Disassembly of a DeWalt 36v Battery
Yesterday afternoon I received a Priority Mail package at my office. I had a pretty good idea what was inside, so I quickly tore the package open. Inside were the 2 new DeWalt 36volt Lithium Nano Phosphate power packs that I had ordered from a guy on eBay. I must say that I scored quite a deal on them, I got the batteries for $80.00 each with $19.99 shipping, so the total cost for 20 A123 cells with shipping was $179.99. If you do the math, that means that I got 20 cells for just a hair under $9.00 each. Not too shabby!
I could not wait to get home from work to try them out last night. Here are the batteries I received, sitting on the floor of my office at home.
There has been considerable discussion on RC Groups as to how to open these up safely to extract the cells without damaging them. With that in mind, I took one of mine apart and took about 40 photos along the way to document the process. Now I can present that information here so the rest of you can take advantage of these new batteries as well.
The key to taking these apart is having the right tool. A hack saw will work, but I would rather not go there! Here is the secret tool that you need right here.
What? A goofy looking block of red rubber?
Ah yes! But it is what is inside the goofy block of red rubber that is important. Let's open it up and see what's inside.
It is a complete set of bits that fit all of the standard "Tamper Resistant" screws that are commonly used on things like GameBoys, X-Box's, VCR's and yes, DeWalt 36 volt batteries!
I have highlighted the one that you need to use for the DeWalt batteries, it is the Torx T-10 bit with the recessed hole in the middle that fits over the little nub that is in the center of the screw heads.
Now I am sure you are all asking where you might get your hands on a set of these wonderful screw bits. Well I will tell you, they can be purchased at one of my favorite stores, Harbor Freight! Yes Harbor freight! Harbor Freight part number 93388 33-piece Security Bit Set. Here is a photo of the packaging so you can see what it looks like.
The normal price of these is $9.99 per set, but Harbor Freight is running a Half-Off sale on many items this week, and as luck would have it, this is one of the items that is on sale. Right now the set only costs $4.99! What a deal! I already had a set, but when I was in there yesterday I bought a second set to take to my office in case I needed any at work. For any of you guys out there that like to tinker with stuff, there bits are a Must Have tool.
Now that we have the right tool, let's start taking one of these batteries apart!
Here is a shot that shows how well the bit fits into the screw heads. It is such a nice fit that the bit will drop down into the hole and just sit there.
Here is another close-up shot that shows the bit in one of the top corner screws.
I took out the screws around the case but when I got to the top 2 holes next to the black latch, I noticed something was not quite right.....
Houston, we have a problem! The top 2 holes have the screws recessed about 1/2" below the surface of the case, and the tip of the bit is just about 1/16" too short to reach the screw. The screw hole is a 1/4" hole, and the bit is a 1/4" hex, so the 6 points around the bit keep it from dropping down into the hole. What do we do now?
There are 2 options. First, you could chuck the bit up in a drill and spin it while you gring the flats off the edges down near the tip. It would not take much, because you only have to grind off a little over 1/16" to get the bit to bottom out in the screw. The second option is to drill the hole out a little bigger to fit the existing bit. Since I was not planning on keeping the case, I went this route. I chucked up a Harbor Freight 9/32" drill bit in my $14.99 Harbor Freight 18 volt cordless drill and opened up the holes a little to allow the bit to fit. You only need to drill down 1/8" to get the clearance that you need. Here is what it looked like.
With that mod complete, I was able to take out all the screws and remove the yellow top cover. Here is the battery with the yellow cover removed.
Here you can see the large die-cast heatsink assembly for the charger/balancer that is built into each battery pack. At the left edge of the heat sink are the charge connectors and the power contacts. In the lower left corner you can see one of the balancer connector peeking out from under the amber colored Kapton tape.
Here is an end view of the battery. I marked in with a couple Sharpie pens the Positive and Negative terminals of the battery pack. At the bottom of the photo, you can clearly see one of the A123 cells peeking out. I took my VoltMeter and measured across the output terminals and measured 33.3 volts DC. The battery has never been charged, so this is probably the resting voltage that these units are shipped with.
Here is a close-up view of the balance connectors. There is one on each side of the pack.
To remove the charger module, you need to unplug both of the balance connectors. The Kapton tape will need to be pulled back to gain access to these connectors. Here is the first one pulled off.
And here is a shot I took after both were pulled off.
Next you need to de-solder the red and black power leads from the cells in 2 places. Here is a photo of me removing the red wire.
Once the 2 wires are de-soldered, the entire module can be lifted off the battery pack. Here is the underside of the charger module. You can see the red and black power leads, a temperature sensing probe and the 2 balance connectors. Looking at the lead wires, they look like they are around 18 gauge, which is a lot smaller than I would have expected. The power tools that use these batteries must be fairly low current to use wires this small.
Back to the battery. If you look down the side of the battery, there is a raised rib on each of the 4 corners that lock into the end caps the hold the batteries. You can see one of these at the very center of the next photo.
You need to pry the end caps up at that point to lift the cells out of the bottom half of the battery case. Slide the tip of a 3/16" wide flat screwdriver down until it hits the ridge shown above, and then push the screwdriver over to pry the end cap up. DO NOT PRY ON THE CELL ITSELF!!! Only pry on the black plastic end caps. The cans of these cells are aluminum and can be dented if you apply too much pressure to them. Here is what I am describing.
Here I have worked the battery up about 1/4". I then went to the other corner on the same end of the battery and brought that corner up 1/4". Then I spun the battery around and pryed up both of those corners about 1/4". I repeated this bringing each corner up 1/4" at a time so I did not twist or distort the battery pack.
After 3 trips around the block, I had raised the pack up about 3/4" and this it what it looked like at that point.
From that point, a couple more tugs and the battery popped free from the bottom case. Here is the battery pack, up and out of the case. Now all the individual cells are visible.
The end caps just sit on the ends of the batteries and are connected with spring loaded terminals. Here is the battery with the end caps removed.
And finally, here are the harvested A123 cells. $90.00 worth of batteries! The terminal on the top center cell is the main positive lead, and the one to the right of it is the main negative lead. All of the cells are spot welded together with rather substantial buss bars.
And here is a shot of the opposite side.
Here is a close-up of the A123 Designation, Logo, Part Number and date of manufacturer.
For those of youthat are wondering about weight, here is the entire pack sitting on my digital scale.
A total weight of 25.9 ounces, or 2.59 ounces per cell. That works out to 73.5 grams per cell. A little over the quoted weight of 70 grams per cell, but the extra weight is due to the heavy buss bars used to connect the cells together.
My plan was to use 6-cell packs in my large ParkJets. I got to looking at the pack more closely, and I noticed that there is one buss bar that is necked down to about 1/2 the width of the others. I marked this point with an arrow in the next photo.
As an Electrical Engineer myself, I believe that this is intended to be a fuseable link that will blow if the battery pack is short circuited for more than a few seconds. A pack of this size could briefly produce over 100 amps of current in a short circuit condition, and this link is about the right size to burn open with that much current.
As I looked at the battery a little more, I also noticed that this link was at the junction of the 4th and 5th cell in the pack. If I were to cut this one link in half, it would seperate the pack into a 6-cell pack and a 4-cell pack, which is exactly what I wanted to do! How cool is that!
So I got out my Dremel tool with a cut-off wheel and used it to slice through the buss bar right at the thinnest point. I did not cut all the way through, I went back and forth a couple times to cut about 2/3 of the way through the buss bar. Here is what it looked like at that point.
Then I picked up the group of 4 cells and rocked them back and forth a couple times and the buss bar split in half. Here is what I am talking about.
What ever you do, DO NOT let the two cells with the solder tabs touch each other or you will short circuit the entire pack! In Hindsight, I probably should have put a piece of tape over the solder tabs to make sure they did not touch, but I was very careful when I did this, so everything worked out fine.
With that complete, I now had a 4-cell pack and a 6-cell pack.
I measured the voltages of the 2 individual packs to see where they were. At full charge, the 4-cell packs should be 14.4 volts and the 6-cell packs should be 21.6 volts if fully charged to 3.6 volts per cell. With the entire pack originally being 33.1 volts I should have 3.31 volts per cell, so the 4-cell pack should read 13.24 volts and the 6-cell pack should read 19.86 volts.
Here is the 4-cell pack.
Not too bad, 13.26 volts. That is very close to the calculated 13.24 volts. What about the 6-cell pack?
This too was what I expected 19.88 volts, very close to the 19.86 I was expecting. These differences are due to the round-off error of the DVM. When I measured the entire pack, I had to use the 200 volt scale, and only had one digit after the decimal place. I was able to measure the 4-cell and 6-cell sections on the 20 volt scale, so there was better accuracy.
One thing you may not have noticed is that these batteries have a polarity that is reversed from the conventional batteries. In this respect they are like Alkaline batteries in that the outer metal can is the positive terminal and the single tip on one end is the negative lead. In this close-up of the last photo you can clearly see that the red lead is going tothe outer can, and the black lead is going to the insulated tip contact.
Here are the weights for the individual packs. 15.5 ounces (439.8 grams) for the 6-cell pack.
And 10.3 ounces (292.3 grams) for the 4-cell Packs.
One good thing that I noticed is that the buss bars are tin plated, and you can solder directly to them with conventional 60-40 or 63-37 electronics solder. I tinned the ends of the contacts on the 6-cell pack to show this nifty feature.
Well, tht wraps up the disassembly of the DeWalt 36 volt power pack. Now I just have to tape up the batteries and charge them up and do a little load testing to see how they perform. More data to come.
Have fun ripping apart your own DeWalt Battery Packs!
Last edited by LBMiller5; Oct 30, 2006 at 03:05 PM.
Thanks so much for this. I have been seraching and searching for a thread to show how to do this. I was surprised that no one had done one yet. This is very timely since I just received mine last Friday. I have been wondering where I could get those anti-tamper bits as well.
When I got mine I was in such a hurry to get it open, I just used whatever I could find.
So you're one of the lucky dogs that got the two for 169.99 deal on fleabay, huh?
I almost bought one of those twofer deals, but as it is I found one for 89.99 buy it now and the guy was a local so no shipping. $8.99 per cell!!!
Thqat was about the best piece of technical writing and instruction documentation I've seen here on RC Groups! Very clear and well done!
Mr. Lin of GWS should hire you to write instructions for him!
Build your pack and document that part, too. Er, I mean part 2!
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