1. Soldering Iron
This is by far the most important tool that you'll need for obvious reasons. Buying a good soldering iron is a must for any RC hobbyist who wishes to play with high performance equipment. Not only is it useful for assembly battery packs, but also for installing power wires on electronic speed controls and motors.
The two most important factors affecting soldering iron performance is wattage and tip size. The wattage is a figure that indicates how much electrical power the soldering iron is using to generate heat. A higher wattage number indicates an increased ability for a soldering iron to reach operating temperature. For example, given two irons that are identical except for their wattage, the one with higher wattage will reach a given temperature faster.
The tip size of a soldering iron affects the iron's ability to maintain its tip temperature. A large and heavy tip holds more heat, so when you touch it to whatever you are soldering, the tip temperature doesn't drop as much. However, a tip that is too large may not be heated adequately if the soldering iron doesn't have enough power. So a combination of a good wattage rating as well as a matching tip is what you should look for.
For soldering batteries, I find that a soldering iron with between 40 to 60 watts of power and a 4mm chisel shaped tip works the best.
2. Solder
This is another obvious requirement. Solder comes in many flavors but the one we are interested in is the 60/40 type with a rosin core. These two requirements are very important so you should not deviate from them unless you are going after something unique and you know what you are doing. The good news is that this type of solder is available at your local Radio Shack in 2.5-ounce spools. Just walk in and say, "I need a 2.5-ounce spool of 60/40 rosin core solder". The drone you've just spoken to should then retrieve for you exactly what you need.
On the topic of silver solder, I'll just leave it up to the readers to make up their own minds. The benefits of silver solder are that it results in a stronger joint, and silver has lower resistance than either tin or lead. But I personally don't use silver solder because it is a lot more expensive, harder to work with, and its benefits have very little practical advantage over tin/lead solder.
3. Battery Jig
When the battery is being assembled, the cells need to be held side by side in a straight line against each other. Trying to do this without a battery jig would be very difficult. There are numerous battery jigs on the market ranging in price from $5 to $20, and I've used most of them in search of the perfect one. I'd have to say that my favorite is the W. S. Deans battery jig for around $14. Those costing less are made from flimsy plastic that looks to be of the same quality as a happy meal toy. Which is an insult to happy meal toys because most of them work better as battery jigs.
4. Battery Bars
Cells in a battery pack are joined together with battery bars. These are strips of solid metal and some times braided wire strands that are soldered from one cell to the next in alternated fashion. There are many different brands of battery bars on the market and most of them are fairly good regardless of price. Which means you should just buy the least expensive one you can find.
For a battery bar to be good, it must satisfy the basic requirements of carrying a lot of current and having very low resistance. Luckily, both of these can be satisfied with inexpensive thick strips of copper. I typically stay away from wire braids because they are harder to work with. Most name brand battery bars are also shaped to fit the contours of a battery's terminals, and many have holes drilled into them for better solder flow. Both of these are good secondary features to look for.
Silver and gold bars? The main benefit of gold is that it doesn't tarnish. So for things where clean contacts are important, such as switches and connectors, gold plating is a good idea. But when we are talking about soldered joints, having gold plating is moot because the contact point is sealed in solder.
I've see two different types of silver bars: silver plated copper, or solid silver. Solid silver is just that, a solid strip of silver. Understandably, this is very expensive and rare. One can argue that silver has lower resistance than copper, but the extra cost doesn't justify the miniscule difference. Silver plated copper bars simply don't make any sense other than looking nice. What little silver gets plated on the bar will not result in any measurable difference compared to bare copper.
After working with different battery bars, I've come to like Deans Pro Bars 2.0. These are available at $10 for a pack of 25, so each bar costs about 40 cents, which is very reasonable.
5. Wire
As long as you are investing the money and time to build yourself a high performance battery pack, you might as well use some good wire. Look for a wire with soft and supple silicone insulation, fine strands of copper with a high strand count, and use as large a gauge of wire as you can get away with. Personally, all my battery packs are built with 12 gauge Deans UltraWire. I also like Team Orion's power wire but they cost somewhat more than Deans and don't offer anything extra.
6. Connector
Some people hard-wire their battery packs to the ESC. Doing so eliminates the need for a connector. Personally, I like the convenience of connectors and I don't drive at a level where I notice the difference between using Deans Ultra Plugs and hard wiring. So all of my battery packs use a pair of female Deans Ultra Plugs.
7. Goop
You can either use Shoe Goo or Carpenter's Goop. Both of these should be widely available at hardware and department stores. They are awesome for bonding the cells to one another. I have yet to have one of my packs come apart once they are properly bonded. Some people get lazy and skip this step. But they ultimately end up with batteries that have the accordion effect as the cells pull apart against the battery bars.
8. Hand Tools
You'll want some hand tools available. I recommend a pair of needle nose pliers, tweezers, hobby knife, and a wire cutter. You'll also need a clean dish washing sponge, and a roll of brown packing paper. The packing paper is available in rolls from Wal-Mart and office supply stores. Lastly, have some fine 400-grit sand paper handy.
9. Discharging Tray
A discharging tray is a device that will individually discharge the cells in a side-by-side battery pack. Everyone who uses side-by-side battery packs should have one of these. They are absolutely indispensable for taking a battery all the way down to empty without the risk of leaving one or more cells partially charged or causing cell voltage reversal.
This tool is helpful when building your battery packs. You'll need one later anyway, so might as well get it now and start using it.
10. Shrink Wrap
A properly built battery pack should have cells that are individually shrink-wrapped. This protects the battery matching labels from dirt and keeps them from lifting off and becoming lost. Some battery matchers sell their batteries with each cell already shrink-wrapped. But annoyingly, majority of them do not. For those that don't, you can buy individual shrink-wraps from Trinity at about $1.50 for a pack of 20 pre-cut segments. If you are going to build a lot of battery packs, they are also available at $5 for a pack of 100. If your batteries need individual shrink-wraps, make sure you have a hair dryer handy.
Preparing the cells for soldering
Step one: Preparation
Meditate, channel your Chi, feel it settle down from your shoulders down into your abdomen, like a river of fog flowing down a mountainside. Now you must catch the pebble…
But seriously, having a clean and organized workspace to work in makes it that much easier. Start by putting down two layers of brown packing paper on your table. This will catch any solder drips so that it doesn't burn the finish on your table. Next plug in your soldering iron and place it on its stand. Get the dish sponge, soak it with water, and then squeeze any excess water out. You'll use this to clean the tip of your soldering iron. Layout all the parts and tools you'll be using on the table so you won't have to get up for anything. A lot of workplace accidents happen because people get up or sit down and snag or trip on things. Also make sure each cell is fully discharged with the discharging tray. Doing so reduces the chance that an accidental short during the assembly process will cause serious damage or harm.
Lastly, you should never work alone. Always have someone around to help in case something goes wrong. If you are careful, nothing should go wrong. But it helps to always be prepared.
Step Two: Preparing the cells
To ready the cells for assembly, use the 400-grit sand paper to lightly buff the positive and negative contacts. Don't go overboard with it. All you want to do is to remove any dirt or tarnish. What I usually do is wrap the sandpaper onto the eraser part of a pencil and use that in a circular motion.
Next, take each cell and shrink-wrap it with the hair dryer. Shrink-wrap works kind of weird in the size large enough to hold a battery cell. You'll find that it doesn't all shrink together but in sections. I usually start in the middle of the cell and work my way out towards the edges in a circular motion.
Step Three: Bonding the cells
Take the prepared cells and line them up in your battery jig as you would want them in the final pack. Because it's a side-by-side pack we are building, each cell in the pack will be turned in the opposite direction as the cells it is next to. So if we were to look at one edge of the finished pack, we should see +, -, +, -, +, -. On the other edge we should see -, +, -, +, -, +.
After making sure that all the cells are in the right order, we can start gluing the cells. Take one cell, apply a thin bead of Shoe Goo or Carpenters Goop along one side, and press another cell against it to glue the two together. Shoe Goo or Carpenters Goop needs at least 6 hours to cure. The Deans Battery Jig I use tightly holds the cells together so I can work on the cells even while the glue hasn't set. If you are using a battery jig that doesn't hold the cells together tightly, you may want to wait until the glue has cured before moving to the next step.
Step Four: Soldering
Take a deep breath. This is the moment of truth. Your life depends on it. Your success will bring honor and recognition to your family name. You will be justly rewarded for your triumph, recognized to be the beginning of a long noble bloodline. But if you fail, your battery packs will run like crap.
Take your soldering iron and wipe it on the wet sponge to clean it. Apply some solder onto the tip of it, let the rosin boil off, then wipe it on the sponge to clean it again. Repeat this until the tip of the iron is shiny and bright with a thin coat of solder. Next use your iron to apply a thin coat of solder to the positive and negative sides of all cells where you will be soldering the battery bars. Make sure that the coat of solder is as thin as possible.
There is a right way to solder, and there is a wrong way to solder. The right way to solder is to touch the iron to the battery, and applying solder to the battery close to where the soldering iron is. As the solder melts, it should form a very shallow bump, kind of like what a mosquito bite looks like. It should not look like a ball, a drop, or a bead. After the solder has fully melted you can use the soldering iron to slowly spread it around a bit, but don't over do it.
Never apply more solder than necessary. The more solder you apply, the more your iron will have to heat and melt. So if you use too much solder, your soldering iron may not have enough heat capacity to melt it adequately.
Never apply solder to the iron and try to use the soldering iron like a brush to "paint" the solder onto the battery. It doesn't work like that. The key point here is that you need the solder to melt against the battery.
Use the tweezers to pick up a battery bar and place it in the appropriate spot. Here is where the Deans battery jig really shines. Unlike most other battery jigs, this one has a clip that helps hold the battery bar in place while it is being soldered. One end of the battery bar should touch the positive side of the first cell while the other end should touch the negative side of the next cell. You should always solder the negative side first because it's easier. Trust me. Anyway, apply a bit of solder to the tip of your iron and press it on top of the battery bar, which in turn presses down on the negative end of the battery where you've previously applied a thin layer of solder.
Once the solder melts, it should bond with the battery bar. Now apply some more solder to the joint until there is enough solder to form a good strong joint. See picture below. Now take your soldering iron away and wait for the battery bar to cool. Make sure you don't move the battery bar while the solder is cooling. Give it at least 15 seconds for the solder to fully cool and solidify. 
Repeat the process for the positive side of the battery bar, which should be easier. You may need to press down on the battery bar with the blunt end of your tweezers to make sure that it sits flush against the battery once the solder has melted.
After you are done, inspect the joints carefully. You should see solder that flows smoothly between the battery bar and the battery. There should not be lumps, beads, or little balls of solder. It should appear that the battery bar has been melted into the battery. If you are unsure about the connection, or if you think it needs more solder, you can reheat the connection and apply some more solder to it. Heat is very bad for a battery so make sure you don't hold the iron on the battery for an extended period of time. Generally, it should not take you longer than 10 seconds to make a connection. So if your iron can't finish the job in 10 seconds, you'll need to get one with higher wattage, bigger tip, or both.
Repeat the process for all the cells. Refer to the picture for placement of battery bars. It is absolutely vital that you follow this patterns because any other pattern may result in a battery that is open or shorted. Check and then double check each time you are about to install another battery bar.
Step Five: Wire connections
If you are going to hard wire the battery to your ESC, all you have to do is solder on two battery bars at either end of your pack. One will be the positive and the other will be the negative. And when it comes time to run the batteries in your vehicle, you'll just solder the wires from your ESC to the correct battery bar.
Otherwise, you can solder on some power wires and connectors instead. Soldering 12-gauge wire is not an easily learned skill. I still have trouble doing it every now and then. So take the following instructions as a good starting point.
I usually start by stripping off a bit of insulation from the wire. 1/8 of an inch should be plenty. Since this wire will be soldered onto a battery, I form the exposed wire strands into a fan shape. Next I apply the soldering iron to the wire and saturate it with melted solder. Lastly, I place the prepared wire onto the battery where I melted solder onto earlier and applied the soldering iron on top of the prepared wire. The iron heats up the wire, melting the solder, which in turn flows down and melts with the solder that has been applied to the battery. After letting it cool for 15 seconds, I inspect the joint to be sure that it is done right.
The picture above also shows some details of the soldering joints. Notice how there is just enough solder at each joint to form small bumps that are spread out. There aren't any lumps or beads of solder.
Installing Deans Ultra Plugs is a process that deserves an article on its own. Besides most people may not be using Deans Ultra Plugs so I'll just show a picture here of the finished product. Just remember to apply the same soldering principles as mentioned before and you should be fine.
Step Six: Finishing up
If your battery hasn't self destructed by now, you've probably done everything right. Give your newly assembled battery pack about 24 hours for the glue to fully cure. In the mean time unplug your iron and give it a full hour to cool off before touching it. Now is a good time to just clean up your work space, put allyour tools away, and toss out any trash you generated in the process.
Summary and closing comments
Your finished battery pack should look somewhat like one pictured here. If it does, pat yourself on the back for a job well done. As you've no doubt learned by now, assembling your own battery packs is not a trivial task. But there is some thing to be gained from the experience. It's like a rite of passage. It's what separates the men from the boys, the wheat from the chafe, and the pros from the wannabes. Having done so, you now have serious bragging rights. Now you can go to the track, point to another man's battery packs and say, "I couldn't help but notice that your solder joints don't look like mosquito bites". A smug look on your face would also help.
I hope this guide has assisted you in reaching your super-sexy-track-stud status. One day, when you are famous, and your face is on every Trinity product from shock oil to diff balls, just remember where you got your battery pack assembly instructions.
Good luck!
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Items you might need in constructing your own
packs:
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Assembling your own battery packs is a very rewarding experience. It can also be an expensive experience if you don't have all the necessary tools already. Buying the necessary tools costs around $100, which puts it out of the affordability ballpark for most casual hobbyists. However once you do get the right tools, building your own battery packs becomes very cost effective.
