• Marine Alternator Tech Tips

    ENGINE HORSEPOWER REQUIRED FOR ALTERNATORS
    For every 23 AMPS of alternator output about one horsepower is required.
    FOR EXAMPLE: A 12 volt,115 AMP alternator requires 5 horsepower.
    (115 divided by 23 = 5 horsepower). A 24 volt unit requires twice the horsepower.

    ALTERNATORS ARE NOT BATTERY CHARGERS
    Alternators are designed to supply current for the accessory load and maintain the charge of the battery.
    Most alternators can safely charge at only two-thirds of their maximum rated output. When trying to
    recharge a dead battery, the alternator will charge at maximum output for extended periods of time causing
    the alternator to overheat. High heat destroys transistors, diodes and windings.

    ONE-WIRE ALTERNATORS CAN NOT BE USED WITH BATTERY ISOLATORS
    One-wire alternators, sometimes referred to as self-exciting alternators, require battery voltage at the output
    terminal in order to charge. Since battery isolators eliminate the battery voltage to the alternator, you must
    use a battery isolator with an ignition excite capability or modifications must be made to the alternator to
    allow ignition excitation.

    ALTERNATORS MUST TURN THE PROPER RPM IN ORDER TO FUNCTION
    Just because the alternator looks like itís turning, doesnít mean itís turning fast enough to charge. Most
    alternators do not start charging until they reach 1,000 RPM alternator shaft speed. 5,000 RPM alternator
    shaft speed is normally required to reach maximum output. If youíre not sure what the alternator shaft
    speed is, you can determine this with the pulley ratio. Measure the diameter of the crank shaft or drive
    pulley and the alternator pulley. Divide the crank shaft pulley diameter by the alternator pulley diameter. This figure would be the engine-to-alternator RPM ratio. A normal ratio would be 2.5 to 1. For example, letís say we have a 7 inch diameter crank shaft pulley and a 2.75 inch alternator pulley. We would divide 7 inches by 2.75 which equals 2.54 to 1. If the engine was turning 1,000 RPM we would multiply 1,000 by 2.54 which would give us 2,540 alternator RPM. Again, with todayís high amperage alternators, belt condition and tension are critical in proper alternator performance.

    IF YOU ARE ABLE TO TURN THE ALTERNATOR FAN BY HAND, THE BELT IS NOT TIGHT ENOUGH.

    ALTERNATORS WILL CHARGE WHEN TURNING IN EITHER DIRECTION

    NEVER DISCONNECT THE BATTERY CABLE WHEN THE ALTERNATOR IS CHARGING
    A common practice with the old generator system was to disconnect the battery cable while the engine was
    running to see if the generator was working. If this procedure is done on todayís transistorized alternator
    systems, severe damage to the internal components of the alternator usually will be the end result.
    This includes using a battery selector switch while the engine is running.

    ALTERNATORS APPROVED FOR MARINE USE
    Alternators that are being installed on inboard gasoline engines must be certified to meet Coast Guard requirements for ignition protection. In order for the alternator to be certified, it must pass the testing procedure, Marine SAE J1171, laid-out by the Society of Automotive Engineers (SAE).

    Brushes inside the alternator cause some sparking when the alternator is charging. This is normal for any alternator. When the alternator is exposed to a flammable atmosphere, such as an enclosed engine compartment on an inboard gasoline application with a fuel leak, the sparking from the brushes in the alternator may cause an explosion.

    The Marine SAE J1171 testing procedure is as follows: A sparking device, similar to a spark plug, is installed in the brush area of the alternator. Another device is also installed in the brush area of the alternator to supply a specified mixture of propane gas and oxygen. The alternator is then placed in an explosion proof test chamber. The chamber and the alternator are then filled with the explosive gas mixture. A high-voltage coil supplies current to the sparking device in the brush area of the alternator, causing an explosion. An explosion must not occur in the test chamber while this test is being performed. This step is repeated nine times. Finally, a spark is supplied inside the test chamber causing an explosion to ensure that the explosive mixture was present during the testing operations.

    This information is from the ARCO Starting and Charging Catalog which can be found on this website.