Ok, I took one apart. They're not AA's, they're slightly larger, and cost about 5 bucks each for exact replacements. Interestingly enough, the newer version of the pack weighs a lot less, so I suspect it uses actual AAs. I left it at work though, so I'll have to check details tomorrow. They're pretty much in series with one resistor between two of the positive contacts, who's purpose is lost on me.

I've got a battery charger that has a spot for 9 volt batteries. Can I just hook that up to the positive and negative contacts? I don't mind if it's not great for the batteries, I suspect I just need to get them slightly charged.

Matthew