My first solution, installed very quickly, did just that. I simply placed a resistor in series with the alternator and when the boat speed was low a switch shorted out the resistor to get maximum charge, when the speed increased I threw the switch to insert the resistor and thus limit the current. Although the system needed attention, it was not as bad as all that; because the charge current was only a few amps I rarely needed to insert the limiting resistor. Obviously something better was needed; a regulator that could be fitted and forgotten and, hopefully, did not waste a lot of energy. When I got the time between trips I found the ideal regulating device — the DC-to-DC converter.
The DC-to-DC converter
This device does for direct current what the transformer does for alternating current, namely change the input voltage to a different output voltage while maintaining the same wattage, apart from small internal losses. It is a fairly recent addition to the family of power electronics. It works by chopping up the input into small packets of energy at high frequency and then recombining them to yield the desired output voltage. Typically the input may vary by a factor of two or three but the output level remains constant. Because switches are used, not resistors, the losses are low, typically less than 20 percent of the power transferred. The unit I selected is the model PH100F 24-15, manufactured by Lambda Electronics Inc. It is rated for 100 watts, with a nominal output of 15 volts and an input range of about 16 to 40 volts. Protection against overvoltage and overcurrent is provided internally. It is possible to vary the set output voltage slightly by means of an external trim resistor. Full details can be downloaded from www.lambdapower.com.
Operation under power
When the boat is under power the shaft rpm increases by a factor of about four compared to the speed under sail. If the shaft generator was left in service under these conditions the thermal rating would be greatly exceeded. This is avoided by connecting the generator output to a relay, which is energized when the engine switch is turned on. In this mode the generator is connected to a fairly high-value resistor; if left open circuit the voltage rating of the rectifier diodes inside the generator may be exceeded. I used an old wire-wound resistor I had lying around but a 25-watt, 120-volt incandescent lamp would be quite suitable to lightly load the generator when under power. When the relay is not energized the generator output is connected directly to the DC/DC converter.
Some practical considerations
The basic schematic of the complete regulator, incorporating the considerations mentioned above, is shown in the figure, and a few details must be added in the practical realization, as outlined in the Lambda Application Notes. The photograph shows the completed regulator mounted on the engine room bulkhead.
Whenever the boat speed was sufficient the ammeter showed current pumping into the batteries. A minor problem, to be corrected when I get home again with the boat, is that the regulator kicks in when the generator output is about 16 volts. The pulley ratio was originally chosen to charge at 12 volts when the boat speed was 4 knots. Thus, charging starts with the new regulator at a boat speed of about 5.5 knots. This is a little too high. But it is easily dealt with by fitting a slightly larger pulley on the propellor shaft. At the other end of the scale, when the boat was exceeding 8 knots in high winds, the regulator worked perfectly and, depending on the state of charge of the battery bank, I never saw the current exceed 6 amps, well within the rating of the generator and regulating components.