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ELECTRICS by Keith Shaw - Model Airplane News, April 1994

ON CONSTRUCTING AN IN-LINE BATTERY PACK

THIS MONTH'S COLUMN may seem an enigma, as I almost exclusively use the "stacked" configuration of cells for the Ni-Cd packs in my models. I wrote an extensive column on this method a few months ago, as I believe it to be the easiest, safest and most reliable way to make a battery pack. Although my scale airplanes rarely lack internal space (especially the Gee Bee R-1 !), I do occasionally build a sleek pylon racer or sailplane. It may be virtually impossible to fit a stacked Ni-Cd pack into these skinny fuselages, so I have to resort to the inline or "stick" arrangement of cells. Also, many kits and ARFs are designed for this configuration to utilize the abundantly available car packs. Most of these stick packs suffer the same shortcomings as the commercial stacked arrangements: thin sheet metal interconnects with poor or insufficient spot welds. For really good performance at high power, the cells need to be soldered together.
The in-line method of assembling cells does involve some risks and safety concerns. You must wear adequate eye protection as there is a chance that you might get hit with a fine mist of molten solder. Although this would cause little risk of a skin burn, it would be very dangerous for your eyes. It is also more difficult to control the heat input while soldering, so the Ni-Cds are at a higher risk of heat damage. I highly recommend that you get out your August '93 copy of Model Airplane News and re-read my column on Ni-Cds, since much of the information is also relevant to this technique.

GETTING EVERYTHING READY

To protect the cell from shorts caused by stray bits of solder or other conductive debris, a "bib" of heavy-duty masking tape will once again be applied. Use a sharpened piece of 3/8-inch-o.d. brass tube to cut a hole in the masking tape. Work on a piece of cardboard while holding the tape adhesive side up. The tape should fit snugly over the positive button of the cell, after which the excess can be trimmed flush to the edge of the case.

For best results, I suggest using cells without tabs, but if yours have them, remove as much of the tabs as possible and file off any remaining burrs. The cells will have to be tinned before assembly, so scrub both ends of each cell with Scotch-Brite, and use a zinc-chloride-based liquid flux, such as Stay-Clean. Carefully tin the positive button and an equal-size spot on the negative can end, using just enough solder to wet the surface. Excess solder will be expelled during assembly; this will make a mess and possibly be a safety hazard. Clean the ends with acetone or lacquer thinner to remove any traces of solder flux.

A simple assembly stage can be fabricated from 1/4-inch plywood. Cut two 6-inch strips, one 1/2 inch wide, the other 1 inch wide, and assemble them with 5-minute epoxy and a piece of 1/4-inch-square spruce to reinforce the joint. You could use a piece of 1/2-inch wooden corner molding if you can still find it, but everything I've found is formed plastic that would melt or break too easily.

Everything is ready to go

The sides of the iron contact the cells to melt the solder

The major difference with this technique is that the sides of the soldering-iron tip will be used rather than the point. It is necessary to tin the sides of the tip if it doesn't come that way. I use an Unger no. 4039 tip that is ironclad, but I tin it anyway before use. Be sure that the iron has at least a 50W rating and a large chisel tip that's 1/4 to 5/16 inch across. The Ungar no. 3310 element will also work well.
ASSEMBLING THE CELLS

Before going any further, you do have on your eye protection, right? Mount two cells on the jig with rubber bands and hold the jig so that your thumb pins down the lower cell while your index finger rests on the positive button of the upper cell. Have the cells spaced just far enough apart to position the end of the soldering iron between them. Gently squeeze the top cell down so that the iron contacts the positive button of the lower cell and the negative can of the upper cell at the same time.

When the solder melts on both surfaces, quickly pull the iron out and press the cells together, holding everything still for a few seconds to be sure the joint has sufficiently cooled. Just before withdrawing the iron, change the orientation so that your eyes and face are out of the "line of fire." If there is too much solder on the cells and/or they are slammed together too hard, the excess solder will be expelled in a fine mist.

After completing each joint, remove it from the jig, and tap the side of the joined cells on the table to dislodge any fragments of loose solder. Check the joint by carefully wiggling it. You should also inspect it using a magnifying glass and a bright light. A good joint will have no spaces or incomplete bonds. A poor bond can be separated by rocking the cells to pop them apart. It can be very worthwhile to practice the motions with a cold iron to get the timing and mechanics down. Then the first few "hot" tries could be done with some old cells to gain confidence and to learn to minimize heat transfer.

Continue adding cells until the desired stick "length" has been achieved. Since several of these sticks are bonded together to make the pack, some protection is necessary to prevent short circuits between them. The plastic shell on the Ni-Cds is not good enough, as I've seen it split or melt under traumatic conditions. Thin cardboard, such as cereal box or artist's railroad board, is quite a good insulator. Cut 3/8-inch-wide strips and glue them between the sticks using Walther's Goo or any good contact cement. The complete assembly can be stabilized with strips of strapping tape or by sleeving the entire thing with heat-shrink tubing. Finally, connect the sticks with heavy-duty copper braid, and attach the wire leads using the techniques discussed in the August column.

The assembly sequence for adding an "odd" cell to the end of a pack.

WHAT ABOUT THE ODD CELL?

No, not the orange one with the purple polka dots! I mean when an odd number of cells is required—most commonly seven. Although a stick of four cells and three cells could be bonded together, the most common practice is to mount the odd cell across the end of the pack. To do this, solder strips of copper braid to the end two cells, but facing away from each other (see photos). Attach the extra cell with a cardboard insulator, and hold it in place with a band of strapping tape. Make the cardboard insulator a little longer at the positive-button end for extra protection. Now fold the braid over the ends of the cells, cut the braid to length, tin it, and solder it into place. Remember that the cells are always connected in series, so each joint is a connection between the positive button of one cell and the negative can of the next. If desired, a heat-shrink sleeve can go over the whole pack.
WHAT'S NEXT?

After all this, everyone should be an expert on Ni-Cd packs. Look for articles about mounting direct-drive and geared motors, testing connectors, wire, switches and fuses and perhaps some simple structural mechanics to help you design and build better electrics. If you would like to know more about a particular topic, write to me at 2756 Elmwood, Ann Arbor, MI 48104, and I will consider it for a future column.