(Maybe add a second model so the heavy holds one in each hand, maybe.)

Rename it: Double-Pounder

Stats:
+25% faster consumption rate
+25% faster recharge rate.
Gains up to 75 hp when used.
Can get up to 2 uses when fully charged. (Think of it like the Thermal Thruster)
Alt-Fire: Throws a small health pack at the cost of 20% charge. (or Reload if do the duel wielding)

If you do add the second, then it can have the same fire mechanic as the Heavy's fists.