"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.

Except for the (very recent) LiIon tool batteries, you will find a bunch
of NiCd or NiMH cells in series in the pack. The cells are 1.2 V each,
so you can easily calculate the number of cells without opening the pack
from the voltage: 12 V is 10 cells, 18 V is 15 cells, etc.

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).

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.

Dave