According to Dave Martindale :
"Mamba" writes:

I encountered a similar situation when I bought a used Dewalt drill. One of
the batteries would hold only a 20-30 minute charge. A friend told me that
essentially the battery pack contains more conventional rechargable
batteries - ie. If you have a battery pack that opens (clips or screws), it
will likely contain 2 or 4 smaller batteries. You can likely find
replacements for the 2 or 4 batteries quite inexpensively from an online
dealer.

The batteries in most tools (including DW) are "SubC" form factor. They're
a bit smaller than C cells. You _can_ get them online, but it's more
a specialty item. The more retail-oriented battery sites don't
carry them.

The connections between cells are welded straps. If you don't have the
appropriate welding equipment, you *can* buy cells that have solder tabs
welded to the cells, and then connect those in series via suitable heavy
wire. You probably can't solder directly to cells without solder tabs,
as the cells get too hot before the solder melts which damages the seals
and the cells leak. (If you have a high-wattage iron and you're really
good at quick soldering, you might succeed, but good luck).

It's possible to solder SubCs with a soldering gun (like a Weller),
but you have to preheat the tip for at least a minute or two first.
Better to use an iron similar to what stained-glass people use - big
beefy thing around 300W.

Yes, you have to be quick.

You need to abrade the contact points first - a dremel with a sanding
drum is best/fastest, but you can do it with (emory preferably) sandpaper.

The other difficulty is what you use for strap. I use #18 guage
stranded wire, with the strands spread flat. However, that still usually
leaves a lump, which can make getting the batteries to fit properly
a bit of a pain.

Next time I'm going to experiment with some copper tape (still soldered).

Caution: while 18ga wire can take nearly 100A before melting,
a single fully charged NiCad SubC battery _can_ melt the wire if you
short it accidentally. Newly charged/fresh NiCads can push a _lot_
of current.

You're generally better to take the pack to a battery rebuilder who has
the welding equipment and a supply of the nickel strap material, and who
will rebuild the pack for you for little more than the cost of the cells
alone in the quantity that you would be buying.

The one I deal with seems to do the rebuild (at least for a 10 cell
DW pack) for just the cost of the batteries.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.