Dissecting DeWalt 36V Packs (A123 Systems) - revised 8/18/06, 8/24/06, 11/0706

DISCLAIMER: These articles are the findings and opinions of one person. Long-term durability of these cells is not known for our application/construction techniques. Any cell technology that we use is potentially dangerous. Do not work with these cells if you do not have a healthy respect for them, or if you lack the requisite skills.

There has been a lot of hype over several months about a new cell technology being developed by A123 Systems and destined for use by DeWalt in their new, ultra-manly 36V tools. Well, they've finally arrived, and while not as good as the rumors implied, they are still a great deal for the money. Another article covers performance and other technical stuff. Here, we will just build some flight packs!

The DeWalt packs are relatively easy to dissect. If you've dissected a Milwaukee V28 pack, you'll find the DeWalt a lot less messy. The trick here is to have the right tool, specifically a "Tamper-proof" T10 Torx driver. The screws holding the pack together have little nipples to keep a standard T10 from working. The Tamper-proof driver has a hole in the center to clear the nipple. I was lucky enough to find one at the local automotive fasteners store, but they're available on the net. Of course you can always brute force drill the heads off the screws.

top removed

The picture on the left shows the pack with the top removed (11 screws).

Remove the latch and spring.

Peel back the yellow tape and unplug the two connectors between the electronic module and pack.

electronic module removal

Lift up the module and rotate it out of the way

Slip the thermistor out from under the tape.

Carefully unsolder the wires from the pack tabs.

It is probably easier to just snip the wires if you don't plan to reassemble the pack.

The module should now be free.

module removed

I am sure the electronic module serves a useful purpose (mainly to control the cutoff of the tool on discharge), but it's not designed to handle our current demands. It also is a bit heavy. Unless you want to put things back the way they were, you can trash this module.

cell module ready for extraction

Now comes the tricky part. With a flat-bladed screwdriver or two, pry the pack module out. This is done by slowly going from [long] side to side, prying up a little at a time. Notice the picture below. You do not want to short any of those wires together!!!

I suppose you could use other means to destroy the lower case to get to the prize.

module removed

The plastic caps slip off easily.

end cap removed

If you don't want to solder directly to the cases, you can leave the tabs intact. Cut the tabs apart where necessary with a razor saw or good tin snips, taking care not to short anything out. I always cover all other contacts with masking tape to avoid undue excitement.

NOTE: for some reason, the buttons can rotate. A123 Systems advises against rotating them. You've been warned.

removing tabs

A123 Systems advises against soldering directly to the cells. I suspect that all battery manufacturers take this position. That being said, how much credibility can a manufacturer have who specs 70 amps continuous discharge and provides only four spot welds to carry the current? We've learned how to solder to NiCads without serious damage. The only way to know if we can do this with these new cells is to try it!

The tabs can be easily removed with needle-nosed pliers, popping each of the spot welds. I used a Dremel tool grinding wheel to clean up the welds. You can choose to keep the cardboard sleeves or discard them and use clear shrink tubing.

WARNING: As explained below, these cells have safety vents that could be compromised by soldering cells into packs. You have been warned.

stacked pack

At this point you can assemble packs with cells side by side using copper battery bars, just like with NiCad and NiMH cells. This gives a lot of flexibility compared to the flat LiPo form factor. If you discard the cardboard, use shrink tubing on each cell. I prefer the thin clear shrink sold by Air Dynamics. You need the 2" width. It's kind of big, but the 1-1/2" is too small. The 3-cell pack on the left shows the flexibility you have with cylindrical cases. I got the battery bars from New Creations. battery bar blue vent

If you really want to be safe, you can use these cells as-is, cardboard sleeves and all. I recommend this. The cardboard sleeves do not weigh much, and the straps can be retained to minimize wiring and avoid soldering to the case. It also makes adding a balancing connector much easier. Click the button for some hints for easy pack construction.

easy pack hints
cell end

It would be nice to be able to do end to end construction per the pack construction article on this web site. When I first saw these cells, I thought end to end would not be possible because of the almose flat button. Closer examination revealed that the case ended short of the nylon insulator. This configuration eliminates the need for the insulators normally needed for end to end assembly. Soldering proved somewhat more difficult than with NiCads. The case appears to be an aluminum alloy of some sort. I was not thinking aluminum when I tinned the ends of the cells. Both ends tinned well, but the case end really sucked away the heat. End-to-end soldering went OK. One joint failed the smack-on-bench test but resoldered OK. You need thermal mass to get the job done quickly and well. I used a #11 blade with tape on one side to clean out loose solder splats. I have subsequently found that some are having trouble soldering to the aluminum case. I did not have a problem but I did not use a Dremel tool to "clean" the case, only Scotchbrite. I assume there is some kind of coating similar to that on the aluminum tab of LiPo cells. Once that coating is gone, solder just won't stick. I have not tried it on these cedlls, but there is a paste that has worked for me on LiPo cells. It is called Solder-It Aluminum Soldering Paste and it is available from ALL e RC.

When I built this pack, I totally missed that the little blue button is a vent! Turns out it's not a vent after all, but where the electrolyte is added. There is anecdotal evidence that these cells will vent when seriously abused, e.g. heavy discharge way below recommended cutoff voltage. I have since learned that there is another vent on the opposite (non-button) end of the case. This vent is large enough as to be outside the normal area for end-to-end soldering. Just be sure to keep solder to about the diameter of the button.
soldered end to end

Believe it or not, this connection is safe from shorting without extra washers or tape.

I charged the stick and then compared individual discharge curves against each other and against previous tests. There was no obvious short-term damage. I was pleased with the results. Clear shrink over the whole stick finishes the job nicely. 3-cell stick

I will use 2 of these sticks side by side in my 40-size eNobler. I am presently using two 3S 3200 packs in series. These cells are only 70 percent of the capacity of the 3200's, but I only use about half of this per flight. The final pack size is slightly wider than a NiCad pack and a bit shorter than a 10-cell sub-C NiMH pack. I used a standard copper battery bar to join the two sticks together. 3" clear shrink holds the two sticks together. 6-cell pack

Pack just fits in the narrow eNobler fuselage. eNobler installation eNobler installation

Pack just fits in the narrow eNobler fuselage. The picture above right compares the M1 pack with a 10-cell CP1700 NiCad pack. The CP1700 pack actually weighs 0.7 oz. more. The eNobler previously flew with two of these packs. The CP1700s provide more voltage but also more resistance. Upping the prop size an inch will take care of this and take advantage of the extra 900 MAH.

eNobler installation

The picture on the right shows the two packs I am currently flying in the eNobler. They are 3S PQ3300. They give 15 minutes of aerobatics but I only use 8. The higher C rating is what I was after. I also use the 3S Kokam 3200 cells [my Gold Standard]. These are 3.5 oz. heavier than the M1 but provide more MAH [unused] and a bit higher voltage.

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DeWalt 36V (M1) Introduction

charging packs

comparing to other cell types

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