Household lamps at sea

Good cabin lighting that puts minimum current drain on the battery bank can be a challenge on a voyaging boat. Most boat lighting uses small, relatively inefficient, 10- or 20-watt incandescent lamps that on average yield about 10 lumens per watt. Task-lighting fixtures benefit from use of the MR-16 series of halogen lamps. These lamps have integral dichroic reflectors and are especially valuable for the task lighting required at the chart table.

The small, usually twin-tube, high-efficiency PL-series fluorescent lamps first introduced almost 20 years ago are among the most efficient light sources. However, as with all fluorescent lamps, they must be powered from a controlled AC source, not directly from the boat’s 12-volt DC bus. While a few manufacturers produce lamp fixtures using the PL series of lamps, incorporating solid-state inverter ballasts, the small size of the marine market limits the available choice and ensures relatively high prices.

In an article "Cabin lighting choices" in the 2001 issue of Ocean Voyager we described how the PL-series of 9- and 13-watt PL lamps can be powered from the 12-volt system using solid-state inverter ballasts made by Iota Engineering in Tucson, Ariz. The combination of the Iota ballast and a PL lamp works very well; however, it is up to the boatowner to build custom light fixtures or convert standard light fixtures to accept the PL lamps. Not every boatowner wants to undertake this task.

The recent introduction of a number of 14- and 20-watt high-efficiency fluorescent lamps presents a new opportunity for innovative cabin lighting. These lamps usually sell for less than $10 and are efficient, producing on average 770 lumens, about 3.8 times as much light per watt as a standard 120-volt incandescent lamp. The lamp assembly contains an integral electronic high-frequency ballast and screws into standard household-type lamp fixtures. These lamps can be excellent area lights for a boat, provided a suitable source of 120-volt, 60-Hz power is available. Fortunately, the advent of small, reasonably efficient and low-cost 12-volt DC to 120-volt AC inverters makes it practical to power these lamps from the battery bank. Since the lamps fit standard sockets, we can take advantage of the many attractive, low-cost light fixtures sold for home use. (Brass, wall-mounting swing-arm lamps can be particularly useful, just be sure to check the "brass" with a magnet; brass-plated steel won’t look good for long on a boat.)

We conducted a couple of tests to determine the relative efficiency of producing light using the method of converting 12 volts DC into 120-volt, 60-Hz AC (so that the lamp’s ballast can then convert that current into the 40- to 70-volt, approximately 25-kHz AC used to power the lamp. Virtually all new fluorescent lamps operate at high frequencies to take advantage of greater light output produced when the phosphor coating on the interior of the lamp is maintained in an excited state through each AC cycle). Although the multiple voltage/frequency changes involved may seem a bit silly, it is quite efficient and much like the power supply operation in your laptop computer or battery charger.

The first test used a miniature 50-watt inverter to power a single 14-watt high-efficiency lamp. The idling current consumed by the inverter with no load applied was 0.06 amps at 12.7 volts, or 0.76 watts. With the 14-watt lamp plugged into the inverter, the DC current measured 1.4 amps at 12.5 volts, or 17.5 watts. Overall inverter efficiency when powering one lamp was therefore about 80 percent. The 770 lumens we obtained from the 17.5-watt battery drain far exceeded the amount of light a similar 12-`olt incandescent lamp, or even a halogen lamp, could produce. Overall inverter efficiency will increase somewhat when two 14-watt lamps are powered from this palm-sized inverter.

Boats that require more than a few general area lights can use a larger inverter to simultaneously power numbers of high-efficiency fluorescent lamps. The overall system efficiency will likely improve so long as enough lamps are on at one time to make up for the somewhat higher base idling current demand of the larger capacity inverter. We tested the idle current of a small 300-watt inverter, measuring an idle current of 0.13 amps at 12.5 volts, or 1.63 watts, twice the idling power of the 50-watt unit. (A substantial part of the increased idling current was due to the operation of a small cooling fan in the inverter). However, at 300 watts, its power rating is six times that of the 50-watt unit. The larger inverter is clearly a better energy choice whenever more than two or three 14-watt lamps are in use. However, given the very low cost of the 50-watt units, it may be desirable to use one for each pair of 14-watt lights or to buy a slightly larger unit capable of powering all the lights needed in one cabin or saloon.

The electrical wiring in a boat equipped with household-style light fixtures using the high-efficiency fluorescent lamps can be set up in a number of different ways. If the boat has a large, general-purpose inverter, the lamps can be simply plugged into the AC outlets. They will draw power from the battery bank via the inverter when shore or generator power is not available. Alternately, a separate 120-volt circuit can be run to a few of these general area lights and connected to whatever size inverter is appropriate for the number of lamps used.

One precaution needs to be taken into account when using any inverter, including the smallest units, to provide AC power. Always assume the inverter is on and remember 120-volt AC is a potentially lethal voltage. Disconnect the inverter’s primary supply wires from the 12-volt system whenever you have to work on the AC feed from the inverter.

The dedicated PL-lamp inverters mentioned in our previous article and made by Iota are still the most efficient way to convert DC power into light. If you have the time and dedication needed to make your own light fixtures, you will enjoy the resulting efficiency. The cold cathode lamps mentioned in the previous article also deserve your evaluation. They cost more but are far superior to any of the conventional, single-tube, 12-volt-powered fluorescent lamps sold for operation from a 12-volt DC supply.

Contributing Editor Chuck Husick is an electrical engineer, sailor, pilot and Ocean Navigator seminar instructor.

By Ocean Navigator