At this point it would be good to mention you should really discharge the pack before you continue. Don't play with electricity unless you know what you are doing!
Step 1: Let's find out what is inside!
Fortunately for me the internals were looking familiar, a simple set of "Sub C" rechargable NiCd cells from my RC car racing days. I used to build my own racing battery packs so I couldn't help applying that here.
I had visions of using batteries that power world championship RC cars and transform my moderately powered B&D into a drill that would make screws and yet-to-be-drilled holes tremble with unspeakable fear.
When you open the battery you'll need to save some plastic bits detailed below. Don't break them.
Step 2: Replacement Sub C Cells
These fellas cost me $1.79 each for a grand total of $21.48 from Budget Batteries. So far so good.
In case you aren't good at math, a 14.4 battery takes 12 1.2V cells to make up a pack.
Step 3: Battery Bars
I only needed 10 to assemble one pack, but I have another drill battery waiting do die. So, that's five bucks for the bars and up to $26.48 total. Still ten bucks shy of the best retail price I could find ($35.00) Incidentally that price was direct from Black & Decker.
Step 4: A Bit of Wire...
Just had this lying around and I only needed about 3 inches so let's call it free.
I would recommend using good quality wire here. Your battery will only be as good as the weakest link. I prefer not to have any weak links. This will also be a likely spot for a meltdown if your wire is not up to the task.
Step 5: On to the Assembly
Don't forget to tin both ends of the battery before you try and attach the bars. A drop of flux and a drop of solder, not too much though.
I should mention that you will need a pretty serious soldering iron to get this job done. Don't overheat the battery...kind of a catch 22 here.
Step 6: Now the tricky part...
The real tricky part is the cell that sits on top of the pack.(red wire attached) This top-most battery sticks up into the shank of the battery case. The little black plastic bit houses the stock connectors that mate with the actual drill and charger...so save that and be careful not to brake it.
The top battery needs to be attached to the battery direcly below it and sit slightly offset. B&D accomplishes this with another plastic part that I tossed. I used one of the stock battery bars that I pulled off the stock battery pack since they were thin and flexible. Both batteries were soldered to the sock bar while they were sitting next to each other. Then, to move the one onto the top, I just folded the battery bar in half...clear as mud...I guess I should have taken a picutre. Finally I wrapped the bottom of the top battery with electrical tape to prevent any shorting.
Step 7: You're done!
You can hear the difference compared to the stock battery. This puppy is just begging for some heavy drilling. Time to pull some screws out of a big crate.