Automotive-style alternators, like this 68-amp Delco-style three-wire model from Sierra, are internally regulated.
Looking for an alternator?
We see customers looking for an alternator for two reasons here at West Marine:
Direct replacement: you’re needing a direct replacement for a factory-installed automotive-type OEM alternator, and we have a selection from Sierra alternators for most common marine engines. Go to our Engine Parts Selector to find the alternator you need.
Upgrade to your charging system: you want to replace your automobile-style alternator with a larger, higher-capacity alternator so you can charge more batteries, charge larger batteries or bring them up to charge more rapidly. In this case, you’re looking for an alternator from Mastervolt or Balmar, and this type of system upgrade is the topic of this West Advisor.
What to consider before you buy
Before you invest in a high-output charging system, save yourself a lot of trouble and dollars by ensuring that you buy the correct system for your application. Charging systems vary by type of boat, as well as a collection of other variables including engine type, engine room space, belt type and size, battery bank size and battery chemistry. Selection criteria you should consider include:
Each type of marine battery, a flooded starting battery, deep cycle flooded battery, gel or Absorbed Glass Mat (AGM) battery has its own unique charging characteristic, which affects the type and size of alternator you’ll need to charge it efficiently. The Acceptance Rate of the battery, what percentage of its total amp hour capacity it can accept, is different for some battery chemistries.
Extra large frame 98-Series alternator delivers current similar to some gensets, with up to 5kW of output.
A standard flooded battery, of either the starting or deep cycle type, can accept charging current equal to 25 percent of its available capacity. Gel batteries can accept 30 percent, AGM batteries can accept 40 to 50 percent, and newer technologies like lithium ion or TPPL (thin plate pure lead, such as Optima) can accept even more. They need to be charged by a large case or extra large case alternator to produce enough charging current, due to their nearly unlimited appetite for amps.
Your alternator should be capable of delivering rated output equal to the maximum acceptance rate of your house battery bank. In other words, the alternator’s rating should be equal to 25 percent (lead acid) 30 percent (gel) or 45 percent (AGM) of your total battery capacity. For flooded batteries, size your alternator toward the bottom of this range.
As we add electrical and electronic appliances to our boats, we notice that our battery banks have become incapable of handling the loads created by these new stereo subwoofers, floodlights, satellite TV systems, refrigerators and other thirsty power consumers. We start shopping for bigger battery banks, or consider adding a battery. Adding battery capacity requires us to take a look at our charging system.
In most cases, your alternator’s output should equal the maximum available capacity of your battery bank. In other words, if your house battery bank is made up of flooded batteries with an amp-hour capacity rating of 400Ah, the maximum acceptance rate of those batteries would be 25 percent, or 100 amps, the ideal alternator would also be rated at 100 amps.
If your house bank consisted of 400Ah capacity AGM batteries, the maximum available capacity would be 45 percent of 400, or roughly 180 Ah. To maximize charging efficiency, a 180-amp rated alternator would be required. Unfortunately, mounting a large-case alternator is not feasible in many vessels.
Compatibility depends on engine model, engine year, engine compartment layout and other factors. Inspect your existing alternator mount and compare to the diagrams shown to determine the appropriate high-output replacement. You may need spacers or modifications to brackets, tensioners and wiring on any high-output alternator installation. Most alternators fall into one of four mounting styles:
Max Charge 614-H has the latest fast processor, seven battery charging algorithms, and a Belt Load Manager program to control field output to ensure that belts are protected from excess wear.
- Single 1" foot spindle mount (Motorola style; Balmar Model 621). Typical engine types are Hino, Lehman, Caterpillar, Atomic 4, Universal, Ford, Crusader, Pathfinder, Westerbeke, other Motorola-equipped.
- Single 2" foot spindle mount (Delco style; Balmar Model 621 with included spacer). Typical engine types are Volvo, Cummins, Westerbeke, Perkins, MerCruiser, Yanmar (6LP), Volvo-Penta, GM, other Delco-equipped.
- 3.15" ID saddle mount (Hitachi style; Balmar Model 60). Typical engines are MerCruiser, Lehman, Yanmar, Westerbeke, Perkins-Sabre, other Hitachi-equipped.
- 4" ID saddle mount (J-180 style; Balmar Model 604). Typical engines are Detroit Diesel, Cummins, Caterpillar, John Deere, J-180 off-engine mounts.
Belt load limitations
Drive belt width is a critical factor in selecting a replacement alternator. Commonly-installed vee-belts and multi-groove serpentine belts have specific limits regarding the amperage and horsepower loads they can support.
As a rule of thumb, 12-volt alternators use one horsepower for every 25 amps of output, or to put it another way, their ratio of output to horsepower load is 25:1. So, when selecting a new alternator, you need to size it for the limits of the capacity of its drive belt. Otherwise, your system will be plagued by premature belt wear, belt slippage and potential damage to your alternator and engine. See the following chart, and note that dual vee-belts and serpentine belts can support much larger loads.