Hi War pony
A battery goes off with the force of a stick of dynamite, a flimsy plastic box will not contain that amount of force (far thinner than the battery case and look what happened there!) You can you the vent caps flew off and yet there was still enough force to break the case.
In the above case there was no charging going on (vehicle just being started) and even with the minimal gas that was present, it still had enough force to blow the case apart.
A contained explosion generates a more devastating force.
You have it the wrong way round...
The optima (and any AGM Battery) only reduces the chances of battery explosion, unfortunately because of its sealed nature if it does explode it is far worse than a normal battery.
I will try to explain this as simply as possible.
In free air an explosion can expand freely, it can dissapate without as much force, the amount of explosive in a grenade would not cause great injury, confine it in a metal case and the force builds up till there os enough to break apart the case, btw having laid the charge from a grenade on the floor and lit it, all it does is flare on a smaller scale consider a hose water runs out, but if you block the hose the force builds up (same principle but bigger scale)
If you want the physics of how thicker cases and confining it increases the violence of the explosion read here. Some of it goes pretty deep but you should be able to get the general picture
http://www.intdetsymp.org/detsymp2002/P ... rk-248.pdf