I won't comment on what some people are saying, but here are some facts:
Car batteries and UPS/Fed Ex usually don't mix. A sealed gel type of battery (Optima, Orbital, Hawker, Stinger, etc.) can be shipped by UPS/Fed Ex. Who gives a crap, right? Well, that is what I'm getting it. Because instead of having lead plates sitting in acid, the cells are made up of a plate and a sponge-like material rolled together. So even if a UPS guy pulls an Ace Ventura on this thing acid won't leak all over the place.
So, the original poster says "My battery is sitting at 12.98 volts, its OK, right?" Well, leave a wet sponge sitting in a hot engine comparment, that acid is going to evaporate. No acid, the cell goes dry.
Batteries that are overcharged produce hydrogen (think Hindenburg). The hydrocloric acid starts boiling, and Hydrogen comes out. Hydrogen is one of the lightest gases out that, but is very flamable. A normal battery won't produce Hydrogen. So it isn't going to blow up if you try to fix it.
I would suggest maybe a get a big patch, and keep the glue away from the cell as much as you can and try to seal it up. That is probably as good as you are going to get.
Measure twice, cut once. The world is littered with little stories like yours (buddy blew up an amp because he used black for power and ground because that was all we had and we didn't want to make a run to the store). I've done the same thing (it was the pully not the fan, but it was a lead acid battery).
Juan