Making your own flight packs.

As most of the flyers from the electric fraternity know, flight packs or cell packs can be very expensive. There are ways however that the cost can be minimised, and saving yourself a few quid can make all the difference between getting that beautiful scale model or not.

Aero modellers rely a lot on new technologies, such as smaller more powerful engines fuel with fewer impurities and of course, Ni-Cad cells. They are used in hundreds of devices from mobile/portable telephones, to cordless power tools. Mobile phone cells aren’t really suitable for the purpose of powering a model aircraft as they don’t have the voltage or the high amperage required. Small cell packs from cordless phones can be cannibalised for use in small park fly or indoor models, as they are light and are the perfect size to squeeze into a small fuselage. This is perhaps all they are good for, as they don’t often have a very high milliamperage, 300mAh being the norm. Cordless power tools on the other hand are an excellent source of sub C Ni-Cad’s in varying amperage. If you know someone who has used these tools, or indeed if you have one or more at home that you believe can no longer serve a useful purpose, then crack open the power pack case, and hey presto, a neat group of sub C cells ready to be turned into a flight pack. The group of cells above were taken from a 14.4v cordless circular saw. As you can see from the image below, each cell has an average amperage of 1200mAh, perfect for an average size model. The amount of cells you get per pack depends on the power of the tool in question. Without a doubt, the most common voltage is 9.6v, although I have seen power packs going up to 24v. With each cell having a voltage of 1.2v, it is just a matter of finding out how much voltage you wish your flight pack to deliver. For most aircraft that use electric flight packs, then I would say that a 10 cell, 12v pack would be the highest you would want to go, with others of 8.4v, 9.6v and 10.8v being very handy for certain situations. A Multiplex Twinstar for instance will fly fine on an 8.4v pack, but slap in a 9.6v or a 10.8v and watch the performance increase dramatically. However if you try a 12v pack, the additional weight will probably decrease the performance. That is not to say it won’t go vertical like a homesick angel though! Anyway sorry to detract from the main article, making cell packs.

To complete one cell pack you will need the following: Required number of cells A hot melt glue gun with glue sticks About 18 inches co axial wire braid* A soldering iron Solder with some flux 70mm wide heatshrink tube or Insulating Tape 27 amp automotive cable 1 male & 1 female Corally plug with heatshrink tube * To obtain the braid, you need to strip the outside sheath of the co axial cable, to reveal the outer core. This is what we are after, as it is braided copper, and perfect for cell packs. Carefully cut the sheath for about 18 inches, then cut the entire wire, and gently pull off the copper braid. Once you have this pull it tight, and you have what you need.

First decide in which order you wish to lay out your cells. Think of commercially available packs, and stick to that. It’s a tried and tested shape with cell packs. If you have an even number of cells, you can lay them out end to end or side by side. End to end Or Side by side By that I mean half the total number of cells laid end to end, the same with the other half, and then laid next to each other.

Once you have decided the above, you need to start attaching the cells to each other. This is done with a soldering iron, a steady hand and some patience! Make sure the positive part of the cell is clean and dry, and apply a small amount of solder to the area only, making sure not to get any outside of the positive area. If you do, it will cause a short and at best ruin the cell, and at worst cause an explosion. Once done, make sure you have the second cell ready, and the negative area is clean and dry. Melt the solder on the first cell and quickly bring them together. If this is done correctly, you will have a semi-strong joint. Repeat this procedure with the third cell, and repeat so that you have two three-cell packs. If you have chosen to lay the cells out in the side by side arrangement, it is best to “tin” the positive and negative ends of the cells before soldering the coax cable, as this make the job a lot easier, and provides a much stronger joint. Lay the packs side by side making sure that one is positive at the top and negative at the bottom, and vice versa for the other one (See diagram below). Take the glue gun, and run a thin bead of glue down the join of the two cell packs. Turn over and repeat. This will give you one cell pack of 6 cells – 7.2v. We still need to add one more cell. Take some co axial braid, and cut off about 1 ½ inches. Bring the fibres together tightly, and make sure it is flat, and solder to the bottom (negative) end of the first cell pack. If there will be contact between the positive and negative area of the cell, insulate with a small amount of insulating tape. Solder the other end to the positive end of the single cell. Repeat this for the other end, making sure that you only connect a positive terminal to a negative one, and vice versa. Once this has been done, hold in place with some hot melt glue on both sides. You should now find that the cell pack is taking shape. Now the final bits. Cut 2 lengths of 27 amp cable, 4 inches long, and strip the ends about 5mm. Solder one wire to the positive cell, and the other to the negative end. It is a good idea before doing this, to cover the ends of the cable with insulating tape to prevent shorts. Once the wires have been soldered, fit the Corally plugs. This is best done with a gas type soldering iron, i.e. the type that relies upon an extremely hot flame to melt solder instead of a hot tip. Grip the plug upside down with a pair of pliers, and place about 1.5cm of solder into the solder hole. Use the gas soldering iron to melt the solder, and place the wire onto the melted solder. When you are satisfied that the wire has a good connection to the plug, insulate with the supplied heat shrink tube.

Now you will have a completed cell pack, but we will tidy it up a little by covering the whole pack with some 70mm heatshrink tube, or wrap some insulating tape around the whole thing and hey presto! The same sort of thing can be done for RX packs, using standard Ni-Cad or Ni-Mh cells. I got hold of some 800mAh Ni-Cad AA size cells from my local market for £1 for four. Before After A bit of soldering and time, and I have now got a new receiver pack, for a fraction of the price I would pay in the model shop.

I hope this article proves helpful to anyone wishing to create custom battery packs for electric flight, receiver applications or just save some money. If you have any questions, Email Matt or post them in the forums.

Article by Matt Carter For Bumpy Green Model Aerodrome
April 2004