Gideon wrote:

Buying a new drill/battery/charger is almost always a better choice. That's
assuming that you don't have a fairly modern, high-voltage, top-of-the-line
drill. Purchasing an individual replacement battery is expensive.
....

As for your existing drill and battery: It usually isn't that difficult to cut
open the existing pack and replace the cells inside. Sometimes you may need a
dremmel tool to fit generic replacement cells inside the case if the
manufacturer purposely used non-standard sizes to prevent this. Sears is one
culprit for such games. The same advise goes for cordless phone battery
packs - often one needs to "Dremmel out" bits of the handset interior to fit
in replacement cells which are slightly larger than the intentionally sub-sized
originals. Any battery pack can be replaced - it's just a matter of how
difficult the manufacturer intentionally made the task, how handy your are, and
the value of your time.

Any sizable town will have one or more places that have the tools and
expertise to rebuild virtually any pack, almost always at reasonable
cost...