How to rebuilt some DeWalt batteries for a quarter of the cost of replacing them

Things you need
You'll need Sub C (or 4/5 Sub C for low profile packs) cells with solder tabs already installed from ebay, a dead battery pack, and a decent soldering iron.  You'll also need some cardboard or fish paper (more on fish paper later) to insulate the main group from the neck cell.  For my pack, I used cardboard .019 thick.  A layer at the bottom under the cells and the insulating layer gave my pack a nice slight compression fit.

Before you start, make a photocopy of the insulating material on the bottom of the factory cells to use as a template to cut your cardboard.  You'll probably need to destroy it to see how the factory cells are connected to each other and the inside of the case has gussets that will interfere with a general shape that just approximates the footprint of the cells.

My soldering iron is a Hakko Dash 25w with a semi chisel tip.  They sell for about $30 on Amazon.  The reason I like that particular model is that the tips attach using a compression sleeve and will not come loose from heat expansion and contraction the way cheapo irons with set screws do.

First bend some tabs
Divide your cells in half and strip the insulator off the positive lead of one half and the negative lead of the other half.  Set one of each aside and bend the stripped tabs of the rest 90° as shown on the three negative-up cells in the back.  Bend them all the same direction.  It doesn't matter which direction as long as all of them are the same.

Pair them up
Rubber band all the ones with bent tabs together in pairs as shown.  It's probably a good idea to tin the leads before doing this step, but I didn't.  I did them this way to ensure the bottom tabs face opposite directions.  It may not be necessary, but it worked for me.

Pre-tinned leads
This photo just shows what the pre-tinned leads look like before the tabs are soldered together.

Solder together the pairs
With the leads pre-tinned, press them together with the iron to let the solder flow between them, then hold them together with  something pointy and heat proof and remove the iron.  This will ensure the tabs don't spring apart before the solder solidifies.

It has been pointed out to me on reddit that a thin strip of protective material should be slipped under the tabs while soldering them.  That will protect the material isolating the terminals of the cells from the heat of the soldering iron.  The way I did it could lead to the tabs shorting together the negatively charged body of the cell with a tab connected to the positive terminal, which could lead to a fire.

Paired cells
This shows all but two cells soldered together at one end in pairs.

Bending the top tabs
Here I'm bending the top tabs so they will connect in the same pattern as the factory battery pack above them.  Before this step, I arranged the pairs of cells in the case the same way as the bottoms of the pairs of factory batteries are arranged.  Unfortunately I didn't get a photo of that step.  Note that I've also included one of the two cells I had previously set aside.  That cell's positive tab gets soldered to the positive lead of the battery pack's connector.  I should have done that at this point, but had not when this photo was taken.

Solder the top tabs
I taped the cells together using a single layer of scotch tape.  For my pack, I taped four together in a square and seven together in a hexagon, then taped those groups together.  The tape just ensures the cells stay snugly together for soldering the top tabs.  Soldering is then done the same way as it was done for the pairs previously.  Note that all the tabs reach each other and overlap nicely with a little creative folding.

Cut out cardboard insulators
Here I've trimmed the photocopy of the factory insulator and traced it to make my insulators the same size and shape to fit nicely in the case.  My cardboard was one of the spacers packed in a big box of dog biscuits from Costco.  Anything about .020 thick will work.  If you don't already have a good way to precisely measure thicknesses, Harbor Freight has decent stainless steel digital calipers for $10 with a coupon all the time.  They are extremely handy for all kinds of things.

Since writing this, I've learned that the material used for insulation by the factory is called fish paper.  It's not actually paper, it's a polymer made entirely of cellulose using processes similar to paper making.  It's a far more heat and fire resistant insulator than plain cardboard and it's available online.  If you want your rebuilt battery to be as close to as safe as original as possible, either save the original fish paper insulators, or get some and make new ones.

Slot your top insulator
Cut a slot and leave a hole for the connector lead to come up through the center of your insulator.  Also cut a slot for the negative tab of the cell that connects to the neck cell.

Solder in the neck cell
Here you can see how I bent and soldered the negative tab of the neck cell to the connector tab that was formerly spot welded to the factory neck cell.  Make sure you understand the exact placement of the neck cell before you bend the final two tabs to ensure they will meet properly when the neck cell is positioned correctly.

My drill came from a storage locker sale and both battery packs were dead.  I also bought three more dead packs on eBay.  My final steps were to erase the word 'junk' someone had written on this pack and remove the 'recycle me' sticker to differentiate it from the ones I haven't refurbished yet.  I bought all new cells for two packs this time around. My next battery adventure will be to separate all the factory cells I have now, attempt to charge them, and get rid of the bad cells.  Then I can use the good ones to rebuild my other packs.  I've heard that these packs die because only one or two cells in them have actually gone bad.  It'd be a shame not to reuse the good ones.

New battery packs cost around $70 each on Amazon.  I bought enough cells to do two packs for $34 on eBay.  There's a LOT of profit built into these things! Also, the cells I bought *say* they are 2800mah.  If that's actually true, these packs should run quite a bit longer between charges than they originally did.