nuts & bolts

craft & technique A Battery Charger Feedthrough Getting the current where it’s needed W I L L IAM RYNONE

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here are many factors that affect the ability of an aircraft engine to start. To list just a few, the ability of a battery to supply sufficient energy to the starter motor, the condition of the aircraft engine, the oil viscosity, the resistance in the starter cables and contacts of the main and starter relays, the volatility of the fuel, etc. Most pilots are familiar with engine starting difficulties due to cold weather. For those who are familiar but not conversant in the technical details, the following may be of interest: cold weather makes the oil more viscous, thus presenting a greater load on the starter motor and its ability to rotate the engine rapidly. One of the recommendations for cold weather starting is to pull the prop through numerous times to break hydraulic locks. This also has the beneficial effect of lubricating the cylinder walls. I have read that the greatest engine wear occurs during starting since some time must pass before oil becomes available to coat the cylinder walls, and the delay in lubrication is exacerbated by cold weather. Of course, preheating is desirable. Both the oil pan and cylinders should be heated. In addition to electric heating pads that may be epoxied to the aircraft oil pan, and/or devices that heat cylinder heads, similar heating pads may be enclosed in the battery compartment. For many plane owners, access to utility power may be impractical, so a heating pad may be ruled out. The fan motors in hot air blower types of preheaters are often 12-volt units, which may be energized from a car battery, either in a car or separately.

However, this article is intended to review what could be done to ensure the battery is able to deliver energy during starting. The ability of a battery to deliver adequate energy to the starter motor is a function of the size of the battery (number, thickness, and size of the battery plates) and the condition of the battery (how old is the battery, how many times has the battery been charged and discharged, how deep have the discharge and charge cycles been, how well has the battery been maintained, how recently has the battery been charged, etc.). With the advent of solar panel cells and the gradual decrease in their cost, the possibility of using a solar panel to maintain the battery charge becomes a viable option to a plane owner. For a lightplane, quite often all electrical connections to the battery are through the master relay. This condition makes it difficult for the pilot to place a solar panel on the plane dash, where it would be protected from rain and snow, and connect it to the aircraft battery. Running a cable from the solar panel inside the cockpit to the battery is a wiring challenge. Also, many light airplane owners choose to protect their instruments and upholstery from sun damage by using a plane cover, thus making it impossible to employ a solar panel on the dash. With the solar panel mounted external to the cockpit, one must decide where to place it and how to feed its electricity into the battery. One possibility is to use Velcro tape to attach the panel to the canvas plane cover. The next challenge is to determine a method of conveying the electricity from the panel to the battery. For a

With the advent of solar panel cells and the gradual decrease in their cost, the possibility of using a solar panel to maintain the battery charge becomes a viable option to a plane owner.

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craft & technique

The feedthrough device makes it possible to close and secure access panels while a solar panel or other device supplies a trickle charge.

battery located in an engine compartment, a wire may be snaked from the solar panel to the bottom of the cowl or through the cylinder air intake in the front of the engine. Neither of these options is desirable, since they would require some effort to string the wire for each charging cycle. If one opens an access panel, leaving it open for an extended period may be undesirable. A simple alternative is to make a battery charger feedthrough device. It is sufficiently thin that the access panel may be closed on the device and the Dzus fasteners or camlocks may be 110

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secured. The feedthrough is constructed from a 1-1/2-inch wide by 0.012inch thick piece of plastic tape, four double hole electrical terminals, two strips of 0.010-inch thick by 1/2-inch copper tape, and a DC power jack and mating plug. To construct the device, the plastic tape is laid sticky-side up on a piece of sheet rock (so soldering can safely occur) and secured in place via two pieces of masking tape. Next, the two strips of copper tape are placed upon the plastic tape, and then the electrical terminals are located at the ends of the copper strips. I used four straight pins to keep the terminals from moving while soldering them to the copper tape. After completing the soldering, a second strip of plastic tape is used as a cover. Then four wires from two connectors are soldered to the four terminals. Be certain to ensure that a red wire “in” is common to a red wire “out” at the other end of the strip, and do the same for the black

Attaching the copper strips to a piece of sheetrock makes for safer soldering. The completed strips are sheathed in plastic tape.

wires. Also, it’s common practice for the center pins of axial power connectors to be the “hot” (red) terminals. Although a solar panel was mentioned as a charging device, a 1-ampere trickle charger would also function through the entry strip. Do not attempt to use a high current charger with the entry strip, and do not use a high current fuse in the fuse holder. High currents would overheat the copper foil strips, causing them to melt the plastic tape and then short to the aircraft sheet metal. If questions arise, feel free to send an e-mail (text only) to the author at [email protected]. William Rynone teaches electrical engineering at Johns Hopkins University and flies a Mooney M20C.

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