Marine Electrical Alternators, Not all are Created Equal

by | Sep 3, 2021 | Marine Electrical Auckland | 0 comments

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How to Install and Maintain Electrical Systems on Your Boat: A blog around electrical systems installation, maintenance, and troubleshooting.

Alternator Regulation

A few years ago a 57-foot single screw cruising vessel, with a 12-volt house battery bank and over 1000 amp-hours of capacity. Would have been considered a huge battery bank, by today’s onboard electrical standards it’s moderately sized for a serious cruising vessel. The response, “the batteries aren’t lasting long enough between charging cycles, in the morning the batteries are nearly dead”, confirmed my suspicion. Indeed, the battery bank required all too frequent re-charging, but not because it was undersized.

The vessel was equipped with a stock alternator supplied by the engine manufacturer, with a 100 amp capacity.

Alternators, Not all are Created Equal

Most marine engine manufacturers follow a similar regimen, the alternator is designed to charge the engine’s own starting battery, as well as supplying power for pre and post-heat systems, instrumentation, electronic engine control if so equipped etc. The average engine start cycle requires less than one amp hour, thus replacing it requires very little effort or time on the part of a stock alternator. A healthy stock alternator’s lacquered copper windings remain bright and shiny, while the windings of an over-heated stock alternator, one that was externally regulated, show clear signs of heat stress, wherein the lacquer coating has been vaporized. The difference between a near-continuous duty high output alternator and a high output stock alternator is, to paraphrase Mark Twain, like the difference between lightning and a lightning bug. Not only is the stock alternator not equal to the task it’s being called upon to carry out, even when externally regulated, its output, contrary to initial perceptions, is simply inadequate.

While 100 amps may appear substantial, it’s anything but when compared to the massive bulk of many of today’s house battery banks. In the case of the 57-foot cruiser, it’s a mere 10% and even if it was a proprietary, continuous duty high output alternator it still would be inadequate. Depending upon the battery type, flooded, gel or AGM, the rule of thumb for the ratio of charge output to battery bank size calls for the charge source, an alternator, in this case, to be a minimum of 25% of the bank’s amp-hour capacity. That is, the alternator output required for this battery bank, in order to achieve reasonable re-charge times, should be at least 250 amps.

Custom pulleys may also be required to fine-tune the alternator’s speed to achieve maximum output at the vessel’s cruising speed. After my inspection of the 57-foot cruiser. We recommended that the stock alternator be replaced by a continuous duty 200 amp unit along with a multi-stage, temperature-compensated regulator and a battery bank amp-hour monitor, which would enable the owner to accurately determine how many amp-hours had been used by the bank, which in turn would dictate their re-charge cycle. With this gear in place, as well as a little training regarding battery bank monitoring and discharge protocols, the perceived need for a larger battery bank evaporated. Although the error was no doubt unintentional, the builder of this vessel never should have installed a mega-battery bank knowing it would be serviced by a mini-charge source.

Battery banks, particularly large ones, must always be treated as an integrated package whose design is based on the electrical needs of the vessel/crew, the desired quiet ship time, and the charge source. Failure to treat each leg of this battery triangle equally nearly always results in a system that fails to live up to expectations.

Alternator Regulators

Among the most important are the methods by which the alternator or alternators are regulated. Most are designed for short duration high output scenarios, to recharge a start battery or perhaps to supply power to an air intake post-heating system for the purposes of reducing smoke upon startup. The solution is to utilize an alternator that’s designed for an extended high output operation. While exceptionally robust and capable of delivering amps galore, they typically lack regulation of their own and proper regulation is especially important when it comes to recharging a large battery bank as quickly and as safely as possible.

Smart, three-stage, temperature-compensated regulators are the heart of a high output charging system and large battery bank. In spite of the alternator’s rated output, when guided by such a regulator, it’s simply not capable of replacing large amounts of energy that have been drawn from a heavily depleted house battery bank or doing so in a manner that ensures the longest possible battery life. One, the battery bank becomes chronically undercharged and two, the voltage that the batteries are exposed to is often incorrect, which leads to poor performance and a shortened lifespan. In addition to correctly sizing a proprietary high output alternator for the battery bank that is or will be installed, it must be controlled by an external multi-stage “smart” regulator.

The owner of the most advanced high output charging system is flying blind unless he or she is able to monitor the battery bank’s state of charge, using an amp-hour meter like the one shown here. The key to effective management of alternator output is to supply it in distinct stages, bulk, which is typically between 14.1 and 14.6 volts, 28.2 and 29.2 for 24 volt systems, acceptance or absorption, usually 2/10 of a volt below bulk, and float, which is typically one volt below bulk. Each battery type has its own ideal charge profile, for which most smart regulators can be programmed. When compared to their conventional brethren, smart regulators are much better able to determine and take advantage of, these states of charge, tailoring the alternator’s output for the greatest charge efficiency, and thereby ensuring the shortest possible recharge time.

Bulk is, as the name implies, the highest output stage of the alternator/regulator. It sends a heavily depleted battery bank the greatest possible amperage that it can safely accept. Finally, once the battery is fully charged the regulator enters a float mode, keeping the battery bank’s voltage high enough to prevent self-discharge and sulfation and low enough to prevent overcharging. Multi-step charging protocols have been too large house battery banks what fibreglass resin has been to boat building.

Without this approach, using large battery banks and recharging them in a reasonable amount of time would be virtually impossible. Simply put, temperature compensation is a must for any multi-stage charging system, whether derived from an alternator or shore-powered charger-inverter/charger. Shore or generator-powered chargers play an equally important role in properly charging large battery banks when the vessel is not underway. We recently inspected a battery bank that was located in Auckland as the vessel cruised where the water temperature was chilly.

The temperature compensation probe enables batteries living in vastly different environments such as these to be charged safely, efficiently and quickly while ensuring maximum battery longevity. Typically, the probe is adhered to the battery case or bolted to one of the battery terminals in the house bank the stick-on probes are notorious for falling off, usually apply a bead of silicone sealant over them to keep them in place. This temperature probe will alert the regulator to an impending overheat scenario, at which point the regulator will reduce the load placed on the alternator. For vessels equipped with twin propulsion engines, the preferred approach, for maximum charge capacity, calls for the installation of one high output alternator on each engine with, once again, both outputs connected to the house battery bank.

If separate, unsynchronized regulators are used, one of a twin independent alternator set up will often prevail, leaving the other in idle mode, essentially nullifying the value of a twin arrangement. Provisions must be made to disable the field voltage to an engine/alternator that is not running, this may be done automatically, using the aforementioned synchronizing device, or via a manual, or oil pressure-actuated, switch. These include start delay, which gives the engine and belts time to warm up before applying alternator load, as well as preventing the alternator from placing a load on a smaller engine during cranking. A remote battery sense enables the regulator to measure and compensate for the battery bank’s actual voltage, thereby taking into account voltage drop between the battery bank and alternator.

The value of battery temperature sensing has already been mentioned, however, the same approach can be used for the alternator itself. By sensing its case temperature, the regulator can reduce the load on the alternator if it becomes too hot. Finally, a belt management program enables the user to intentionally de-rate an alternator’s output, thereby reducing the load on the alternator and engine crankshaft pulley. This can be especially useful in small engine applications, where crankshaft, as well as belt loads, must be managed, or where, for longevity purposes, over-sized alternators are used, a 400 amp alternator, for instance, is limited to 250 amps will run cooler and last longer than a 250 amp alternator running without limitation.

Too much of a good thing can be problematic, however, as over-speeding an alternator will result in its self-destruction, pulley seizure, and engine stoppage. Make certain you, or your electrician, do the pulley math before proceeding with a high output alternator installation. This installer failed to ‘do the math’, causing the alternator to overheat and seize.

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