How To Build a Battery Pack from A123 Cells
I promised to do this last week, so here it is. As many of you may have already read in other threads, I have purchased several of the DeWalt 36v Power Tool batteries from people on eBay over the past few weeks. I have purchased 4 so far, and have a bid in on another pair of them right now. For my larger ParkJets, and other large planes needing 600 to 2500 watts of power or more, I think that these A123 cells represent the best value on the market right now.
So far in the auctions I have won on eBay, I have paid an average of $8.94 per cell, including shipping and handling charges! The 6-cell A123 pack that I will build in this How-To cost me a grand total of $54.87 including the cost of the 6 cells, the Deans Connector, the 12 gauge wire and the tape used to hold everything together. If you decide to add a balance connector, this will add about $2.00 to the cost of the build.
The 6-cell pack built here has a open circuit voltage of 6 x 3.6 volts per cell for a total pack voltage of 21.6 volts, and has a capacity of 2300mah. The pack can deliver 30C continuous (69 Amps) with burst capability of 40C (92 Amps). I am currently running them in my 150% F-4 and 166% F-18 ParkJets and in both planes I am pulling 35 Amps (15.2C), and getting 5-6 minute flights with 300-400mah left in the pack at the end of the flights. At this current draw, the A123 6-cell pack delivers 17.2 volts or 2.867 volts per cell. In this application, the packs are delivering 17.2 volts x 35 Amps or 602 watts, which is pretty good for a battery pack that weighs just 1 pound!
Well, that is enough background information, let's get to the build!
I already wrote an in-depth thread on how to disassemble a DeWalt 36v battery pack and remove the cells in another thread, so instead of repeating that information here, if you need to know how to get the cells out of the case, check out this thread. Disassembly of a DeWalt 36v Battery Pack
Once you have the battery pack taken apart, and removed the cells, you will be left with 10 cells that are spot welded together, and look like this.
The DeWalt packs have one of the buss bars that interconnect the cells that is a little bit smaller than the rest of the cells. This is to form a built-in fuseable link that will burn open in the event that the battery somehow gets shorted out. Looking at the photo above, this particular buss bar is located just to the right of the center cell in the bottom row. If you start at the positive end of the battery, this buss bar is located at the interconnecton between the 6th and 7th cell in the pack. Since I wanted to make 6-cell packs for my application, this was a natural place to cut the pack apart to seperate the cells into a 6-cell group and a 4-cell group.
To cut the pack apart, the best method that I have found is to use a cut-off wheel in a Dremel tool and grind about 3/4 of the way throught the buss bar. Once this is done, you can rock the pack back and forth at the cut and the buss bar will snap in half. By cutting the buss bar this way, you can cut the buss bar without nicking the end of the cell. Here is a close-up of the buss bar after it was cut with the Dremel tool.
To seperate the pack, hold the 6-cell group on the table and rotate the 4-cell group up to a 90 degree angle and wiggle it back and forth a few times. After 3-4 times back and forth, the buss bar will snap in half leaving you with a 6-cell group and a 4-cell group of cells. Here is what the packs look like after splitting them apart.
There are many different combinations of cells that can be created form these packs. You can cut the one buss bar between the 5th and 6th cells and get two 5-cell flat packs, or make several 3-cell packs or 4-cell packs. It is up to you depending on your application.
The next modification that I made was to remove the excess solder tab that hangs off the positive end of cell #1. In the DeWalt packs, the Positive lead from Cell #1 and the Negative lead from Cell #10 stick about 1/4" past the edge of the cell to provide an attachment point for the power connector and charger module. I did not want this extending out past the edge of the cell, so I cut it off. Just like I did before, I used my Dremel tool with a cut-off wheel to cut about 3/4 of the way through the tab and then bent it back and forth a few times to snap it off. Here is the tab after I scored it with the Dremel tool.
And here it is after I bent the tab back and forth a few times and broke it off the cell.
The next thing I did was to mark the positive end of each cell with a number. This was to identify the cells, so that when I added the leads for the balance connector later on, I would not mix up the cells and solder a wire on the wrong cell. Here is the top side of the battery pack with the positive ends of cells 1, 3 and 5 marked. You will notice that these cells are "Backwards" from conventional cells in that the outer can of the cell is the positive lead, and the button tab is the negative lead.
After that was done, I flipped the battery pack over and marked the positive ends of cells number 2, 4 and 6 like I had done on the first side.
With that done, it was time to start putting the pack together. To begin, you will want to tape the pack to hold the cells together while you work. For this, you will want to use nylon strapping tape in either 3/4" or 1" width. I had a roll of 1", so that is what I used. For a 6-cell pack, you will need 2 pieces of tape that are 13" long each as shown below. This will be enough tape to go all the way around the pack with a little overlap.
To install the tape, lay the pack on it's side and start one strip of tape about 1/16" up from the bottom edge. Be sure that you get the tape lined up straight, because if you don't, it will not match up when you wrap it around the pack.
Here is the tape as it comes aroung the entire pack. It looks like I got it started straight, because the tape is going right back over the original layer as it comes around. I love it when it works out like this!
Once the tape is all the way around the pack. Press the tape down good so it holds tight against each cell in the pack. Once the first tape strip is done, the battery pack should look like this.
To hold the top of the pack together, I repeated process, and put an additional strip of tape at the top end of the battery pack. Once that was done, the battery pack looked like this.
Now that the pack is taped together, it is time to start wiring it up. Since these packs can deliver almost 70 amps of continuous current, you should use 12 gauge silicone insulation hi-flex wire for the output leads. I cut my leads 4 inches long, since this is enough for all of my planes. If you look back at the photo above, you will notice that the solder tab on the negative terminal of the battery points straight to the end of the battery, but the tab on the positive terminal of the battery points sideways. To compensate for this, I cut the the positive lead wire 1/4 inch longer than the negative lead wire as you can see in this next photo.
To get the 12 gauge wires ready to attach to the battery pack, the ends need to be tinned. Strip 1/4" of insulation from one end of each of the lead wires and coat them with solder as shown here. Make sure that the wire is tinned all the way around, and that there is a little extra solder on the wire when you are done.
To prep the pack to receive the lead wires, tin the tabs on the positive and negative outputs on the batteries. If you put an X-Acto blade under the tab and bend it up slightly, it will tin easier to solder, since the end of the cell will not absorb as much heat. Here is what it should look like once the battery tabs are tinned properly.
With the tabs tinned, now we can attach the power leads to the battery. I have developed a technique that I use to solder these types of leads that works very well, and I will share it with you now. To start, tin the end of your soldering iron with enough solder so that there is a little blob of solder left on the tip of the iron. Now hold the tip of the soldering iron about 1/4" above the tab of the battery and lay the end of the wire lead on top of the tip of the soldering iron. This will melt the solder on the end of the lead wire. Once the solder on the lead wire is molten, lower the soldering iron tip so it hits the solder on the battery tab. In a few seconds the solder will melt and start to flow on the battery tab. When it melts, slide the soldering iron out of the way and set the wire down into the pool of molten solder. Then set the tip of the iron on top of the wire, press it down for a second to make sure you have a good bond, and then remove the iron and wait for the solder to solidify. If you do it right, the whole process takes less than 5 seconds, and you minimize the time you have the iron on the cell to limit the heating on the battery. When you are done, your solder joints should look like this.
Pulling back a little bit you can see why I made the red lead 1/4" longer than the black wire. With the wires dressed to attach to the Deans connector, you can see that they are exactly the same length.
Depending on how you attach your leads, you may need to adjust the length of the leads to acommodate the layout of your pack. You can leave the wires a little long and then trim them when you are done to meet up ffor your output connector. If you do cut the wires after soldering Do not EVER cut both wires at the same time! This will create a dead short and create a bunch of sparks and probably damage the pack and ruin your cutting pliers! Always cut one wire at a time, and even when you cut one wire, make sure you hold the wires apart so the strands that poke out the end don't accidently hit your pliers when you cut the other wire.
Now that the leads are attached to the back and trimmed to the proper length, it is time to solder on your connector. You can use whatever connector you choose, but I will be using a Deans connector. Before you solder on the connector, remember to put a couple pieces of shrink tubing on the wire leads if your connector requires them.
To prep the wire leads to solder to the Deans connector, cut 3/16" of insulation from the end of one of the leads and tin it well, leaving a little extra solder on the wire as seen here.
Here is another little trick that I use when soldering connector ends to my wire leads. This will work for Deans connectors or bullet connectors just as well. Take a pair of needle nose pliers and wrap a rubber band around the handles to hold the jaws shut. Then you can pry the jaws open, insert a connector, and let it go. The rubber band will hold the jaws shut and hold the connector firmly in place. Here is what it looks like with a Deans connector ready to solder.
I always like to start with the positive lead when I solder on my connectors, so I put the Deans connector in the pliers with the positive terminal up. I used my soldering iron and tinned to end of the positive lead as shown here.
Now that the Deans connector is tinned the read lead from the pack can be soldered on. I use the same technique I described earlier to attach the wire to the connector. I put the soldering iron in between the wire and connector to heat both parts at the same time. Once the wire is soldered it will look like this.
After the solder cools down, slide the piece of heatshrink tubing down and shrink it down with a heat gun or butane lighter. By putting on the heatshrink now, it helps prevent the end of your soldering iron from shorting out the pack if you should happen to slip while soldering. When you are done, the connection should look like this.
When using 12 gauge wire, or any other stiff wire, sometimes it is hard to add the second wire to the Deans connector because when you pull on the second wire, it tends to pull the pack at the same time and move things around. To keep the connector from moving, it helps to set something heavy on the pliers. It just so happened that I had another 36v battery pack sitting on my bench, so I used that to hold the pliers down on the bench. This photo shows what I am talking about.
Here is another little soldering tip. When you tin the contacts on a Deans Connector, you get a much better joint if you tin the top surface and the sides and end of the contact. To do this, after you tin the top side, turn your iron up 90 degrees to the contact and run the tip of the iron around all 3 edges of the tab. If you do this, when you solder the wire on later, the solder will flow around the edges giving you 4 times the surface area for the solder to bind to. After I soldered on the black lead on the Deans connector, I turned it sideways to take the photo so you can see how this works. Here you can see how the solder not only holds the wire to the top surface of the contact, it wraps around the edges of the contact as well. This gives a VERY strong solder joint.
Once the solder on the black lead has cooled off, you can slide down the piece of heatshrink and shrink it around the solder joint. When you are done, your finished connector should look like this. Pretty good job if I do say so myself!
If you do not plan on adding a balance connector, you are basically done with teh pack now. All that is left to do is to tape the pack to cover all the bare cell ends and you are good to go. For this How-To, I will be showing how to add the balance connector to the battery now.
To make it easy to solder the wires and wire the connectors, I will be using different color wires for each cell in the pack. For those of you that are not familiar with the colors used in electronics, there is a standard that has been developed for colors that represent numbers in electronics. These are the little colored bands that you see on resistors and other electronic parts. The color code used goes like this.
0 = Black
1 = Brown
2 = Red
3 = Orange
4 = Yellow
5 = Green
6 = Blue
7 = Violet
8 = Gray
9 = White
For my balance harness I will be using 7 different colors of wire. Since Black is also universally used for ground, I will be using a black wire for the pack ground. For each of the other cells I will be soldering a wire to the positive ends of the cell, and I will use a color to represent the cell number. Cell #1 will have a brown wire, cell #2 will have a red wire, cell #3 will have an orange wire and so on as laid out in the chart above.
One source that I have found for good wire is old burned out computer power supplies. Since I work on all the computers in my office, and I do computer consulting on the side as well, I get 2-3 burnt out power supplies per month. I usually part them out, saving the good stuff for other projects. To give you an idea of what I mean, here is the wiring harness that was salvaged from a 350 watt ATX computer power supply.
As you can see, there is an abundance of red, yellow and black wire on the drive connectors, and the motherboard connector also has green, blue, white, violet, gray and occasionally brown wires. Depending on the size and quality of the power supply, the gauge of these wires can vary from 24 AWG up to 18 AWG wire. Since I have bags of these harness laying around, it is easy for me to find the sizes and colors that I need. You can also buy spools of wire if you plan on making a lot packs. You do not need to use different color wires, you can put numbered tags on them if you want, but for me, it is easiest to just use different color wires.
As another source of wire, I also have some multi-conductor Data Communition cable that has 6 twisted pairs of 22 AWG wire inside. Since I have a bunch of this, I deceided to use this wire for the harness. I cut off a foot long piece of the cable and slit it open to remove the wires inside. After I was done, this is what I had.
The 22 AWG wire is UL rated for over 2 amps and is capable of handling up to 5 amps of current with no problem, so it is plenty big enough for use in the balance connector. Now that I have the wire selected, it is time to attach it to the battery pack. To prep the battery pack to receive the wire, I tinned a spot next to the numbers that I wrote on the positive end of each cell. Here is the top side of the pack after I tinned the spots for the balance leads.
To attach the balance wires, I removed 1/8" of insulation from the end of the brown, orange and green wires and tinned them. I only removed 1/8" of insulation because when you tin the smaller wires like these, the insulation melts back a little bit and you end up with about 3/16" of tinned wire when you are done. Here is the top side of the battery pack after the balance leads for Cells number 1, 3 and 5 were soldered on.
With that done, I flipped the pack over and soldered the balance leads onto cells number 2, 4 and 6. These wires were dressed up to the top side of the pack through the openings between the cells when they were installed so all the balance leads exit at the top side of the battery. Here is the bottom of the pack after the wires were installed.
With all the leads for the balance connector attached now, it was time to dress the leads and get them to neatly exit the pack. Here it the top of the battery again, with all of the wires coming out. There are two 12 gauge power leads and 7 22 gauge balance connector leads as seen here.
I wanted to get the balancer leads neatly lined up in the proper order to prepare them to go into the connector. I lined up the wires and taped them with a piece of masking tape to hold them in the proper alignment.
When I cut the balance leads wires, I purposely left the wires a little long so I could trim them to length. The wires that go to the bottom of the pack are around 2 inches shorter than the other wires since they have to travel further to get ot the bottom of the pack. I decided to cut the wires off so they were the same length as the power leads.
IMPORTANT SAFETY NOTE: When you cut the wires to length, do NOT cut all of them at the same time! You MUST cut the wires one at a time. If you try to cut more than one wire at a time you will short out one or more cells, and since these cells can deliver well over 100 amps of short circuit current, they will almost instantly burn up the balancer lead wires and melt off the insulation!
When I cut my wires, I cut them one at a time, and then put a small piece of masking tape over the end of each wire to cover the end. This way, there would be no chance of having any wires short against one another. Here is what the harness looked like after I had finished 3 of the wires and just cut the 4th one.
After all the wires were cut to length, it was time to add the balance connector. The connector that I will be using is a common size that is used on circuti boards, and is readily available form places like Mouser, Jameco or Digi-Key. The connector pins are on a 0.156" spacing, and the contacts can take up to around 5 amps of current. The other reason I chose to use this connector was because I already had a bunch of them left over from a project at work. Here is a photo of the connector and the contacts that go with it.
The connector body already has the contacts numbered on the end, so I figured that I would stick with convention and put the balance lead from cell #1 into socket #1 and the lead from cell #2 into socket #2 and so on. Here is the end of the connector so you can see the numbering.
The contacts for these connectors are best installed with a crimp tool that is made for these cotacts, but they can also be installed with a simple pair of needle nose pliers. I will crimp the connectors with a pair of needle nose pliers just to show how it looks. The connector contacts have 2 sets of wings that fold over, and these can be seen a couple photos above. The smaller set of wings fold over the bare wire at the end and the larger ones wrap around the insulation. To use the pliers, I just fold one wing over first, then fold the second one over the first one. Here is a close-up of one of the contacts crimped on to the first brown wire lead from the balancing harness.
If crimped properly, these wires hold very well, but for just a little extra insurance, I went ahead and used the very tip of my soldering iron to put a tiny bit of solder into the bare wire part of the crimp. Here is what the connector contact looked like after I had soldered it.
After the solder cooled off, the contact is simply pushed into the connector shell until it clicks into place. Here is the first lead installed into the connector shell.
This process was repeated six more times for each of the other wires in the balancer lead harness, and each contact was snapped into place as it was finished. Once I was done, the balance connector looked like this.
This completes the wiring of the pack. Now it is time to tape it all up to protect the cells and keep the battery from shorting out against anything. I have found a tape that I really like for sealing up battery packs. I have used nylon strapping tape in the past, but it does not stretch at all, and does not co around curves very well. The new tape I found was purchased at Lowes and it is made by Scotch. The tape is called "Transparent Duct Tape", and here is what the roll looks like.
It looks white in the photo, but it is actually a milky translucent color. It has filaments running in both directions, and it sticks real well to the cells and to itself. I like to use 2 layers of tape applied at 90 degrees to one another to protect the ends of the cells. To begin, I put a piece crosswise on the pack starting at the center of the pack and working to the edges. Since there is less stuff on the bottom side of the battery, I will start there with the taping. Here is the first piece of tape being applied.
In order to get the tape to form around the corners of the pack, I make a couple small cuts in the tape to make it wrap easier. Here is what the cuts look like.
To make the corners neat, first fold in the small middle flaps as shown here.
Next fold down the center flap and smooth it down against the end of the pack.
And finally fold in the two end flaps. Once this is done the tape sort of melts together leaving nice smooth corners.
To complete the first layer on the bottom, I cut an identical piece of tape and butted it up against the edge of the first one. Then I cut the tape and folded the edges around just like I did on the first piece. Once that piece was sealed down, this is what the bottom of the pack looked like.
Now it is time to add the second layer of tape over the first. This time the tape runs the long way on the battery pack and is installed just a little past center on the pack. Here is the positioning of this next piece of tape. It is tough to see in the photo, but the tape is cut so it extends about 3/4" over the ends.
SInce this layer will show when finished, I dressed the corners a little different that the first layer. This time I cut the corners into two flaps and trimmed off the bottom edge to even them out. Here is the tape after the cuts were made.
To seal this layer, the two corner flaps are stuck down first and smoothed out as shown here.
Once that has been done, the end piece is folded down. Don't fold down the long edge yet. Now, go repeat this process on the other end of the tape sealing down the 2 corner flaps and then the end flap. Finally, to complete this piece of tape, fold down the long edge on the side and rub it out smooth. When you are finished, the tape will look like this.
Spin the pack around and put the second piece of tape on the bottom of the pack. This piece should overlap the first piece about 1/4" down the center of the pack. Here is the second piece positioned on the back before any cuts were made.
Repeating the process that was used on the earlier piece, this second piece of tape is sealed down to the pack. When that is done, we are finished with the bottom side of the pack. Here is what the pack should look like at this point in time.
Now it is time to flip over the pack and tape up the top side. The first thing to do is to add a couple small pieces of tape to tops side of the cells to keep the balance wires from accidently shorting out against one of the other cells in the pack. The first piece of tape I cut is 1" x 2" and is positioned as shown here.
Next, I cut another piece of tape that was 1" square and put in place between the power leads. To make room for the red and blue leads to come up from the bottom of the pack, I cut a small slit in the first piece of tape and pulled the wires back into this slit. Then the second piece of tape was installed up against these wires as shown below.
To hold the balance wire leads in the proper order, I cut another piece of tape that was 1" square and used it to hold the balance wires down once they were dressed to the proper order. Here is how things looks after this piece of tape was installed.
To dress the balance leads down away from the power leads, I folded the wires down along the front side of the pack and put a 3" long piece of strapping tape over the wires as seen here.
Now the pack is almost done. We just have to repeat the taping process used on the bottom side of the pack on the top side and we will be ready ro rock and roll! By now the pack should look like this.
Just like we did on the bottom of the pack, two pieces of tape are applied crosswise to the pack. Here is the first piece of tape installed and cut, ready to seal down.
The tape flaps are sealed down one at a time, starting with the corner pieces, then the end flap, and finally the two side flaps. After all the flaps have been sealed down the pack will look like this.
The other end of the pack is a little tougher, since the power leads are in the way. You just need to make a couple more cuts in the tape to get it to wrap around the cells and the wire leads. Here is the second piece of tape installed and cut, ready to seal down.
After folding all the tape flaps over, the end of the pack will look like this.
To help add a little strain relief to the power leads, I cut another piece of tape about 2" square and applied it to the end of the pack. two cuts were made in the tape strip to allow the power leads to pass through.
This piece of tape was folded up over the top of the battery pack and over the top of the lead wires as seen here.
To finish the tape job, the two final pieces are applied lengthwise on the pack just like the ones that installed on the bottom side. Here is the firsti one being put into place.
The tape was cut at the corners and wrapped around the pack like we did before. after that was done, the other side was installed, cut and sealed down around the battery. With that last piece installed the pack is now complete! Here is the finished product, the 6-cell A123 battery pack, complete with a balance connector.
And to complete the build thread, for those of you that are interested, here is the final weight of the completed pack.
The completed weight is 16.7 ounces. Not too shabby!
Well hopefully, this battery how-to will answer all of your questions on how to build an A123 battery pack. As always, any comments or questions are welcome. Believe it or not, the time it took to build the pack, shoot all the photos, process the photos, write the article and proof-read it took almost 6 hours! My, the time flys when you are having fun! To actually build the pack should take around an hour, so it is not that tough a project to take on.
So go on, get your cells and build your own packs!
Now, what is next? Well, since there are no commercially available balancers for the A123 cells right now, my next project will be to design and build a custom balancer that can be plugged into the balance connector to keep each of the cells at 3.6 volts during the charge cycle. I will build a 10-cell balancer so I can use it with my 6-cell packs, and be ready to balance larger packs in the future if I decide to move up to larger planes.
That wraps up this How-To article. I hope that you all find it informative and helpful. Let me know what you think.
See you all next time,
Last edited by LBMiller5; Nov 21, 2006 at 10:21 PM.
May I suggest the use of Kapton tape or black electrical tape in the place of the packing tape. I would still use packing tape to wrap the cells for strength, but Kapton tape would probably be safer for insulation, considering the potential energy of those cells.
I do 3-cell packs "inline" by bending the original tabs, easy and nor a problem if you start by bending them around a screwdriver or something round before "puhing" cell to cell.
Great work, why are you not in my office?
Btw, one advice: to me the original steel tabs are definitely a "no-no" as you sey there, for currents above 10A, let's forget 30 or 40A. At least, you can recommend to solder some copper cable or braid over the steel to reduce resistance there. I do my packs that way.
From this Thread
I guess someone needs to run a 50 A and perhaps a 80A discharge test through one of these links and do a voltage drop test.
Last edited by everydayflyer; Nov 20, 2006 at 09:37 AM.
Fact is in years past we all used fuses.
If you have a motor bearing,gear box or such to lockup then you may wish the battery was fused.
Wonder which will fail first ,the series bars or the end caps connecting links?
Last edited by everydayflyer; Nov 20, 2006 at 09:49 AM.
Then I will advice to:
1-solder copper braid on tabs to lower R on the pack and have one less variable.
2-solder fuse of know A-rating on ESC connector (say a 50A one for a 42A static measured setup). They are really cheap on any automotive mart, and they come calibrated.
But this is advice for ANY electric plane, btw, not just A123 cells.
You are still running that power through 4 small spot welds. sectional area probably less than the fuse link. Someone awhile back mentioned wicking solder under the tabs to increase contact area. Then braid or bars added.
[ I don't have any so I can't say, just want you guys to figure it all out for me so I don't have to, before I invest in these. ;P ]
So far no problems up to about 48 amps.
Some have advised that the vent should no be covered. A123 Racing takes the standard stand that the cells should not be soldered to.
FYI : I have soldered to the cell casing itself(positive ) using aluminum flux and 64-36 solder and it was very easy to do so. I used a 40 Watt Weller with a chisel tip and the adjoining area of the cell was barely warm.
Then on the other hand I will solder to most any cell.
Last edited by everydayflyer; Nov 20, 2006 at 04:21 PM.
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