Battery charging under sail Part II – Regulation

In a previous article (“Battery charging under sail,” Issue No. 155, July/August 2006), I described the evolution of a generating system running off the propellor shaft to charge under sail. I discovered over many thousands of miles of sailing that the most reliable generator was a brushless alternator using a permanent magnet field.
A problem with this type of generator is that that regulating the output voltage for varying shaft speed is not as simple as the more common wound field alternator. For this type the regulator senses the output and adjusts the field current automatically to maintain the output at a desired level; typically 14.4 volts for a lead acid battery. Although in principle a mechanical solution is possible using a constant speed drive, this is complicated. To control the output electrically the regulator must be inserted between the alternator and the battery. The trick is to do it without wasting a lot of energy.

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


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

Because the converter uses high-speed switching circuits Lambda recommends filter networks on the input and output connections. For details see the Application Notes on the Lambda Web site. The converter dissipates its electrical losses as heat. I mounted the device on a thin sheet of aluminum, which also carried a conventional finned heat sink. It is mounted on a vertical bulkhead with small rubber spacers underneath so that air can circulate on both sides. The generator and converter must be protected from excessive currents caused by over speeding or internal component failure.
The generator is protected by a 10-amp circuit breaker and the converter is connected to the battery bank via a diode, adequately rated for the maximum charging current. The diode prevents the possibility of any reverse current flowing into the regulator; it also causes a roughly one-volt drop between the converter and the battery, thus the 15 volts at the converter terminal are reduced to about 14 volts, which is OK for a 12-volt battery.  The external trimmer mentioned in the application notes could be used to bring the charging level to 14.4 volts if desired.

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.



I began work on the regulator problem about three months before the start of my 2006-2007 voyage to Antarctica. Unfortunately, I then discovered the delivery time of the Lambda converter was about 10 weeks and, since my order had not been delivered by the time I sailed, I reconciled to using the limiting resistor as the means of regulation.
In November 2006, I left the boat in Uruguay for a couple of weeks and flew home. Amongst a vast stack of mail awaiting my return was a small packet with two converter chips. I mounted one on the heat sink and brought it back with me. When I got the chance I substituted the regulator for the limiting resistor and turned on. To my mild surprise there were no debugging problems, the device worked just as advertised, and I did, indeed, fit and forget it.

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.

By Ocean Navigator