelatinous silica slurry to suspend the electrolyte conductor, which surrounds the battery's plates. Gel batteries can accept as much as 40% of their available capacity during the charging process.

Absorbed glass mat batteries -- use a densely packed fiberglass matting to hold electrolyte in suspension. Estimates regarding AGM battery acceptance rates vary depending on the manufacturer, but 40% seems to be a realistic expectation.

How big an alternator?

The alternator's rated output should meet or exceed battery acceptance rate, so once we know 1) what daily energy demands are, 2) what battery capacity is required, and 3) what battery technology is being used, it’s likely that the standard alternator will be woefully inadequate for the house battery loads. Suppose that 400 amp hours of flooded batteries suits the vessel's needs, and the acceptance rate of our batteries is 25% of available capacity, we know that the alternator needs to provide as much as 100 amps of charging current. If we're using gel or AGM batteries, an alternator capable of 165 amps may be better equipped for the task.

What limits alternator capacity?

There are some limiting factors to consider when setting up your charging system. First, the engine must be capable of driving the alternator. In terms of horsepower load, an alternator will require roughly one horsepower for every 25 amps of output -- so a 100-amp alternator under load will demand about four horsepower from the engine. Most engines are equipped with a drive belt that's matched to the needs of the standard alternator. As a result, the output of the replacement alternator will be limited to the capacity of the existing belt. For most applications, the following guidelines will ensure proper operation, with minimal undue engine or belt wear:

3/8" Single Belt - 80-amp max. alternator load

1/2" Single Belt - 110-amp max. alternator load

1/2" Dual Belts - alternator loads greater than 110A

Serpentine belts - alternator loads greater than 110A

Note: Belt width, belt wrap, belt tension, engine rpm, engine compartment temperature and belt alignment can have an effect on alternator performance and system health. "Drop in" compatibility is not assured in all applications.  Some modification may be required to ensure proper alternator fit, pulley alignment, or to ensure satisfactory cooling. Belts should be inspected for proper tension every 30 minutes when new, and before each engine operation thereafter.  Should your belt show signs of slippage (glazing and/or excessive black dust on the alternator or engine) it may be necessary to reduce maximize field potential with the Max Charge regulator’s Amp Manager function. See details in the Max Charge Installation Manual.