A Reserve Tank (リサーブタンク Risābu Tanku) is a type of expansion exclusive to Super Metroid. There are four Reserve Tanks to be found on Zebes in Brinstar, Norfair, the Wrecked Ship and Maridia.
As the name suggests, each Reserve Tank provides an "extra" energy reserve of 100 points, with a maximum reserve of 400 once Samus finds them all. They can be set to be used automatically once her base energy is reduced to zero, or to be used manually from the Samus Screen.
Recharging Samus' energy at her ship can refill the Reserve Tanks, though using an Energy Charge will not. She must otherwise collect Energy Capsules from defeated creatures to charge them, once her base Energy Tanks have been filled to capacity.
Super Metroid manual
"Reserve Tanks act as Samus’ emergency energy source. Reserves Tanks are empty when you collect them."
Trivia
- The Reserve Tanks are identical in function to the Sub Tanks from the Megaman X series.