Understanding the Three-Stage Regulator - SailNet Community
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Understanding the Three-Stage Regulator

A multistage regulator is often on the wish list of cruisers, but do you really need one?
The de rigueur companion to the high-capacity alternator, which we took a look at last month (Alternators—How Big is Big Enough?), is the multistage regulator. Let’s turn our spotlight on this piece of cruising gear and see what we can determine.

Regulators 101         Almost every kid has made iron filings or some small metal object move “magically” across a piece of paper by passing a magnet beneath the paper. Electrons in a piece of wire can be induced to move in the same way—by passing a magnet close to the wire. This is how we generate electricity. The essence of a generator is simply a magnet spinning past a coil of wire. Alternators operate on the same principle, except that the magnet is an electromagnet. Unless the electromagnet is energized, there is no output from a spinning alternator (because there is no magnet).

We need to know this because energizing the electromagnet is the function of the 12-volt regulator. The basic regulatormanufactured by the millions for the automobile industryis nothing more than an electronic switch. Closed, the switch provides current to the alternator’s electromagnet, inducing alternator output. The charging of the alternator drives up the voltage in the charging circuit, and when it rises to some preset level, the switch opens, turning off the alternator. This causes output voltage to fall, but when it drops below the regulator’s set voltage, the switch closes again. This switch is flicking on and off at a phenomenal rate—hundreds of times per second—with the effect of nearly constant output voltage and a pulsating current. As the voltage of the batteries rise, it takes less time for the alternator to elevate the charging circuit voltage to the cutout level, shortening “on” time. Voltage remains fixed, but shorter current pulses reduce the average current output.

"The first rule is that charging current should not exceed 25 percent of battery capacity."
Allow me to remind you of the appropriate charging regimen for a flooded deep-cycle battery. The first rule is that charging current should not exceed 25 percent of battery capacity. A healthy battery will accept this level of charge until battery voltage rises to around 14.4 volts. At this point the battery will be around 75 percent charged and the cells will be gassing. Now we want to maintain the voltage at 14.4 volts and allow the charging current to fall naturally. When the current drops below 4 percent of battery capacity, the battery will be around 90 percent charged. To fully charge the battery, we should now feed it a constant charging current of less than 4 percent of capacity until battery voltage stabilizes at its highest level—typically around 16.2 volts. This is known as equalization. Once the battery is 100 percent charged, we want to stop charging.

In the real world the equalization step is usually omitted because it takes up to four hours to raise the battery from 90 percent to 100 percent charged. For this reason, cruising boat batteries generally operate in the range between 50 percent and 90 percent of full charge. If the electrical system is configured to allow it, the batteries can be equalized periodically for their health.

Three-stage “smart” regulator            How closely does a so-called smart regulator paired with a high-capacity alternator match the desired charging pattern? Nearly all do a pretty good job. Borrowing last month’s example of a 440 amp-hour battery bank discharged 50 percent, we want a constant 110-amp current from the alternator until we have pumped in around 132 amp-hours—110 amp-hours to raise the charge level from 50 percent to 75 percent plus another 22 amp-hours for the 20 percent battery inefficiency. In a perfect world, a 110-amp charging source could get us to this point in an hour and 12 minutes, but that level of sophistication is expensive. The typical three-stage regulator starts limiting maximum current earlier than ideal, extending the time to reach the second stage by 10 percent to 20 percentperhaps 15 minutes in this example.

In the second stage, called the absorption stage, the regulator holds the voltage constant at 14.4 volts and the current begins a slow decline. We need an additional 79 amp-hours of charge (including inefficiencies) to take our bank to the 90 percent level. Here the three-stage regulator closely matches alternator output to battery acceptance. It still requires an hour or more to reach our target charge level. Some regulators terminate this stage based on time rather than charging current. The real effect of this alternative protocol is insignificant.

The third stage is called the float stage. If you continue to run the engine (to turn the prop, for example) and alternator output remains 14.4 volts, the already-gassing electrolyte will begin to boil away, and the positive plates inside the cells will begin to oxidize—corrode. Corrosion of the positive plates is the number two cause of battery death. At the end of the absorption stage, a three-stage regulator reduces alternator voltage to around 13.2 volts, allowing the alternator to maintain a full charge on the batteries without overcharging.

This regulator offers simple, single-stage regulation that’s easy to install and to use, but how does if fare compared to a three-stage regulator?
Standard Regulator   
The standard regulator that comes with nearly all marine engines is essentially an automotive regulator. In the distant past, 12-volt regulators were mechanical and prone to problems, but for nearly half a century now regulators have been solid state. Modern regulators exhibit two highly-desirable characteristics: they are cheap and astonishingly dependable.

How does the performance of a standard regulator compare to that of a three-stage regulator? Not very well if the regulator is an automotive unit with a cut-out voltage around 13.8 volts. A 13.8-volt regulator may be satisfactory on a powerboat, but it does a lousy job of charging sailboat batteries. Raising the battery charge level from 50 percent to 90 percent with a 13.8-volt charging source takes more than five hours. Few of us can stand to run the engine at anchor for that long, so unless there is an alternative charging source, the batteries get fully charged only when the boat is under power for a long period. The rest of the time the batteries remain undercharged. The inevitable result is a condition known as sulfation, which is the number one cause of battery death.

A higher cut-out voltage dramatically improves the charging performance of a regulator. Since we want to drive the battery voltage up to around 14.4 volts before we reduce alternator output current, it stands to reason that the appropriate cut-out voltage for a standard regulator to be used to charge batteries on a sailboat should be 14.4 volts. How does a 14.4-volt regulator perform? In our ongoing example, our 110-amp alternator should deliver close to 110 amps until the battery voltage reaches 14.4 volts. At that point, the regulator holds the charging voltage constant while the current begins a slow decline. We might expect the 50-percent-to-90-percent-charge cycle to require around two and a half hours. If you are thinking “Gee, isn’t that more or less the same as a three-stage regulator?”, you get a gold star for paying attention. A standard regulator is, in effect, a two-stage regulator, maximizing current output while the battery voltage rises to the set level, then holding voltage constant and allowing output current to decline. Given flooded batteries and the same alternator, a smart regulator in a head-to-head competition will reduce charging time not more than around 15 percent over a 14.4-volt standard regulator.

The fuel savings of reducing engine hours 20 minutes a day can pay for a smart regulator, not to mention the value of reduced engine wear or of 20 extra minutes of quiet.
Does that make a three- or four-stage regulator a waste of money? No. Just the fuel savings of reducing engine hours 20 minutes a day can pay for a smart regulator, not to mention the value of reduced engine wear or of 20 extra minutes of quiet. And let’s not overlook the third stage. Reduced float voltage is kinder to your batteries. That said, however, unless you motor for days on end, damage from overcharging with a 14.4-volt regulator is likely to be minimal. This is because regulators are temperature compensated to lower output about 0.01 volts per degree centigrade of ambient temperature, so if the engine compartment warms by 50 degrees C (90 degrees F), a 14.4-volt regulator actually holds the alternator output at 13.9 volts—not ideal, but not beyond the tolerance of most deep-cycle batteries as long as you pay attention to their water level.

The bottom line is that the major benefit of a smart regulator is that it is matched to the needs of your batteries, which will maximize battery life. Unfortunately, most sailors I meet are contemplating this expenditure in hopes of a significant reduction in charging time. If you are in this group and your boat is already equipped with a 14.4-volt regulator, you are going to be disappointed.

Equalization—The Fourth Stage

Many “smart” regulators have an operator-initiated fourth stage that feeds a fixed current of less than 4 percent of amp-hour capacity to the battery until battery voltage rises to its maximum natural level—generally around 16.2 volts.

The purpose of this treatment is to restore batteries to full capacity by converting sulfate deposits in the plates back into active material. Equalization normally takes several hours. Batteries should not be equalized more often than every 30 to 50 discharge cycles.

Equalization is only appropriate for flooded deep-cycle batteries. It is likely to do more harm than good to thinner-plate starting or “dual-purpose” cells. Gel cells and AGM batteries must never be equalized.

Don Casey is offline  
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