Reserve Tank

A Reserve Tank (リサーブタンク Risābu Tanku) is a type of expansion exclusive to Super Metroid and Metroid: Samus Returns. 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 units, 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 and the Crystal Flash 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.

The Reserve Tank returns for the first time in over 20 years in Metroid: Samus Returns. If the Zero Suit Samus Super Smash Bros. amiibo is used, a Reserve Tank will be unlocked. When Samus's energy is depleted, the tank will automatically replenish 299 energy. The tank has indefinite usage and cannot be discarded unless a new save file is started. Using the Smash Bros. Samus and Samus Returns Samus amiibo also contain Missile and Aeion Reserve Tanks that will replenish Samus's Missile Ammo (50 regular Missiles, or 10 Super Missiles) and Aeion.  Dying due to damage from the purple liquid will not cause the Reserve Tank to activate.

Official data

Reserve Tanks in Samus Returns on the Map screen.

Super Metroid manual

"Reserve Tanks act as Samus’ emergency energy source. Reserve Tanks are empty when you collect them."

Metroid: Samus Returns amiibo warning

"Once a Reserve Tank is unlocked, this function cannot be deactivated until you start a new game. Unlock?"

Trivia

• The Reserve Tanks are identical in function to the Sub Tanks from the Mega Man X series. However, Sub Tanks in Mega Man X do not automatically refill energy when health is depleted and must be manually used.