How to exchange the dead Ni-Cad battery of a cheap chinese import cordless drill? That was the question and challenge I had to solve. The solution is simple. You can exchange the nickel-cadmium of a 12Volt, 14.4V and 16,8V cordless drill by a cheap 12V - 1.2Ah lead gel battery. Perhaps there is lesser speed, instead it produces higher torque on the drill head. I bought the lead gel battery in an electronic shop for 10 euro or 12 dollar. The specs are: Nominal voltage: 12V, Rated capacity (10h rate): 1.2Ah, Size (L x W x H): 97 x 43 x 52mm, Total height: 58mm, Weight: 0.61kg (approx.).
I tested this battery also with higher voltage drills and it functioned surprisingly well. Beside that you can put this battery in most battery boxes from the old pack. The charger of the 12V cordless you can continue to use. For higher voltage drills you have to change the charge adaptor by a 12 till 14V 400mA DC unstabilized one. Don't use the old network adaptor. Most charge stations don't have, for Ni-Cad battery packs, electronics build in . I hope that many people go to fix there cordless drills. It must be ten of thousands laying around in garage and workshops. They are mostly in perfect shape and have still years to go; only that battery...
Addition: To avoid wasting the old adaptor, you can use a charger IC. It is the PB137. You need only 2 capacitors extra. See the diagram. The input voltage can be between 16 and 40 Volt DC. This circuit fit's easy within the charger unit.