PS Advisor October 2008 Issue

Wanted: A DC-DC Converter

Reader looks to downsize his battery bank.

I’ve created a 24-volt system by connecting two 200-amp-hour 12-volt batteries in series to drive an electric outboard as auxiliary power for our 25-foot sailboat. The 24-volt bank will be charged using a 24-volt charger on shore power and by a 24-volt series of solar panels when mooring.

I would like to eliminate the 12-volt batteries. I bought a 24- to 12-volt converter to stand in place of the 12-volt batteries, but I learned that the converter is not compatible with driving any kind of motor due to the back-voltage created by the collapsing field when the motor stops. I have a freshwater pump and a motorized outboard-motor bracket, so this particular converter is out of the question.

Do you have any suggestions? Must I maintain a 12-volt battery for all the 12-volt equipment or is there a step-down technique?

Michael Hoffman
Concord, Mass.
Puffin, Com-Pac 25

Unfortunately, you are working with electric motor circuits in your electric outboard, its lifting bracket, and your potable water pump.

In electrical systems, motor circuits fall into a special category generally described as "inductive" loads. This load type has several characteristics that make the use of a typical DC-DC converter as a replacement for batteries essentially impossible. Besides the back EMF (electromotive force) issue that you mention—which is quite correct—there is also the matter of motor start-up current measured in amps, as well as the typically high amperage that motors need to run continuously. Start-up current for any motor, AC- or DC-powered, can often run as much as four to six times the continuous running current or amperage. So let’s say the running current draw for a motor is 5 amps; its start-up current draw might be as high as 30 amps. A motor that draws 50 amps continuously could conceivably draw as much as 300 amps on initial start-up. Granted, this start-up current draw is usually measured in milliseconds. None-the-less, the power-providing equipment has to be capable of delivering the needed current just to get things going.

In our search of DC-DC converters, we found no such equipment available. Batteries as power-storage devices, with high current discharge capability—typically measured in either CCA (cold cranking amps) or MCA (marine cranking amps)—are the only way to power your system.

Sealing LPG Fittings

Planning to change my propane regulator and fittings, I began looking for the best sealant to use on the brass NPT fittings. After a half-hour on the Internet, and talking to a couple of chandleries and other boaters, I was surprised at the varied information I received.

What do you think is the best way to seal the brass fittings for marine use? I tried the "use nothing, brass NPT will self-seal" approach and can say that, in my experience, it does not.

Frank Tansley
Grace, Hans Christian 41/T
Ventura, Calif.

According to Trident Marine, maker of marine hose and LPG systems, the only self-sealing NPT fitting is the 45-degree SAE flare fitting. For others, the company suggests Teflon tape, a Teflon thread compound, or Loctite.

Brad Clarke at Force 10, which manufactures top-rated LPG galley stoves (July 2007) and marine grills (June 2006), said you must be sure to use a high-temperature acrylic sealant. He recommended Loctite 567, a smooth and creamy, one-part thread sealant. To apply, clean the fittings with a compatible solvent. Distribute a 360-degree bead of the 567 to the male fitting and put the fittings together. Properly sealed fittings will seal instantly. A minimum 24-hour cure time is required.

For more on Loctite and its application, visit the manufacturer’s website,

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