The combination of lithium-ion batteries and more powerful and efficient alternator-style charging devices has reshaped how we generate energy for house loads on many boats. For decades, batteries were the limiting factor in these systems, but that distinction has shifted to the alternator. With that change in mind, we’ll focus on the challenges of alternator installations—some longstanding and relevant to any system, and others driven by recent technological advances.

Determining Output

Looking at any charging system, our first step is to distinguish real alternator output from rated output. An alternator is normally given an SAE (Society of Automotive Engineers) rating that describes its maximum output at a given temperature—77°F (25°C)—and speed of rotation. But in practical operation, an alternator will rapidly heat up, causing its output to decline by as much as 25%. Thus, the meaningful rating number for a systems designer is a hot rating (sometimes known as a KKK rating). Although I have found some SAE-rated alternators with matching operating and rated outputs, for design purposes I derate an alternator by 20% if a hot rating is not available. Manufacturers provide curves or tables illustrating alternator performance (amps) as a function of speed of alternator rotation (rpm). Comparing these may reveal that models with the same nominal peak (e.g., 150 amps) might require very different speeds of rotation to achieve it. The lower the speed at which an alternator reaches full output, the more desirable it is for most marine applications, because it maximizes charging when the engine is idled solely for battery charging or is run slowly for harbor maneuvering. The rate at which an alternator builds output is a key factor in designing a system.

Another important element is pulley sizing. Standard engine pulleys, and those supplied with high-output alternators, often don’t allow an alternator to reach target output at a common engine rpm. That’s because most alternators and pulleys are geared for automotive applications, where engines typically run at 2,500 rpm or higher; marine engines operate most frequently anywhere between idle speeds of 700 rpm to 800 rpm, and cruising speeds no higher than 2,000 rpm.

Because boat engines and usage vary considerably, you must specify alternator pulley size based on individual applications, using the following steps:

  • Determine alternator rpm needed to produce the target alternator output. Unless the alternator is specifically designed for high-temperature operation, as a guard against overheating, the target continuous alternator output should not exceed 75% of rated output.
  • Find the alternator’s maximum safe operating speed (usually 10,000 rpm). • Determine minimum engine rpm during normal operation.
  • Establish an alternator pulley ratio that will achieve the target output at this minimum engine speed.
  • Check that the alternator will not overspeed at maximum engine rpm. If necessary, power down the pulleys so the alternator reaches its maximum rated speed only at maximum engine rpm.

As a general rule, if alternator rpm needs to increase for a given engine rpm, you should increase the engine pulley diameter rather than decreasing the size of the alternator pulley. The latter is highly loaded, and decreasing pulley size exacerbates belt slippage and other issues.

An additional complication is that some engine tachometers operate by sensing the internal phase frequency of the alternator (those are the ones with a wire run from a terminal on the alternator to the tachometer). If a nonstandard pulley ratio is used to drive the alternator, it will throw off the tachometer calibration. There’s not always a means of adjusting for this, and with some multistep voltage regulators the tachometer may also trip out or flicker on and off when batteries are nearing a full state of charge and the charge controller trips to its “float” setting.

Alternator Installation

In many cases, an ideal installation does not replace the existing alternator but instead adds a high-output alternator to it.

High-output alternators commonly come in a small frame, which is the same size as most automotive alternators, and a large frame, which is highly desirable to optimize heat dissipation but may be hard to mount in some circumstances. There are several standard mounting arrangements in general use. Whatever its size, a replacement alternator should use the same mounts as the original. The most common are saddle mounts (also known as dual foot), on which a long bolt runs through two lugs spaced at either 3.15″/80mm or 4″/102mm (J180- style); and single-foot mounts, with one heavy-duty lug either 1″ or 2″ (25mm or 51mm) thick.

High-output replacement alternatorson older engines are often driven by a single V-belt that also powers the engine’s freshwater pump. Unless the pulleys and belt are replaced, the factor limiting alternator output will be the existing belt size. As a rule, a single 3 ⁄8″ (10mm) belt should not be used to handle much more than 1 kW of alternator output (e.g., ~75 amps at 14V); and a single ½” (13mm) belt not much more than 1½ kW (e.g., ~100 amps at 14V). Note that the belt tension needed to power higher loads may damage bearings in the water pump.

Traditional V-belts come in two profiles: classic (based on 1930s U.S. standards, also known as wedge and conventional); and narrow, the most common on engines with conventional pulleys. (Metric versions of both profiles exist.) Belt quality varies markedly between manufacturers. In my experience, there is no way to tell the level of quality just by looking at a belt. I havehad excellent results with Gates Green Stripe belts, and thus always specify them. Note that the bible for a detailed description of V-belts and associated design factors is the Gates Heavy Duty V-Belt Drive Design Manual (Gates document number 14995; get the latest edition).

Serpentine belts eliminate many traditional pulley and belt issues, and can also support extraordinarily high belt loads, with the limiting factor on many systems now being the bearing in the water pump if it is included in the circuit. Retrofit serpentine pulley kits are available for most popular marine engines and are an excellent investment if installing high-output alternators.

When specifying serpentine belts, the automotive industry currently uses “K” section “micro-V” belts and compatible pulleys. The marine world is largely following suit. However, the industrial “J” section sometimes appears in marine applications even though finding replacement belts is nowhere near as easy as replacements for “K” section belts.

Typical belt geometry on a marine engine results in no more than 90° of belt wrap at the alternator pulley. Given the high loads of powerful alternators, considerable belt tension is needed to prevent slipping in that configuration. Adding an idler or tensioner pulley to increase the alternator pulley belt wrap to as much as 180° will accommodate high loads with significantly lower belt tension and less risk of slippage. Such a pulley should always be located on the low-load side of the alternator pulley, where the belt feeds onto the pulley, not the side from which it is pulled off the pulley by the crankshaft pulley.

Engine Overload

Powerful alternators can easily overload a relatively low-powered engine when under way, particularly at idling and low speeds and when approaching wide open throttle (WOT). If this is a problem, you’ll need to limit the alternator’s output at specific engine speeds, which requires a more-sophisticated charge controller than is normally installed. In the likely absence of specific detailed information from the manufacturer, what follows is a rough method to determine whether you need one:

  • Obtain a graph of the engine’s full power curve and a nominal propeller load curve (commonly published online by the engine manufacturer) with the engine power and the propeller load expressed in either hp or kW. Print out this graph in a reasonably large format.
  • Obtain a copy of the alternator output curve expressed in amps at a given speed of alternator rotation
  • Multiply the amps by the charging voltage (e.g., 14V, 28V) to determine the maximum output in watts (W) at any given alternator speed of rotation. Divide the watts by 1,000 to get to kW(e.g., a 150-amp output @ 14V = (150 x 14)/1,000 = 2.1 kW). If the engine power and propeller load curves are expressed in hp, multiply the kW by 1.34 to convert kW to hp (e.g., 2.1 kW x 1.34 = 2.81 hp).
  • If you know the alternator’s efficiency, convert this to a decimal (e.g., 60% efficiency = 0.6) and divide this into the alternator’s kW or hp output at any given alternator speed of rotation to determine the approximate crankshaftpulley load at that speed (e.g., at 60% efficiency, a 2.1 kW output = (2.1/0.6) = 3.5 kW crankshaft load; a 2.81 hp output = (2.81/0.6) = 4.68 hp). If you don’t know the alternator’s efficiency, assume it is no more than 60%.
  • Determine the engine rpm at which these crankshaft pulley loads occur by dividing the pulley ratio between the crankshaft pulley and the alternator pulley into the alternator’s rpm at any given output point.
  • Add the alternator load at a specific engine rpm to the propeller load at that rpm. This can be done by adding the alternator loads to the propeller curve graph you printed out. We are looking for a rough indication of the combined alternator and propeller load rather than a precise number. If there are two alternators, do the same calculations for both; add both loads; and add the sum to the propeller load
  • If the combined propeller and alternator load ever approaches the engine’s full power curve, reduce the alternator load at that engine rpm. It can be done manually in some cases but most likely will require a sophisticated charge controller. In general, until you approach WOT, at which point ideally any powerful alternator will be shut down, the combined alternator and propeller load shouldn’t exceed 70% of the engine’s maximum power rating (the full power curve) at any given speed. This accommodates calculation inaccuracies and unanticipated loads while simultaneously coming close to optimizing engine fuel efficiency.

The calculations above provide a rough idea of the alternator loads added to the crankshaft pulley at any given engine speed. Convert them to torque values as follows:

  • Nm = (9,549 x kW)/rpm • In-lb = (63,025 x hp)/rpm • Ft-lb = (5,252 x hp)/rpm

where kW and hp are the crankshaft pulley load as calculated above, and rpm is the engine rpm.

On smaller engines (e.g., below 100 hp) these loads can exceed published allowable side loads; engine manufacturers may void the warranty on new engines. This has been an ongoing issue for many years. In correct installations there are few instances of engine damage that can be attributed to an alternator, and engine manufacturers have been increasingly forced by consumer demand to accept these installations. I am not qualified to give legal advice, but in the U.S. the Magnuson-Moss warranty act, passed in 1975, specifically bans the voiding of an engine’s warranty because nonbranded parts have been used in servicing and maintaining it.

Alignment and Tension

With the correct pulleys, brackets, and belts on hand, the mechanical installation of most alternators is straightforward. Alignment is determined by the mounting brackets, so it either works or it doesn’t—there’s no means of easily making any adjustment. You can check pulley alignment with various laser tools prior to fitting a belt. For the K-section serpentine belts, a couple of easy-to-use and accurate devices are available from Gates (model numbers 91075 and 91006). Although belt tension can be checked more-or-less satisfactorily by hand, if the belt is to be driven hard, it is best to use a tension meter. The best one I’ve seen is a Gates Sonic U-508 model, expensive at $700 but worth it for the professional installer.

Cable Sizing, Stranding, and Connections

After the mechanical installation, wiring is next. There is more to this than meets the eye, especially with higher-performing alternators. Below I describe some obvious requirements and some not-so-obvious ones that I see violated from time to time.

In accordance with the American Boat & Yacht Council (ABYC) E-11 standard, cables should be sized to allow a maximum 3% voltage drop in the alternator’s full rated output. Depending on the length of the cable run, with high-output alternators this can easily require 2/0 (70mm²) cables, and sometimes even larger. Additional considerations include:

  • Cables should always be at least Type 2 stranding to withstand the inevitable vibration without work hardening and fracturing.
  • Depending on the cable size required to meet the standard, your installation may include a large cable connected to a relatively small output stud on the back of the alternator. It is essential that cable terminal and studs be exactly matched and mated, and that the retaining nut includes either a lock washer or a nyloc nut. The former is preferable, as the temperature on the back of an alternator (see below) can exceed the rated locking temperature (250°F/121°C) of the plastic insert in a nyloc nut.
  • The cable should be supported with strain relief close to the alternator to prevent it from working loose. If it does, a resulting arcing fault can burn through the output stud, allowing the cable to drop off entirely. If the loose cable end contacts the engine block or any other grounded surface, a dead short across the batteries to which it is connected could burn through the engine oil pan or start a fire.

Thermal Issues

Conventional alternators with peak efficiency of no more than 60% get hot when they run hard. To put this in perspective, a 60%-efficient nominal 12V alternator running at 150 amps supplying (150A x 14V) = 2,100 W = 2.1 kW of electrical energy to the boat’s electrical system will also generate 1.4 kW of heat. Alternator case temperatures frequently exceed 212°F (100°C} and sometimes go significantly higher. For example, the factory temperature setting for many Balmar alternator temperature sensors, which are bolted to the alternator case, is 226˚F (108˚C), with an option to raise this temperature setting as high as 248˚F (120˚C via Balmar’s charge controller software. I have set it at 237°F (114°C). High operating temperatures have several consequences:

To prevent it from burning out, the alternator may require a temperature sensor tied to its charge controller, especially when charging lithiumion batteries (see below).

  • The output cable(s) from the alternator, in particular the positive cable (most alternators are grounded through the case without a negative output cable), carry the alternator heat, with the section immediately afterthe attachment to the outputstud approaching the temperature of the case. Cables with insulation temperature ratings as high as 392˚F (200˚C) should be employed here, yet I have not seen them in use. Common in the U.S. is UL 1426 “Boat Cable” (also known as BC5W2) with a “dry temperature rating of 221˚F (105˚C). An alternatorrunning hard will transmit enough heat to the cable terminal to push a cable to its maximum rated temperature. At this point, according to ABYC ampacity tables, no amps should be put down the cable, but the alternator is delivering outputs of well over 100 amps. At present there is no good standards-compliant solution to this other than to size cables very conservatively; never use cable rated for less than 221˚F; and never support the cable close to the alternator output stud by strapping it to the alternator case. The wire needs to be in free air to dissipate heat asfast as possible. Note that many European boatbuilders install cables with a temperature rating below 221˚F—completely inappropriate for many alternator installations. To compound problems, the cable is frequently not labeled with its temperature rating.
  • Automatic engineroom-fire-suppression systems trigger at 174˚F (79˚C). To avoid nuisance triggering, install the temperature sensor away from high-output alternators that will be run hard. Never mount the temperature sensor directly above the alternator or in the exit path of its cooling air. Advise boat owners to invest in the more-expensive gas-filled extinguisher option, not the dry powder, which can cause irreparable engine damage if triggered with the engine running.
  • Following installation, an alternator should be run at its full rated output, held there for at least 10 minutes, and then all the connections in the associated cabling—from the alternator output stud to the batteries being charged—should be photographed with a thermal-imaging camera to ensure that there are no hot spots. If you nd any, clean and reestablish the ‑ awed connections. (­ ermal-imaging cameras that plug into smartphones can now be purchased for about $200. ­ is device is an essential component of any marine electrician’s tool kit. A laser heat gun can serve a similar purpose for less than $20, but the area image of a camera provides a better overall picture of what is going on.)
  • In an increasing number of applications it makes sense to remove the diode pack from the back of an alternator and mount it separately. Diodes contribute significantly to heat generation and simultaneously obstruct air- ‑ ow through an alternator. Removing the pack improves alternator cooling, while the pack itself can be mounted in a cooler environment for better heat dissipation. With this change, instead of a single positive cable coming off the back of the alternator, with perhaps a negative cable, there will now be three phase cables between the alternator to the diode pack, with a positive and negative cable thereafter.

Overcurrent Protection

Under ABYC E-11, an alternator wired back to a starter motor solenoid (the most common installation) with a cable run of less than 40″ (1m) does not require overcurrent protection (OCP). But if the alternator is wired to any other point in a boat’s electrical system, it may require OCP, sometimes at both ends of the cable. The determining issue is whether the alternator’s maximum possible output is “self-limiting” (in practice, it will be) and whether the ampacity of the output cable is as high as, or higher than, this maximum possible output. If the 3% voltage drop table is used for sizing cables based upon the alternator’s full rated output the cable ampacity will be high enough.

It is important to follow the positive cable electrical path back to the batteries being charged until you find an OCP device. If this device has a rating higher than the ampacity of the cable from the alternator, an additional OCP device, rated according to the ampacity of the output cable from the alternator, is required at the point where the alternator output cable connects to the rest of the system. ANL or similar fuses are commonly used here and elsewhere in high-current DC circuits. The connections to these are almost always made with stainlesssteel fasteners, which have low electrical conductivity. It is essential that cable terminals are in direct contact with fuses, or with the conducting surfaces of busbars, and that no stainlesssteel washers are placed between conducting surfaces. Such washers accidentally included in the circuit and subjected to high continuous currents can generate sufficient heat to blow fuses (at which point the diodes in the alternator may be destroyed) and even to start fires. I recommend slow-blow fuses to minimize the chances of nuisance blowing.

Lithium-Ion Batteries

As lithium-ion batteries gain wide acceptance, the tendency is to think of them as a straightforward replacement for lead-acid batteries. For a number of reasons, some directly related to alternator installations, they should not be treated this way.

In principle, a well-discharged lithium-ion battery can drive charging devices to full continuous output for extended periods, but if an alternator is not rated for continuous duty, there must be a way to derate it. For example, Yanmar requires that, when their Valeo alternators are used with lithium-ion batteries, the output current is limited to no more than 75% of their rated output. Another derating mechanism is to track alternator temperature and control the output based on a set temperature limit.

Note that not all lithium-ion batteries are rated for high charge rates. In fact, recommended charge rates as low as 0.3C (i.e., a charge rate in amps that is 30% of the battery’s Ah rating; for example, a 200-Ah battery would have a recommended maximum charge rate of 60 amps) are the norm. It is important to match the battery’s characteristics to those of the alternator.

Even though lithium-ion batteries are 90%–95% e‑ cient, if they are cycled at high C rates, the remaining 5%–10% can generate significant heat. If the internal temperature rises beyond a certain threshold, a battery management system (BMS) is likely to disconnect the battery from the boat’s electrical system. Thus, alternator charge rates must be coordinated with internal battery temperatures.

­ e BMS on any lithium-ion battery for marine applications is likely designed to forestall the battery entering a potentially unsafe condition. A range of voltage, temperature, and state-of-charge thresholds will trigger protective measures, which usually result in the battery being disconnected from the boat’s electrical system. If this occurs with an alternator running, it may create a voltage spike throughout the boat that damages all electronic equipment turned on at the time, and it may destroy the alternator by blowing out its diodes. To prevent this, the BMS should be programmed to shut down any alternator before disconnecting the battery. ‑ is feature is frequently absent in so-called drop-in batteries designed to replace existing lead-acid batteries.

The two predominant lithium-ion chemistries in marine batteries are lithium iron phosphate (LFP) and nickel manganese cobalt (NMC). It’s a peculiarity of NMC batteries that they will permanently lose capacity if maintained for extended periods in a full state of charge, such as when a battery charger is plugged into a dockside power supply. Similarly, if a generator or propulsion engine is running 24/7, continuously charging NMC batteries, a charge-control mechanism must be programmed to maintain the batteries below a full state of charge.

LFP chemistry is not as sensitive to continuous full charging but will still bene­ t from being maintained below a full state of charge. ‑ is is the opposite of how we aim to manage leadacid batteries.


For at least the past three decades pretty much everything we have done to advance onboard DC systems design has been to work around the inherent limitations and weaknesses of lead-acid batteries. Increasingly, adoption of lithium-ion batteries is eliminating this roadblock, and the stress point in our DC systems is shi- ing to the generating side, notably alternators.

Some excellent solutions in development include high-output alternators married to extremely sophisticated control systems, but until the capabilities of such solutions ­ lter down to the broader alternator marketplace, to avoid unhappy customers we need to pay close attention to the ­ ner details of conventional alternator installations.