What is EL15? (Google says it's a battery, but there are no batteries in H2.)
The answer to your question is that the excess energy is in the vibration of the H-H bond. If you think of the potential energy curve, two hydrogen atoms approaching each other have sufficient energy for the H2 molecule to fall apart within the first vibration unless some energy is lost. If within the first vibrational period the new molecule collides with another molecule and transfers some of its vibrational energy to that, it is now in a stable bound state, and the other molecule has increased kinetic energy.
(It's analogous to the case where you put a mass on the end of a vertical spring and let it go. It doesn't stop at the equilibrium displacement, but oscillates about this point. To get it to stop you have to absorb that extra energy (with your hand, or by air resistance etc.)