Three Ways to Build a Battery Pack

If you fly electric models, one thing is certain: sooner or later, you'll want to build a battery pack. There's no magic involved, and the following steps provide you with three approaches to the process. Most of you already have the necessary equipment, and by building your own packs, you can configure them any way you want. You can also remove and replace defective cells without replacing the whole pack. Be sure to check the voltage of each cell before you solder it into a pack; you don't want to use a dead cell by mistake.

1 BRAIDED PACKS

When cells are soldered upright, they have to be restrained. Place a drop of hot glue between the cells to keep them in position. Each cell can then be tinned with a bit of solder. To help prevent the battery braid from melting the cell's shrink-wrap and shorting the cell during soldering, I place a small piece of heat-shrink tubing around the center of the battery braid. Some modelers prefer to use a small piece of cardboard under the braid. Make sure that you've oriented the cells properly so that you're connecting them in series (positive post to negative post). Double check the cells before you start to solder.

Place the heat-shrink tubing on the braid and then shrink it. As you attach each braid, the solder joint should be bright and shiny; if it isn't, you've got a "cold solder joint," and you need to redo it. If you look closely at the photo (below left), you can see the hot glue drops that secure the cells.

The finished pack (below right) has two packs of 4 cells connected by a solder joint on top of the 2 cells at the far right. Once a pack is complete, I always charge it the first time at 1/10C to equalize the cells.

2 STICK CONFIGURATION PACKS

Now we'll build a stick pack in which the cells are soldered end to end. I placed tape over the ; cell on the right to make a bib and then cut its center out with the sharpened brass tube. You can use scissors or a sharp hobby knife to trim the outer edge so it fits the cell. The bib is placed on the positive end of the cell (button), and that helps prevent little bits of solder (slag) from shorting the button to the cell's case. Before you start, tin each cell lightly. Use a hammerhead tip on your soldering iron; it makes building these packs much easier. They are available from Hobby Lobby and New Creations R/C.

The cells must be aligned before you solder them; use a slotted channel board instead of drops of hot glue to secure them. Place two cells in the channel (positive post to negative post). Place the hammerhead between them, push the two cells against the iron for a few seconds and then pull the iron out. This step happens very quickly, and if you use too much solder, it will splatter out from between the cells. Be sure to wear your safety glasses! As soon as you pull out the hammerhead, quickly and firmly push the 2 cells together and hold them in place until the solder has hardened. This takes only a few seconds. Repeat this step for each cell you add to your stick.

To clean any slag between the cells, I use dental floss and then hold them up to a light to ensure that I have a good, clean joint. If there is a large blob or small chunks of slag between the cells, they'll have to be re-soldered. Next, you need to solder the connector leads to the pack. To reduce strain on the leads, attach the leads and run them down the center of the pack before you add the shrink-wrap.

TOOLS OF THE TRADE

To solder cells to each other, you'll need a 40W iron with a wide tip, as it applies a lot of heat to the ends of the cell in a short time. The ends of the cells must be thoroughly cleaned before you solder; I use a Scotch-Brite pad for this. Never use steel wool because the steel fibers can shed and cause a short circuit. After I've scuffed the ends of the cells, I wipe them with acetone.

Stick packs are configured with the cells end to end, and they need to have a "solder bib" between the cells to prevent a short circuit. I use masking tape to make bibs and the sharpened end of a brass tube to cut the centers out of the bibs. To hold side-by-side packs together, I use narrow tape, but you can use shrink-wrap. For the least resistance, I use flat battery braid to connect the cells. When you build a pack with the cells end to end, use a hammerhead tip to heat two cells at the same time.

You need only basic soldering skills; just make sure that everything is clean and tinned. Don't forget your safety glasses-they're a must when soldering.

3 SOLDERLESS POWER TUBES

What if you don't want to solder, or you're just not very good at soldering? No problem, thanks to Solderless Power Tubes (SPT) from MEC (Model Electronics Corp.). Pete Peterson has been selling this system for years, and he continues to perfect it. With SPTs, you can easily assemble end-to-end packs of any size in a matter of minutes, and if you have a defective cell, they allow you to disassemble a pack and replace it easily. Let's see how it's done.

Once again, be sure to clean the tops and bottoms of each cell. MEC has recently begun to include silver paste with SPTs to coat each cell before pack assembly to reduce current loss by pressure-fitting them instead of soldering them. A little paste goes a long way, so only use enough to make the cell's button look dull; apply it to the top and bottom of each cell before you slide them into the tubes.