Bonus Energy is automatically restored at the rate of 1 every 30 minutes. You can restore up to 10 Bonus Energy in this way.
There are also other ways to restore your Bonus Energy:
Rank Up
Restores 10 Bonus Energy.
Use Crystals
You can restore 5 Bonus Energy for 50 Crystals.
Use a Bonus Energy Drink
Bonus Energy Drink (L) restores 10 Bonus Energy.
Bonus Energy Drink (S) restores 1 Bonus Energy.