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The Heart of an Outboard Electrical System By Clark Beard, CDI Electronics, and Bill Grannis, Am-Tech Charter Member

When electric starters first became commonplace on outboard motors back in the mid 1950’s, batteries only supplied the “juice” for starting and perhaps to power the boat’s running lights. At that time there were no charging systems, so the boater of 50 years ago had to charge the battery every few weeks. Soon, generators became available to replace the power that was used for starting, and in later years, flywheel alternators provided increased output. This improvement became necessary as more and more electrical items were added to boats. These additional accessories severely strained the early electrical systems. Today’s motors with electronic fuel injection (EFI) and direct fuel injection (DFI) systems, along with their electric fuel pumps and power hungry computer modules, make the battery an integral and very important part of the system. Any glitch in the electrical system on a modern motor can leave the boat dead in the water instantly.

BATTERY TYPES Unfortunately, most boaters make their battery selection based on price instead of what works best for their motor. Therefore, it becomes your responsibility to not only educate your customer on the proper battery for their system, but to dispel the many common battery myths that are running rampant as a result of the Internet boating forums.

Every outboard manufacturer recommends a starting type battery as the main electrical power source. These batteries have numerous thin plates that are designed for quick bursts of power. When they are operated below 50% discharge, however, their life is severely shortened.

Deep-cycle batteries, on the other hand, have fewer but thicker plates, and are designed for long-term, low current draw, such as powering electronics, cabin lights, stereos, pumps, etc. They can be discharged and recharged repeatedly, but they are not designed to deliver quick bursts of power.

There are combination starting/deep-cycle batteries that have both good starting characteristics, as well as some deep-cycle capacity that can be used for either application, but they only offer a compromise and may not last as long, or work as efficiently as a dedicated battery for each particular purpose. Having a main starting battery and a secondary deep-cycle battery on a selector switch offers the best of both worlds.

The most common boat battery is the traditional lead-acid “flooded-cell� design that has removable caps for adding distilled water as needed. These batteries tolerate higher charging voltages, especially from unregulated outboard alternators. The maintenance-free battery has calcium or antimony added to the plates to minimize gassing during recharging, but these batteries are subject to early failure and/or electrolyte loss if the charging voltage is not regulated, or if it becomes severely overcharged. Since there is no easy way to replenish the lost water, the electrolyte level diminishes and the plates are exposed to air, lowering the battery’s power output. Most outboard companies do not recommend a maintenance free battery except for their engines that have better charging voltage regulation.

The sealed battery style comes in two flavors, AGM (absorbed glass mat) and gel-cell. These batteries cost roughly two to three times that of traditional lead-acid types. They are very different internally and both types are super sensitive to charging current, and both require special voltage regulation to keep from shortening their life.

AGM batteries have their electrolyte contained in boron-silicate glass mats suspended close to the active material on the plates. This increases efficiency making them the long lasting batteries, as long as they are not discharged below 50%. The exception would be the deep-cycle versions. These batteries are more tolerant to voltage and overcharging compared to the gel-cell models, but still are more sensitive than traditional batteries.

Gel-cell batteries use a thickening agent such as silica to solidify the electrolyte to the consistency of Jell-O, thus making it truly spill proof. This design makes gel batteries very good for deep-cycle use, but they are very sensitive to charging currents and voltage limits. If these limits are exceeded, bubbles could form in the gel, which insulates the electrolyte from the active material on the plates. This results in the loss of power output and shortens the battery’s useful life.

POWER RATINGS Marketing departments of battery companies have a tendency to make specification numbers sound big, giving the consumer the impression that bigger is better than another company’s product. They are actually reporting the battery output at higher temperatures. This increases the internal chemical reactions but it skews the results compared to industry standardized testing.

CCA, or cold cranking amps, is the maximum current output when tested at 0°F for 30 seconds without the battery dropping below 7.2 volts. All batteries use this procedure as an industry wide standard of testing.

MCA, or marine cranking amps, is the same test except at a temperature of 32°F which results in a higher number than the CCA specification.

RC, or reserve capacity, is the time in minutes that a battery can deliver a constant 25 amps until the voltage is decreased to 10.5V.

AH, or amp hours, was the standard for battery capacity years ago but is not normally mentioned in specifications today. If the reserve capacity (RC) is multiplied by 60%, the answer will be a close approximation to the AH.

CHOOSING BATTERY SIZES Outboard and inboard manufacturers list a minimum battery size for each of their engine models. For example, a Mercury Optimax requires an 800 CCA, or 1000 MCA, battery, while a 50 hp Johnson only needs a 360 CCA, or 400 MCA. Yamaha recommends a 512 CCA, or 675 MCA, battery for their 250 hp 4-stroke outboard and advise against using sealed, gel-cell, or maintenance free batteries with their motors. Always follow the manufacturers recommendation for best results.

Keep in mind that the minimum specification is only for the motor and does not take into consideration the boat’s electrical requirements. An offshore fishing rig with a console full of electronics, multiple live-well pumps, and radar will require much more battery capacity than a runabout with the same size motor. A good rule of thumb is to add an additional 20% battery capacity over the engine manufacturer’s minimum specification.

Industry group numbers designate physical battery size and terminal placement. A group 24 is the most common marine-battery size and nominally measures 10 3/4” long, 6 ¾” wide, and 9 ¾” tall. Deep-cycle batteries are often larger in size and come in group 27 and group 31 designations.

TESTING There are various methods and meters to test a battery, and most technicians become familiar with the procedure with which he or she was taught. In the past, the hand-held load tester was handier than the larger carbon pile version that was around since the turn of the century. For wet-cell batteries, a digital voltmeter and a hydrometer will do. The new electronic testers are much easier and quicker to use. They generate a signal through one post of the battery, and sense the output from the opposite post. Sulfation, bad cells, or internal damage distort the signal and the meter displays either a green or a red light indicating the battery’sl health.

A quick method to test for battery condition, as long as it is fully charged, is to crank over the engine for approximately 15-20 seconds with the ignition disabled. Monitor the voltage at the battery terminals; it should stay above 10.5 volts for the entire cranking time if the battery is in good health and of the proper capacity.

UNREGULATED SYSTEMS Many older motors, and most of the smaller and mid-size engines, do not have regulators on the charging system. Actually it is the battery that acts as a regulator for these motors, the larger the battery, the better the voltage regulation. Luckily most of these systems are only 4 to 9 amps output, so they won’t boil away the battery electrolyte. Some battery powered CD engines had unregulated charging outputs as high as 16 amps, but the ignition consumed up to 10 amps, leaving the remainder to charge the battery.

As a battery charges, heat is produced and the voltage starts to climb. It is not unusual to see readings in the 16 to 18 volt range during extended running. Years ago this was not a problem, but on today’s digital electronics, over voltage can trigger shutdowns or activate an alarm. Turning on the running lights helps absorb some of this current, but installing a heavier duty battery can lower the excessive voltage easily. A group 27-style, compared to an inexpensive group 24-size, will control the voltage better on an unregulated system. Another way would be to install a second battery in parallel, which will accomplish the same thing.

MAINTENANCE Clean and secure battery connections are required for a dependable electrical system. Loose connections can cause intermittent and voltage spike problems. OMC/BRP has recommended for years using lock washers and hex nuts tightened with a wrench. They strongly advise against using wing nuts which can loosen. Yamaha’s latest service manuals make the same recommendation.

Corrosion is an ongoing problem, especially in saltwater areas. Be sure the tops of the batteries are kept clean and the connections are sprayed with a non-hardening electrical sealer for protection. In the past, the use of grease or Vaseline did the job, but messy, dirt attracting substances, melt in the heat, making cleanup a real chore.

High temperature is the battery’s enemy, yet many boat companies install them inside the engine compartment. Not only do the summer temperatures affect them, but the heat from a nearby engine helps to shorten their life.

CONCLUSION Include battery maintenance as a part of your customer’s annual tune-up or seasonal check-over, and use a battery tester whenever any type of electrical

servicing is done. Inform your customer that he should have the largest capacity battery that he can afford. This will minimize future problems and disappointments. Having a powerful enough battery of the correct style, combined with proper maintenance, will insure your customer’s safety and enjoyment on the water.

Understanding Batteries  

The heart of an Outboard Electrical System

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