How to Make a Battery Pack

      First go on the internets and buy some LiFe cells. I specified LiFe cells because those are the ones that I used. You can buy some directly from the A123 website. I used A123 lithium ion cells for my battery packs. They carry 3.3v nominally and 2.2aH. Use a voltmeter to make sure that the voltage across every cell is 3.29v-3.30v. Make sure that all of your battery cells are at the same voltage. Your battery pack is as strong as its weakest cell so if some of your cells have 3.29v and others have 3.3v your battery pack will die when the 3.29v cells die, regardless of whether or not your other cells still have charge.

So many batteries!

 Then you should use a hot glue gun to glue together your batteries. I did a 5s2p pack which stands for five in series and 2 in parallel. This means that each battery pack has a total of 16.5v (5x3.3v) and 4.4Ah (2x2.2Ah). I first glued together five pairs of batteries and made sure that they all had the terminals facing the seam direction.

Two cells in parallel

two pairs in parallel

small packs of 2s2p

 Now begin gluing the pairs to each other and make sure that their terminals are facing away from each other. This is because we are about to connect the pairs in series. We wil use copper braids to make this connection; I'll explain it later in the tutorial.

Two 5s2p packs; 

Positive, negative, positive, negative, positive...

 You should now be done gluing the cells together and you should be left with 5 pairs of alternating polarities hot glued together.

Use sandpaper to clean up the terminals

 Before we start soldering on the batteries we must clean the terminals using sandpaper. This is to brush away the corrosion on the terminals.

Notice the first pair of batteries have two negative terminals connected with copper braid. This is the pack's main negative terminal.

After sanding the terminals you should solder strands of copper braid in between each battery terminal. I used 12 gauge equivalent grounding braid from mcmaster. Make sure that you are connecting terminals with opposite polarity. The only exceptions are the first and last pair of batteries. Those will be your main positive and main negative terminals. This is the only occasion in which you should have a strand of copper braid connection terminals of the same polarity. To solder this braids you will require two soldering irons: a huge one as wide as your pinky and uhh... a normal sized iron. You'll use the huge iron to solder the negative terminals since they are much larger than the positive ones and therefore have more space for heat to diffuse. For the positive terminals you should just use the normal sized iron because the big one might heat up the terminal too quickly and you could damage the cell. You might want to buy some flux from radioshack or mcmaster to aid you in the soldering although it is not necessary.

Note the thin cables connecting the parallel nodes together.

 You should also solder some thin wire in between  each parallel node as seen in the picture above. Theoretically, all of the cells should be at the same voltage so there should be no current flowing through these thin wires but if there is a slight discrepancy in voltage across one of these cells. Current can flow from one cell to the other and hopefully equalize the voltage across them.

Finally finished soldering two packs.

 Now it's time to make a balance connector. Balancing is very important. It will make your battery pack way more efficient. If your cells are not properly balanced then your pack will die as soon as your first cell dies.

Balance Connector

For the balance connector you will need two things: the connector housing and the crimp connectors. This the most annoying and monotonous part of soldering your battery pack. When choosing a connector housing you gotta make sure that it has the correct number of ports needed for your battery pack. The number you need is one more than the number of cells in series in your pack. So for my 5s2p pack I used a 6-port connector housing.

Balance connector housing

XH Crimp Connector

If you don't have a crimping tool you can just solder the wire onto the crimp connector

 If you do decide to solder the wire to the crimp connector you must make sure that you use very little solder. If you use too much solder, when you try to connect your finished balance connector into the female port on the charger you won't be able to plug it in. I messed up my first connector doing this and I've heard that a lot of people make the same mistake their first time. The red wire should go from the "arrow" port on the connector housing to the main positive terminal on the battery pack. The blue wires should each go to a different battery terminal and they must be in order. So the red wire starts in the main positive terminal. The second wire (or the first blue one) should go to the negative terminal opposite the main positive terminal. The third wire (or the second blue one) should go to the positive terminal opposing the terminal you just did, and so on...

Make sure you connect the red wire on the arrow

I realize that the balance wire connections might have been confusing for people so I hope this diagram will help you guys visualize it better. The whole point of these balancing connector is that the charger has to be able to measure the voltage drop across each cell. After you're done you should be able to get a voltmeter and measure each cell's voltage by probing the inputs on the white balance connector housing.

Balance wires and main positive/negative wires complete.

For the main positive/negative wires you must use thicker wires since these wires will be taking on most of the current. I used 10gauge wire I believe. Very important: you should solder a polarized female connector to the end of the power wires. They should be polarized so that nobody can accidentally connect your batteries backward and short them. I like to use XT-60 connectors, but a lot of people use dean's connectors. Personally I think you should just choose one connector and stick with it so that you can use any of your battery packs for any of your electric vehicles. Make sure that the female end of the connector is soldered onto the battery power cables. This will make it harder for your batteries to short accidentally.

Heat shrink the battery pack

After you make sure that all of your balance wires are soldered to the right spots and everything is soldered correctly it's time to insulate the battery pack. There are many ways of doing this. You could do the legit way which would be to get huge heat shrink tubing and a heat gun to insulate the pack. Or you could do it the candace way and just wrap a ton of electrical tape around it.

Done!

Before you actually use the battery pack you must make sure that you use a balance charger to balance the cells. A balance will charge each cell separately to a pre-set voltage. However, after a pack has been balanced it will remain balanced for a while so you can just use a normal charger on it. I own the 1010B iCharger. It's expensive but it's powerful and can charge and balance up to 10s LiFe battery packs. It was recommended by several people who have built many electric vehicles, such as Charles, Shane, and Eli.

Anyway, I hope my tutorial was helpful! Good luck!