Is Your Marine Alternator Up to the Job, or Not?

Many marine engine alternators are inadequate to handle the charging demands of modern boats. Typically derived from automotive designs, these alternators struggle to perform effectively when paired with large deep-cycle battery banks, falling short as a reliable charging solution. While high-output alternator options exist that can meet these demands, they are seldom straightforward drop-in replacements.

If your current alternator is falling short, here are the key considerations for choosing a high-output alternator and how you can optimize its performance.

Why upgrade your alternator?

While the electrical demands of modern boats have increased dramatically, stock marine alternators have failed to evolve to meet these needs. Although some engine manufacturers now include high-output alternators, these internally regulated options are far from optimized for use as primary charging sources.

Internally regulated alternators generally fall into two categories: “dumb” and “super dumb.” Dumb alternators typically support two-phase charging (bulk and absorption), while super dumb alternators only provide absorption. Both types are fundamentally flawed in their ability to charge effectively and safely in demanding marine environments.

These alternators underperform for several key reasons. For one, their fixed voltage setpoints cannot be adjusted to suit specific battery types or conditions. Second, they fail to account for battery temperature, a critical factor in proper charging. And third, their output directly depends on the engine’s rpm. Consequently, when paired with large, deep-cycle battery banks, these limitations can lead to overheating issues.

Another common issue with internally regulated alternators is that they measure voltage at the alternator itself, not at the batteries. This setup frequently results in batteries receiving insufficient voltage, leading to premature failure. Voltage drops caused by long cable runs, some diode isolators, and corroded cable lugs can reduce the alternator’s output below the required 13.6 volts for flooded batteries and 14.4 volts for AGM batteries. Compounding the problem, many older internally regulated alternators were designed to provide a charging voltage of only 13.6 to 13.8 volts, making them even less suitable for modern onboard battery systems.

Because some boaters are ditching their generators in favor of battery systems, a proper high-output alternator and inverter setup can reduce their yearly maintenance and maximize the amortized cost of running their engines.

Factors to consider

When selecting a high-output replacement alternator, it’s important to consider several factors, including the mounting saddle, belt size and the overall clearance around the alternator.

Alternator mounting saddles generally fall into several distinct types, as outlined below. While most engine manufacturers tend to use a consistent mounting style, it’s always a good idea to verify the dimensions of your mounting saddle and confirm there is sufficient clearance for the new alternator.

After identifying the saddle type, the next step is to evaluate the alternator belt. Alternator belts come in various sizes, each with a general amperage load capacity. When installing high-output alternators, if possible, upgrading to a serpentine belt is highly recommended. Serpentine belts offer superior efficiency at 99 percent compared to 92 percent for V-belts. Serpentine belts can also handle higher output levels.

Companies like Balmar and Electromaax offer kits to convert existing pulley systems to a serpentine belt setup. However, if a conversion isn’t feasible, an alternative is to upsize your alternator pulleys. Increasing pulley size enhances the surface area. This reduces the likelihood of belt slippage and allows higher-output alternators to operate above standard ratings. Charts are available to help determine appropriate pulley sizes. To ensure the proper pulley ratios are maintained, may require replacement of the crank pulley.

If you opt to keep your stock pulleys, ensuring proper pulley alignment after installation is essential. Even if the new alternator uses the same mounting foot style as the old one, that doesn’t guarantee that the pulleys will align perfectly. If you encounter misalignment, shims can be added to the alternator mounts to correct the issue and prevent future belt problems.

Alternators are available in various case sizes, including small, large and extra-large. While high-output, small-case alternators may offer a direct size replacement for your existing alternator, they are typically not designed for constant-duty applications. Whenever possible, upgrading to a large-case alternator is recommended for better performance and durability. The same can also be said for selecting an alternator with a higher amperage than required and then derating the output.

Everyone makes mistakes, and the same is true for alternator manufacturers. Always double check the dimensions of a new alternator to ensure a proper fit, especially when changing case sizes.

How big of an alternator do I need?

When selecting a high-output alternator, several factors must be considered, including your battery’s charge acceptance rate (CAR or C-Rating), the alternator’s output rating (hot vs. cold), desired engine run time and onboard electrical loads.

The charge acceptance rate indicates how many amps your batteries can accept during charging. Typical CAR values are between 30 and 40 percent for AGM batteries, 20 to 30 percent for flooded lead batteries, and 20 percent for gel cell batteries. Thin Plate Pure Lead (TPPL) AGM batteries particularly require a minimum of 40 percent of their capacity for charging. These values can vary by manufacturer, so it’s important to verify them. For example, with a 500 amp-hour AGM bank, the maximum recommended charge output would be 150 to 200 amps.

Next, consider whether the alternator’s output is rated for hot or cold temperatures. Hot ratings are typically lower due to heat in the engine room, which can significantly reduce performance compared to cold ratings. Quality marine alternators usually provide both ratings, but your engine compartment may run hotter than the conditions the alternator was tested in.

If the alternator will serve as your primary charging source, calculate how long you’ll need to run the alternator to fully charge your batteries. Keep in mind that at idle or low engine speeds, the alternator’s charging capacity may be significantly reduced.

From avalanche diode packs to increased cooling and external rectifier packs, choosing a high-quality alternator when upgrading is critical to ensuring you are getting what you pay for. Alternator brands that we have recommend include Balmar, Arco, Electromaax, and American Power Systems. 

Maximizing your charging investment

Installing a high-output alternator will significantly improve performance, but to truly maximize its potential, there are additional upgrades to consider.

While high-output alternators can come with internal regulators, switching to an external regulator will provide a notable performance boost. External regulators, such as the WS500 from Battleborn adjust for battery temperature, monitor amperage output, alternator temperature, and regulate the field windings to optimize output for all measurable parameters. While the WS500 is our preferred regulator, additional options include the Balmar 618, Arco Zeus, the Revatek Altion and the E-Maax by Electromaax. When selecting an external regulator, it is important to be aware of whether your alternator is a P- or N-type. Some models like the Wakespeed and Arco are capable of working with both, whereas the Balmar is only compatible with P-type.

Since alternator output is directly tied to engine speed, adjusting your pulley ratio can further enhance charging performance. Stock pulley ratios are typically 2:1, with the alternator spinning twice as fast as the engine. Altering this ratio can improve performance across the rpm range, but it’s crucial to ensure you don’t overspeed the alternator at maximum engine rpm.

Upgrading to a high-output alternator is a crucial step in meeting the growing electrical demands of modern boats. By carefully selecting the right alternator, optimizing the installation with external regulators, and adjusting pulley ratios, you can significantly enhance your charging system’s performance. With the right setup, your boat will be better equipped to handle modern electrical loads and the addition of large battery banks. By taking the time to consider all factors, from mounting styles to charge acceptance rates, you can ensure that your new alternator performs at its best and maximizes your investment for years to come.

Mike Garretson owns Sea & Land Yacht Works in Wakefield, Rhode Island.

March 2025