Having gauntlets that replenish grenade energy works better. Every punch gives you energy. Don't need your special melee for it to work.

EDIT: This is apparently wrong (discussion below). I will verify tonight, and delete this post if is. I'd anyone has video proof, I will delete this sooner of course. :) The next time you do it, keep an eye on your grenade recharge timer. When you energy drain, you recover a chunk of time at once. It's nor massive though, I would guess around 12% or so (just based on visual approximation). It doesn't increase the rate of recovery. Example, I'd you need 100 "grenade points" to throw a grenade, you recover 1 point per second when you have thrown one. So until you can 'nade again, you have 100 seconds. I'd you energy drain, you recover 12 "grenade points" at once, and now only need another 88 seconds to wait.