Nah, that's the wrong way to think about it. Oxidation (well, burning) of lithium releases far more energy than the electric charge of the battery. Electric (dis)charge merely gives the initial heat to break structural integrity and set lithium on fire.
And even then the problem is not in the amount of energy, but in that it's almost impossible to extinguish a lithium fire. And the fumes from battery are thick and toxic.
> I think the density of the energy is what is most dangerous in my mind. A whole room full of lead acid batteries might not be very portable...
Nah again. Lead-acid just does not burn so spectacularly. And about the energy density - filled fuel tank of a regular car has much higher energy density than Tesla battery, but is significantly harder to ignite, easier to extinguish, and when burning, releases the energy slower.