Batteries store energy in a reversible chemical process. In so doing their ability to discharge that energy is limited by the rate at which that reaction can occur internally. The reaction is exothermic so the batteries get hot and sometimes catch on fire. Lithium ones are particularly nasty as the materials are reactive with oxygen in the air so once the battery begins to disassemble itself the additional reactions are started.

Capacitors on the other hand simply store charge. There is no chemical process involved and so their internal resistance is much much lower (it is basically the metal conductivity of the plates and the wires to the plates). So a capacitor with with the equivalent of 2mJ of charge, can release it all in picoseconds. [1] Such an energy release would be indistinguishable by people observing it from any other high explosive detonation. Further is would have no warning, batteries at least get hot, then start smoking, and then catch on fire. This would simply consume what ever conductor had shorted it and immediately turn it into a raging ball of plasma.

[1] I once considered a supercap as a "fun" battlebots weapon with the 'fork of dooooom' which when it hit your bot would vaporize that section.