I've noticed that most seem to use a Dremel for this, but my Dremel died years ago.
I thought I'd ask my brother what he had in his shed that I could use (He's retired, and his hobby is customizing Chev Pickups and building Hot Rods), his back shed is twice as big as the the duplex house we share, and includes things like central V-Twin air compressor, fully ventilated spray painting booth big enough for dual-cab pickups, permanent ramp for working under pickups etc.) and he suggested a tool to use.
It's massive overkill but it did the job very quickly - a Chicago Pneumatic Die Grinder and it could shorten a cell if you wanted.
This is it http://www.sears.com/chicago-pneumatic-die-grinder/p-00998346000P
They are $10 at Harbor Freight, but I wouldn’t think of using that tool for this job, it’s way too much power for this job…really!
The metal battery casing is thin and you want to be careful not to remove any material from the casing itself and weaken it or create a trench in the metal.
A Dremel is capable of fine work, but can also easily remove too much material if you slip and create a hole or weak spot.
I use a Dremel or Swiss needle file to get the nub down close, and a sheet of 400 grit sandpaper wet on a sheet of glass to smooth it out.
Thats a nice diegrinder xv. With the right tool in it this would be ok but as suggested carnage will result if your not extremely careful. Safety glasses are an absolute must if not a full face shield when using this tool as its very easy to get very small slithers of metal in your eyes. If I may also suggest that when using any tools like this that if your not 100% comfortable (working and it feels awkward) mistakes will happen. Your brothers shed sounds like a good place to be.