I was wondering if you could elaborate more on how to make your own battery packs... I can get the cells/connectors, but I'm not sure how to put them together?

From : Don Stackhouse

Chris, there are a lot of different schools of thought on this, and for some folks it approaches a matter of religion. However, there are several approaches that seem to be generally accepted.

1. Call E.H. Yost, SR Batteries, Garrison Aerodrome, etc. and have them solder up a pack for you. It's a bit more expensive, but these folks do a good job for the money, and it also leaves you with more time to fly!

2. Buy cells with solder tabs. The serious electric fliers are likely to turn up their noses at this, since the solder tabs can add resistance to the connection. For an electric powered model pulling a few dozen amps, this can cause some very significant power losses, which show up as extra heat in the battery pack. Definitely not a good thing. For battery packs that only have to power the radio, the current is much lower and the losses aren't that significant.

When using solder tabs I usually bend the tabs to be joined out to the side of each cell, then lay the cells side-by-side with the tabs overlapping each other. Remember to double check the polarity, you want to join the cells plus to minus, NOT plus to plus, or minus to minus! A touch of the soldering iron (a 25 watt iron is normally sufficient) and some rosin core solder joins the tabs. I then fold the two cells back in line with each other to form an in-line pair. For a 4-cell flat pack I make two of these pairs, then lay them side-by-side and join the two pairs together at one end (using the solder tabs), then solder the connector cable ends to the other end of the pack. Once again, be sure to check polarity on these connections. The connector should have the red wire connected to the battery's positive end, and the black wire should go to negative.

After getting all of this hooked up, protect the battery with a shrink-tube sleeve. This protects against electrical shorts, and also keeps the individual cells from squirming around and fatiguing their solder joints.

3. Put together the pack using end-to-end solder joints, with no solder tabs. This is generally considered the ultimate joint from both a weight and an electrical resistance standpoint. If done right it's a thing of beauty, and will baffle all of your friends who aren't familiar with the method, sort of like the mystery of a ship in a bottle ("Now how on earth did he do that??!!"). If done wrong, it can easily destroy a cell, since you will be soldering directly to the cells themselves, with no solder tabs to protect the cells from overheating.

The procedure requires some special tools: a piece of angle big enough for the cells to lie in the "V", and a special "hammerhead" tip for your soldering iron. This tip looks something like a capital letter "T", with the upright portion screwing into your iron, and the actual soldering taking place at the ends of the crossbar. Check magazines like "Sailplane and Electric Modeler", I've seen them advertised occasionally, It's also possible to make your own if you're used to working with copper bar stock.

First clamp the angle in a vise or any appropriate holding fixture. Test fit the cells to be joined, and make sure to double check the polarity. Lightly sand and clean the ends of the cells to be joined. Now carefully tin the ends of the cells. This should be done QUICKLY, so the absolute minimum of heat gets transferred to the cell. This is where a low-power iron is BAD, because its low heat will require longer to make the joint, and will actually heat the inside of the cell more. A 100 watt iron is probably adequate for most typical cells. In any case, you should need no more than 2 SECONDS to tin the cell end. Any more than this and you risk melting the nylon seal inside the cell, allowing venting and damaging the cell. You might want to practice this on some old, dead alkyline cells first.

Let the cells cool COMPLETELY, then set them in the piece of angle, with a space between them. The angle iron will keep them in line with each other during the joining process. Now use the special "T" soldering tip to heat the ends of BOTH cells simultaneously (remember, 2 seconds max!), then pull the iron out from between them and simultaneously slide the two cells together while the solder is still molten. Voila! one perfect end-to-end solder joint!

At the end of the pack where the two rows of cells are joined to each other, use a short piece of wire, or some copper braid. The braided copper de-soldering strip that Radio Shack sells works well for this. For smaller, low-amperage packs, some ordinary connector wire will do. Solder the cable and connector to the other end of the pack, just as before, but remember the 2-second rule on all of these joints too! A piece of shrink tubing finishes the job.

So how do you get one of these packs apart if one cell goes bad? Just pop it in the freezer and get it good and cold (which makes the solder joint brittle), then grab the cells on either side of the joint and give it a quick flex. The solder joint should pop apart nicely.

Don Stackhouse
DJ Aerotech