Permanent Magnet Generators and Diversion Loads Info

Permanent Magnet Alternators and Diversion Loads  

Unlike current exited alternators, the kind in your car or powered by your boat’s engine,  those using permanent magnets such as  are found in wind and water generators, can not be “shut off” by switching off the field. They are “on” all the time and if turning above a threshold RPM, they are producing power. If your batteries are fully charged, the generator will continue to force current into the batteries, causing excess gassing, heat and eventually the batteries dry out and are destroyed.  

Consequently, it is necessary to “do something” with the un-needed power the generator is now producing.  One approach is to monitor the system and when the batteries are fully charged, shut off the generator. In the case of a wind generator this can be done by furling the unit off the wind so it can no longer turn and produce power, or on some models shorting the output by connecting a switch of suitable current carrying capacity across the positive and negative output conductors. Water driven generators can just be pulled out of the water, easy with a product like the DuoGen but not so easy with a towed generator and its snarled rope. You cannot just open the circuit because you may over speed and damage the alternator.

There are some problems with this easy and seemingly inexpensive solution;  

1. Nobody has a good enough memory to be a charge current monitor/regulator.  

2. Since you now may not have a charging source, assuming no AC charger, alternator or solar panels your batteries will be constantly cycled shortening their life.  

3. If you leave the boat for any length of time with the wind generator running, the batteries will over charge, gas and dry out. In the case of AGM batteries the overcharge can trigger thermal runaway with potential catastrophic results.  

I said seemingly inexpensive above but if you have to replace an entire battery bank or clean up the mess, it won’t seem so cheap.  

The appropriate solution is a purpose designed regulator like the Flex-Charge, see link below, and a diversion load which will absorb the excess power, in the form of a bank of appropriate sized resistors or a water heater element. There are advantages and disadvantages to both.  

Charging Source Conflicts: In applications with multiple charging sources like a cruising sailboat, conflicts can occur between the various chargers, alternator, AC charger, wind generator, water generator and solar panels. With charging sources requiring a diversion load to absorb the power being generated by the device when the batteries are fully charged, wind and water generators being the most common, it is necessary that the diversion load be connected only to the generator and not to the battery as is common with most regulators (even if the regulator has a diversion option).  Otherwise, when the engine driven alternator or other charging source is operating, without diversion isolation, the other regulators will "see" the diversion load, which may be as much as 500 watts for larger permanent magnet generators and continue to put out up to 40 amps trying to meet the demands of the diversion load. In other words, the alternator or other charger never stops charging which leads to overheating and premature failure, as well as wasting power and/or fuel and creating a continuous heat source from the diversion load as long as the other charge source is operating. The solution to this problem is the FlexCharge regulator with charge divert.

Pretty it may not be, but the NCHC will regulate PV, Wind, or any other current limited charging source with the highest efficiency possible 

The NCHC line of controllers are series type regulators for use on flooded lead acid, gel, or flooded NiCad battery technologies. The regulation voltage is adjustable through a wide range.   Available as an option with this controller is CHARGE DIVERT which allows you to use the excess unregulated energy from your charging source for other electrical jobs. The NC25A Regulates charging inputs from PV, Permanent Magnet Wind and Water Generators, or any other current limited DC charging source. Standard unit is for 12V systems.  Special order for 24V, 36V or 48V systems.  (1000A max charging current in this configuration). 

Link to Flex Charge Regulator information

Resistor Bank: The simplest arrangement is a bank of wire wound ceramic resistors of appropriate ohms resistance to absorb the maximum wattage plus a safety factor the generator is capable of producing. The advantages are simplicity, reliability and durability.  The disadvantage of the resistor bank is that it can get hot. In the case of a wind generator capable of producing 400 watts such as the D-400, if all the batteries are charged and there are no other loads to absorb the produced power it is like having four 100 watt light bulbs side by side. As a practical matter that is only rarely going to occur because there are almost always loads, a pump, the refrigerator lights, propane valves, etc. The most likely occurrence is if the boat is left with the wind generator operating and no loads, but even in that case the worst that can happen if the resistor bank is properly installed and ventilated is that the temperature will go up. Since with most wind generators, wind speeds of 15 knots are only going to produce about 100 watts the temperature effect will be a lot less. Bench tests at 385 watts would produce resistor temperatures as high as 360C (680F.) although the bus bars and heat sinks were considerably cooler. The composite base of the bus bars was 60C. (140F.), or the temperature of hot water in a tank.  Actually they were only barely warm to the touch. It is of course a rare situation when all batteries are completely charged or there are no loads, so there is always some reduction in current available to the diversion load. If a load comes on such as a refrigerator at 5 amps we will reduce the current to the resistors by about 70 watts. On the average boat with a consistent 15 amp load, or about 200 watts, temperatures would fall accordingly. With only 100 watts of power production and usage in excess of that, the diversion load will actually seldom come on.  

Water Heater Element: Another attractive solution is to use a low voltage water heater element in your on-board hot water heater. Now if you are making excess power you can use it for something more useful than drying socks. Without going deeply into the math, 300 watts will produce slightly over 1000 BTU/hr. One BTU will raise one pound of water 1 degree Fahrenheit. A 10 gallon water tank contains about 84 pounds of water. To raise 84 pounds of water from 70 to 110 degrees F. a 40 degree temperature rise, requires 3360 BTUs /Hr. divided by our available 1000 BTU/hr tells us that we would need 3.36 hours to attain our desired temperature. Unfortunately nothing is free or easy in the world of thermodynamics. The 1 BTU/pound/degree relationship is only accurate at the maximum density of water which occurs at 39.1 Fahrenheit and so the time will be slightly different. More important is tank loss. The average 10 gallon marine water tank will lose about 15 BTU/hour so in 4 hours we have lost about another 60 BTU or a required total of 3420 BTUs. Assuming we have 300 watts available which with most small wind generators would require 30 knots or more of wind, will result in 1023 BTU/hr. 3420/1023=3.3. The wind will still have to blow for about 3.5 hours before the shower is hot if there are no other loads on the system and the batteries are fully charged. With the average wind generator of the size commonly found on boats,  even in the  Trade Winds or the Caribbean with 15 knot winds we would only have about 100 watts available (7.5 amps at 13.5 volts), with no other loads and the batteries fully charged. Now we could stand around for 10-12 hours waiting for that 5 minute shower. In a cold climate the wait will be even longer. (With 100 watts of available power each 1 degree increase in water temperature will take a little more than 5 minutes.) This is not to say that a water heater element is necessarily a bad idea, but we can’t change the laws of physics.  

A different consideration is if the boat were left unattended for an extended period of time in high winds the water in the tank could boil, creating steam and potential explosion and damage. Even if the tank has a pressure relief valve, if we lose the water the element will go dry and burn out and the generator will now be looking at no load at all with potential damage to the generator itself. The long and short of it is that the boat should not be left unattended for an extended period with out disabling the generator.  If you do decide to use a water heater element you should choose one with a wattage capacity 20% greater than highest  potential generator output as a safety margin. For example, a 400 watt maximum generator should have a 500 watt element, we use 600 watts with the D-400.  

Another issue is that most marine water heaters have only one element, so if you replace the AC heating element, the only power source is DC. If you have an AC generator or are plugged in to shore power, and an adequate battery charger (you will need about 50 amps of charger output to run the element). Although there is a combination AC/DC element available, the DC wattage is not sufficient to absorb the output of a wind generator with the capability of the D-400 but could be used with very small generators of 200 watt output or less.

Copyright 2007-2008 Laurence L. Janke JD, Sc.D

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