I have a 12 volt Ni Cad Screwdriver battery (to be exact, it is a Black & Decker Firestorm) I would like to find the amp-hours of the battery. The battery doesn't say anything on it (except that it is 12 volts), and I couldn't find any info online. I have no amp meter for testing amps drawn/time, but I tested a 12 volt 21/5 watt bulb and it lasted around 45 minutes. I did the division of (21/5)/12 and got .35 amp hours if it lasted an hour, but it was only 45 minutes and that would be less than .3 amp hours. I don't think this is true. So am I doing anything wrong and if so, what should I fix, and how can I find the amp hours?

edit: The model number of the pack is ps160 12v type 3 power pack

edit 2: I just figured out that the 21/5 means there is 2 filaments and doesn't mean "21 divided by 2" so I probably used the 21 watt because I used the brighter one.

  • \$\begingroup\$ Working backwards through your math, you had 0.35Ah with a 1 hour discharge, suggesting 4.2W. This is neither the 21W filament, nor the 5W filament. Which filament did you use in your discharge test? \$\endgroup\$
    – HikeOnPast
    Dec 3, 2012 at 4:26
  • \$\begingroup\$ oh I thought it 21 divided by 5! silly me!! \$\endgroup\$
    – skyler
    Dec 3, 2012 at 4:30
  • \$\begingroup\$ I used the brighter one, which was probably the 21 watt \$\endgroup\$
    – skyler
    Dec 3, 2012 at 4:31
  • 1
    \$\begingroup\$ Bingo. 21W, 12V, 45min = 1.31Ah \$\endgroup\$
    – HikeOnPast
    Dec 3, 2012 at 4:32

1 Answer 1


The Black & 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.

  • \$\begingroup\$ +1. Good bit of detective work, and 1.2Ah matches nicely with the rough calculation in comments for the 21W bulb. \$\endgroup\$
    – user16324
    Dec 3, 2012 at 10:41

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