Finding the Amp hour of battery
The Black &amp; Decker Firestorm 12 Volt drill / screwdriver FS1202D seems to match your description, I have the same product. This model uses a 12 Volt NiCd 1.2 Ampere-hour battery pack.

If your experiments indicate a much lower ampere-hour rating, that could be due to battery aging, charging issues or the battery being loaded beyond the current / continuous operation parameters it is rated for.

If you could provide the screwdriver model number, a more definitive answer could be obtained.