Mailport August 15, 1998 Issue

Mailport 08/15/98

Wiring Batteries
As you stated in your May 15 answer to the question regarding 6-volt vs. 12-volt batteries, four smaller 6-volt batteries can be more easily installed than two larger 12-volt batteries. However, your concluding statement that “…a 6-volt battery of the same approximate size/weight as a 12-volt battery will have about twice the capacity, say, 210 amp-hours instead of 105 amp-hours…” needs clarification.

The problem is in your use of the term “capacity” when referring to the amp-hour (AH) rating of each of the batteries, which gives the impression that the 6-volt battery has twice the energy content of the equal size/weight 12-volt battery. The energy content of a battery can be measured by its watt-hour capacity, which in turn is a product of volts x amps x hours. In your example, the 12 volts x 105 amps x hours (1,260 watt-hours) for the 12-volt battery equals the 6 volts x 210 amps x hours (1,260 watt-hours) for the 6-volt battery. There is no free lunch!

In addition to the easier installation, it is also possible to save money using four 6-volt batteries because of the wide variety of 6-volt batteries available through the golf cart market.

Mark Chramiec
Newport, Rhode Island


Under normal circumstances, two 6-volt 220-amp-hour batteries in series will perform exactly the same as two 12-volt 110-amp-hour batteries in parallel. In a 12-volt parallel installation, if one of the batteries develops an open cell, the system voltage will not be affected and you will continue to operate normally but with the capacity of a single battery. In a 6-volt series installation, an open circuit or defective cell in either battery will completely disable the entire battery system. Twelve-volt parallel operation is the preferred method if only two batteries are used and reliability is paramount. The ultimate system would employ four 6-volt golf cart batteries connected in series/parallel with a battery selector switch. You could then enjoy the longevity of 6-volt batteries without having to worry about an open cell creating a total system failure.

The real benefits of 6-volt batteries lie in their construction. A 6-volt battery has massive plates compared to a 12-volt battery and can be cycled through approximately 700 charge/discharge cycles compared to the 350 or so that you get from a typical 12-volt deep-cycle battery. This doubles the life of your system for approximately the same cost.

John Branch
Chandler, Arizona


Inverter Tests
Regarding the February 15, 1998 test of inverters, I have been cruising a sailboat for the past seven years, in hot and cold climates. Three years ago, I purchased a ProWatt 800 inverter to recharge the batteries on a notebook computer. The inverter fulfilled this role while at sea, but upon arrival in Alaska, while living aboard in cold water, it quit working.

The inverter was bolted to an inside bulkhead, in the heated cabin, as instructed. However, as everything inside a boat sweats when in cold water, and heated below, the inverter began to sweat also, as it is mounted on a metal plate of its own.

A week after hand-carrying the inverter to ProWatt’s parent company’s factory, StatPower, a message on my home telephone answering machine said that “water had been pouring through the unit.” When I was finally able to speak to one of the technicians, he stated that water spots within the unit were proof that it had been leaked upon, and that it required a new circuit board at a cost of $350 (a complete replacement from West Marine costs only $399!).

Apparently, StatPower’s products cannot withstand any kind of moisture. My advice is to find something of marine quality.

Robert Jans
LaGrange, Illinois


Unfortunately, we are not aware of an inverter that is specifically made to be waterproof. If one exists, we’d like to hear about it.

As for the replacement cost of the circuit board, we found Ivan Hills’ letter in the April 15 issue, “The High Cost of Spare Parts,” very interesting, as he explains the corporate thinking behind such apparent anomalies.


Some months ago, I purchased a PortaWattz 300-watt inverter from West Marine. The catalog and the label on the item both indicated that the unit would produce 300 watts AC power. It comes, and is shown in the catalog, with the insert for a cigarette lighter outlet.

What is indicated only in a brief reference in the information accompanying the unit is that it will produce only half the stated output when plugged into a lighter outlet. In practice, I find that it has difficulty supplying a lap top computer that normally draws just over 50 watts. Unlike the West Marine catalog, which implies that it will work down to 10.6 volts, mine cuts out at about 12.4 volts, meaning that I must often run the engine if I am to use my computer.

I recall having read in Practical Sailor a while ago that there was some dissatisfaction with small inverters, for the reasons I have given.

Michael Jarvis
West Vancouver, British Columbia


You’re right about reduced output when using a cigarette lighter, which makes a good case for using another, higher-rated type of plug. As for the inverter cutting out, either your inverter isn’t working properly, or you may not have sufficient voltage at the receptacle; it may well be less than the 12.4V measured at the battery.

Also, Tom Hale of ABYC recently warned us that most pocket inverters do not include a GFCI. He said that if a ground fault occurs in a tool powered by such an inverter, and the operator makes contact with ground (engine, mast, etc.), he, too, becomes a path and may suffer a shock.


Why use an inverter for TV, SSB, computers, when they can be purchased in 12-volt versions?!

You also dismissed the square wave inverters as of “limited use with…sensitive electronic equipment.” This may be true, but see the previous paragraph. I use my inverter to run a sewing machine and handheld tools like an electric drill (which could be purchased in 12V versions), which are not very sensitive to the wave form and function very well on a low wattage, inexpensive inverter. My inverter is very inefficient as far as a conversion coefficient is concerned, but it cost less than $200, which brings me to a third point.

Why spend the $300 you mentioned for an add-on, which is a battery charger? What is going on here? Isn’t the inverter running off the ship’s battery bank? Are you selling us a perpetual motion machine? Perhaps you want the readers to believe that a very efficient inverter can actually charge the battery it is using for a power supply? Whoa! I can sell my solar panels, get rid of the water generator and buy an inverter that will supply power to my sailboat’s battery! Now that is being a Practical Sailor!

Lary Wasserman
Coronado, California


You’re not fooled are you? Actually, the battery charger supplied with some inverters is intended for use with AC shore power, not from the ship’s batteries, which as you note, would be ridiculous.


Where Credit Is Due...
To Aqua Signal: “I sent an eight-year-old fluorescent cabin light back to Aqua Signal in Batavia, Illinois, for repair. It came back promptly, in working order, marked ‘No Charge.’ My thanks to Aqua Signal.”

Bruce Beh
Port Townsend, Washington


To ITT Jabsco: “I never expected a new unit. ‘Just repair it and send me the bill,’ I wrote to ITT Jabsco when I shipped them my six-year-old macerator pump after the housing cracked. Their response to my problem made my day!”

Jeff Olson
Sarasota, Florida

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